Critical Care Medicine

Respiratory Failure in Obesity

Acute Respiratory Failure in the Obese Patient

Hypercapneic Respiratory Failure in the Obese Patient

Acute Hypoxemic Respiratory Failure in the Obese Patient

Postoperative Respiratory Failure Following Bariatric Surgery

Postoperative Respiratory Failure Following Non-bariatric Surgery

Respiratory Failure in the Bariatric Surgery Recipient

Failure to Liberate from Mechanical Ventilation in the Obese Patient

Synonyms

Acute Respiratory Failure in the Obese Patient

Hypercapneic Respiratory Failure in the Obese Patient

Synonym: Type II Respiratory Failure, Ventilatory Failure

Acute Hypoxemic Respiratory Failure in the Obese Patient

Synonym: Type I Respiratory Failure

Postoperative Respiratory Failure Following Bariatric Surgery

Synonym: Type III Respiratory Failure

Postoperative Respiratory Failure Following Non-bariatric Surgery

Synonym: Type III Respiratory Failure

Respiratory Failure in the Bariatric Surgery Recipient

Failure to Liberate from Mechanical Ventilation in the Obese Patient

Synonym: Weaning from Mechanical Ventilation

Related Conditions

Obesity Hypoventilation

Obstructive Sleep Apnea

Chronic Obstructive Pulmonary Disease

Asthma

Diastolic Heart Failure

Systolic Heart Failure

Cardiogenic Pulmonary Edema

Negative Pressure Pulmonary Edema

Venous Thromboembolic Disease, Pulmonary Embolism

Hypercapneic Respiratory Failure Acute Hypoxemic Respiratory Failure

Postoperative Respiratory Failure

Failure to Liberate from Mechanical Ventilation, Weaning from Mechanical Ventilation

1. Description of the problem

What Every Clinician Needs to Know

Defining Obesity

Fat, stored throughout the body, cannot be measured directly. The measurement most often used to quantify body fat is the body mass index (BMI), a measurement of weight relative to body surface area. The BMI is easy to calculate (weight in kilograms divided by the height in meters squared), correlates with body fat in most people and has defined risk categories.

The National Heart, Lung, and Blood Institute (NHLBI) guidelines use patient BMI to guide the following labels:

Underweight: less than 18.5 kg/m2

Normal weight: 18.5 to 24.9 kg/m2

Overweight: 25 to 29.9 kg/m2

Obesity: 30 to 39.9 kg/m2

Morbid Obesity (also known as Extreme or Severe Obesity): 40 kg/m2or greater

The correlation between BMI and obesity is very good in most middle-aged adults but can be misleading in individuals engaged in weight training with increased muscle mass and in individuals with significant edema.

Definining Obstructive Sleep Apnea

The diagnosis of OSA is based upon the presence of symptoms of disturbed sleep as well as the frequency of respiratory events during sleep (ie, apneas, hypopneas, and respiratory effort related arousals) as measured by polysomnography or portable monitoring.

In adults, OSA is confirmed if either of the two conditions exists:

1) There are 15 or more apneas, hypopneas, or respiratory effort-related arousals per hour of sleep (ie, an apnea hypopnea index or respiratory disturbance index ≥15 events per hour) in an asymptomatic patient. 75% or more of the apneas and hypopneas must be obstructive.

2) There are five or more obstructive apneas, obstructive hypopneas, or respiratory effort related arousals per hour of sleep (ie, an apnea hypopnea index or respiratory disturbance index ≥5 events per hour) in a patient with symptoms or signs of disturbed sleep. 75% or more of the apneas and hypopneas must be obstructive.

Defining Obesity Hypoventilation Syndrome

Obesity hypoventilation syndrome (OHS) is defined as the combined presence of obesity and arterial hypercapnea (paCO2 > 45 mmHg) while awake, in the absence of other causes of hypoventilation.

Diagnostic criteria for OHS include the following:

1) BMI greater than 30 kg/m2

2) Awake arterial hypercapnia (paCO2 > 45 mmHg)

3) Exclusion of other causes of hypoventilation

4) Polysomnography revealing sleep hypoventilation with nocturnal hypercapnia with or without obstructive apnea/hypopnea events

Patients with OHS may have obstructive sleep apnea (OSA)/hypopnea syndrome with hypercapnia, sleep hypoventilation syndrome, or a combination of sleep-related breathing disorders.

Obesity in the ICU

An ever-increasing percentage of people in developed countries are obese or overweight. Approximately one third of intensive care unit (ICU) patients are obese and nearly 7% have morbid obesity, frequencies that are predicted to increase as the prevalence of obesity in the general population rises. Understanding the challenges of obesity is quite relevant to the practicing intensivist.

Overweight, obesity and extreme obesity are associated with all-cause mortality among the general population. Obesity is associated with multiple comorbid conditions including diabetes mellitus, cardiovascular disease, hypertension, and cancer. Likewise, physiologic derangements, including a proinflammatory state and insulin resistance, are associated with obesity. These factors, combined with physical limitations and pharmacokinetic alterations, complicate acute illness and may impede the implementation and efficacy of evidence-based interventions in the ICU. Although most studies looking at outcomes of mechanical ventilation in this obese group have found an increased ICU length of stay, a recent meta-analysis showed there is no difference in mortality between obese and nonobese ICU patients. For this reason, it is important that those caring for obese patients in the ICU are not unnecessarily pessimistic in their outlook, despite the known relationship between obesity and ill health.

Compared with BMI-matched eucapnic obese persons, OHS patients have increased cardiovascular and respiratory morbidity, higher healthcare utilization, higher likelihood for hospitalization, and a significantly lower postdischarge survival rate.

Key Management Points

Acute respiratory failure in the obese patient

1. Evaluate for the acute nature of the problem.

The usual examination can be quite limited in the obese patient. Markers of volume status (neck vein assessment, precordial impulse, distal pulses), chest radiographs, electrocardiograms, blood pressure measurements (appropriate cuff sizing), and even bedside ultrasound can all be limited. As a result, a healthy suspicion for respiratory failure, ventricular dysfunction, and pulmonary hypertension is warranted even in patients with nondescript symptoms such as lethargy.

Early assessments should include

- arterial blood gas measurement

- electrocardiogram

- bedside ultrasound (including estimates of ultrasound collapse point of jugular vein, limited view echocardiography, lung and pleural evaluations)

- consideration for early central venous access

- phlebotomy, including

   - chemistry (bicarbonate, kidney injury)

    - cardiac panel

    - LFTs

  - lactate

    - BNP

In the unresponsive patient:

   - ammonia

   - UDS

Historical clues:

- polysomnogram

- non-invasive ventilation (NIV) setting

- sleep patterns

- prior respiratory failure, difficult intubation

- prior venous thrombotic disease

Criteria for ICU monitoring and management:

- acute acidemia (pH < 7.30)

- decreased level of consciousness

- hemodynamic instability

- refractory hypoxemia

- intolerance to NIV

Precipitants to deterioration:

- acute illness (ischemia, infection, or thrombosis)

- respiratory depressant effects of alcohol, other sedatives, and opioid analgesia

- acute kidney injury

OHS may deteriorate secondary to a concurrent acute illness or to the use of aggravating substances (alcohol or sedatives), so that the patient develops acute hypercapneic respiratory failure (AHRF) with significant hypoxemia, uncompensated respiratory acidosis, mental status changes, and possible coma.

Patient populations at particular risk:

Prior studies of OHS patients in ICUs have described at-risk patients as being middle-aged (mid-50s) and morbidly obese (BMI > 40) with daytime hypercapnia, hypoxemia, and acidemia (pH <7.30). Although OHS is more prevalent among OSA patients with higher apnea-hypopnea index (AHI), AHI is not a good predictor of which OHS patients are likely to develop AHRF.

2. Assess the airway and ensure patency.

External examination should include investigation of the upper airway (apply Mallampati score), neck circumference, and BMI.

- Assess airway reflexes.

- Auscultate over the neck for air entry and possible stridor.

- Observe respiratory pattern, evaluating for paradoxical breathing, either persistent or periodic.

- In the lethargic patient with poor air entry, consider jaw thrust.

3. Support breathing, implementing non-invasive ventilation (NIV) early.

NIV is an established treatment for chronic conditions such as OHS and OSA and for acute exacerbations of obstructive pulmonary disease and cardiogenic pulmonary edema. Given the high prevalence of these conditions within the obese population, NIV may be utilized as a first-line therapy in the support of obese patients with respiratory failure. It confers unloading of the respiratory muscles, decreases atelectasis, and addresses sleep-disordered breathing while maintaining an interactive patient. Contraindications to NIV include decompensated shock, active myocardial ischemia or gastrointestinal hemorrhage, intolerance to the mask (claustrophobia or facial trauma), or the inability to adequately protect the airway. Given the additive increased risk of aspiration with obesity, airway reflexes must be carefully evaluated.

For the patient experiencing obstructive events without fatigue, CPAP may be utilized. CPAP does not directly augment ventilation independent of maintaining upper airway patency.

For most patients with acute respiratory insufficiency, BiPAP will likely be necessary. During BiPAP therapy, an inspiratory positive airway pressure (IPAP) and an expiratory positive airway pressure (EPAP) are independently set and titrated. Tidal volume correlates with the difference between the IPAP and the EPAP. Alveolar ventilation is enhanced by a larger tidal volume assuming that the respiratory rate is constant. Advantages of BiPAP compared to CPAP include a lower mean airway pressure (which may lead to better tolerance of therapy) and better rest of the ventilatory muscles.

Most patients require a full face mask for support of acute distress. Nasal devices often yield pressure leak - distressed patients often open their mouth and maintaining mouth closure is difficult when applying higher (rescue) ventilatory pressures.

4. Optimize noninvasive positive pressure ventilation (NIPPV).

Mask leak may be the result of either poor mask fit or an inadequate EPAP/CPAP to splint open the upper airway. Once fit is established, the EPAP/CPAP should be uptitrated to ensure that the chest is rising and falling with inspiratory efforts. Titration of IPAP in BiPAP should be dictated by the adequacy of minute ventilation and carbon dioxide elimination.

Patients may become less responsive after NIV, either the result of sleep (as they often present in a sleep-deprived state) or decompensated ventilatory failure. Surveillance of minute ventilation coupled with sequential blood gases or end-tidal carbon dioxide measurements will be necessary to delineate the two conditions.

Absence of improvement in arterial blood gases within 2 hours should prompt invasive ventilation.

Airway management

This situation can frequently be complicated by severe hypoxemia and hemodynamic decompensation.

To avoid derecruitment, continue noninvasive ventilation until the intubation sequence.

Triage the acuity of the emergency:

Recognize the potential for a difficult airway.

Assess all potential risk factors.

Generate a preplanned strategy, including back-up plan. If non-emergent, seek help and carefully plan.

Gather and check necessary equipment.

Optimize patient position.

Optimize mask ventilation or continue NIV.

Discuss merits of neuromuscular blocking agent (NMBA)

Obesity and Difficult Airway

There is only a modicum of difference in how the airway is approached between obese and non-obese patients. Studies focusing on obesity as a risk factor for airway problems have reported a wide range of risks. Obesity does not seem to be an independent risk factor for difficult intubation but is one of the several factors that need to be considered as part of an airway evaluation. However, when an obese patient cannot be intubated, maintaining an adequate airway is more likely to be challenging and may lead to a "cannot intubate, cannot ventilate" situation.

Risk factors for difficult intubation include: Mallampati score, thyromental distance, cervical spine extension, interincisor distance, upper lip bite test, neck circumference, and BMI. None of these factors has a positive predictive value of greater than 50%, but all have a negative predictive value greater than 95%. If a patient tests positive for any risk factor, there may be a difficulty in intubation. The more risk factors, the more likely the patient is to have a difficult airway and intubation. Patients who test negative for all risk factors are unlikely to have a difficult airway and intubation.

When encountering the airway of an obese patient, a high index of suspicion is sensible; the American Society of Anesthesiologists recommends that a preplanned strategy is put in place, including an equipment check and the preparation of a back-up plan.

Other challenges contributing to difficult intubation sequences:

Excess soft tissue around the neck and in the oropharynx can make mask ventilation difficult or impossible. When bag-mask ventilation is difficult, oxygenation, not intubation, is paramount and may require 3 hands for bag-mask ventilation and oral and nasopharyngeal airways. Adequate preoxygenation is vital and, despite this intervention, obese patients will desaturate more rapidly and recover from hypoxia more slowly than non-obese ones.

Additionally, increased intra-abdominal pressure from abdominal adiposity increases the risk of regurgitation and aspiration.

An increased neck size can make retrograde wire intubation and emergent surgical airways procedures difficult.

Essential equipment:

Endotracheal tube with stylets, functioning suction, bag-valve device connected to an oxygen source, laryngoscope and blades, oral/nasal airways, and device to detect end-tidal carbon dioxide are essential.

Consideration for endoscopic assistance:

The use of adjunctive devices such as flexible bronchoscopes, supraglottic devices, and lightwands requires the acquisition and practice of a specific set of skills.

Some individuals prefer using a short or stubby laryngoscope handle when dealing with obese patients. These individuals should also be proficient with at least one alternative method of airways management, such as flexible fiberoptic bronchoscopy, supraglottic devices (LMA, COBRA PLA, Engineered Medical Systems, IN, USA), lightwand, or video laryngosocopy. There are several types of rigid stylets as well as more invasive techniques, such as transtracheal jet ventilation, retrograde wire techniques, and emergent cricothyrotomy.

Both the American Society of Anesthesiologists and the UK Difficult Airway Society suggest the laryngeal mask airway as their primary rescue in a "can't intubate, can't ventilate" scenario. If successful, some LMAs can then be used as a conduit for a fiberoptic bronchoscope. These masks should only be employed by operators experienced in their use, and there remains a risk of aspiration, large ventilatory leak, and laryngospasm with these devices.

Upright positioning:

Preoxygenation with sitting may help. Positioning obese patients so that the ear is aligned with the sternal notch (the ramp postiion) seemed to facilitate tracheal intubation. Elevation of the upper body with pillows may also improve the view of the larynx and the ability to bag-mask ventilate if a paralyzed technique is used. Consider early central venous access and vasoactive drug availability (especially those to recruit the failing right heart–acute pulmonary resistance changes).

Controversy regarding whether hypnotics or NMBA should be given:

PRO: Paralysis improves mask ventilation and laryngoscopy.

CON: Assisting patient's respiratory efforts, but if paralyzed and failed intubation, the airway has to be maintained until the intubation completed or patient returns to spontaneous respiration.

For patients with refractory hypoxia, ventilatory failure or postoperative respiratory dependence

5. Mechanical ventilation support:

Low tidal volume ventilation (ie, 6cc/kg ideal body weight), a mainstay of treatment for acute lung injury (ALI) and acute respiratory distress syndrome, can be challenging to implement in this population.

The landmark trial, performed by the acute respiratory distress syndrome network (ARDSnet), excluded patients who weighed more than 1kg/cm of height. This does not correlate simply with the BMI but is likely to have excluded many obese patients, especially morbidly obese ones. However, a secondary analysis of patients included in ARDSnet trials identified over 200 patients (BMI>30); similar outcome benefits of low tidal volumes were seen in this subgroup.

To overcome the pulmonary effects of increased abdominal pressure and reduced respiratory system compliance, higher levels of positive end-expiratory pressure (PEEP) and plateau pressure may be necessary to achieve adequate tidal volume and may not be injurious, as the transpulmonary pressure is not increased.

The Lung Open Ventilation study used higher PEEP levels and a plateau pressure of up to 40cmH2O with an outcome similar to patients ventilated with the ARDSNet protocol, but obese patients were also excluded.

An elevated plateau pressure, reflecting the compliance of the respiratory system (including both lungs and chest wall), may better reflect the weight of the chest wall rather than the presence of alveolar overdistention. Some authors argue that obesity is protective in this manner.

We advocate measurement of the plateau pressure in a semi-upright position, and permit some elevated values (ie, up to 35 to 40 cmH2O) in the morbidly obese. Although beneficial to those patients with ALI, low tidal volumes are not proven beneficial in other etiologies of respiratory failure and can precipitate further atelectasis, further worsening oxygenation.

The potential for development of intrinsic PEEP should always be considered and airflow limitation treated. Obese patients may have asthma, precipitated by low lung volumes and other triggers of airway hyperresponsiveness accompanying obesity.

There are limited data regarding other modes of mechanical ventilation such as high-frequency oscillatory ventilation, but this mode provides the benefit of increased mean airway pressure and has been used safely in these patients.

Mechanical ventilation in the prone position for severe respiratory failure in the obese poses a "Herculean" task for the ICU team, but it is possible and may be beneficial in refractory cases. An experimental study in eight mechanically ventilated pigs turned from supine to prone showed that oxygenation and pulmonary gas exchange improved significantly. The improvement was more pronounced in the presence of abdominal distention. Data from 10 obese patients without lung injury revealed that changing from supine to prone position during MV leads to improved pulmonary function, increased functional residual capacity (FRC), better lung compliance and oxygenation. There is only one case report on the use of prone position in a morbidly obese patient with ALI.

It is important that the position is established by supporting the upper chest and pelvis without compression of the inferior vena cava or femoral veins. The abdomen must hang freely.

6. Stabilize hemodynamics.

7. Treat the underlying disease.

A substantial number of patients may present with acute hypoxic respiratory failure and no prior diagnosis. If a diagnosis of OHS is suspected, treatment should be started immediately without waiting for confirmation by a sleep study. A sleep study can be performed once the patient's condition is stable to confirm the diagnosis and to prescribe the appropriate PAP therapy. However, sleep studies in acute care areas have been described.

A pulse oximeter with fast sampling rate

8. Remain cognizant of abdominal pressures.

Common Disorders in Critically Ill Obese Patients

  1. Thromboembolic disease

  2. Aspiration

  3. Abdominal Compartment Syndrome

AHRF in the Obese Patient:

PEEP application

Inhaled vasodilators

Deep paralysis

Ventilatory management for respiratory failure should be lung protective and aim at early weaning. Unfortunately, most therapeutic options have not been formally evaluated in obese cohorts. Most ventilatory strategies studied in obesity are derived from studies in obese patients (without lung injury) undergoing bariatric surgery.

The tidal volume for the obese patient has to be adjusted to the ideal body weight, not the measured body weight, to prevent the lung from harmful overdistention.

This suggests that obese patients with respiratory failure can be ventilated according to the protocol of the ARDS Network study: Vt: 6mL/kg ideal body weight; plateau pressure less than 30cm H2O; ventilation mode: assist control; respiratory rate 6 to 35/min (aim to keep pH: 7.3 to 7.45); and oxygenation goal paO2 55 to 80mmHg or oxygen saturation 88% to 95%; PEEP: 5 to 24 as specified in the referenced "PEEP table."

It may be difficult to stay within the recommended plateau pressure limits. Overdistention of the lungs is limited by the large thoracic fat masses and the elevated intra-abdominal pressure that may act as a cuirass. Therefore, application of high plateau pressures may be less harmful in obese than in non-obese patients.

Obese patients have a great potential for atelectasis. PEEP and lung recruiting ventilatory strategies can open atelectatic lung regions and prevent them from re-collapse. Pelosi et al measured lung volumes and mechanics in nine obese patients (BMI > 40) and nine non-obese patients postoperatively after abdominal surgery. A PEEP of 10 cmH2O significantly improved respiratory mechanics and oxygenation in the obese but not in patients with normal weight. In morbidly obese patients, PEEP level up to 15cmH2O may be indicated to prevent loss of ventilated lung volume and maintain a normal functional residual capacity.

PEEP alone, however, may not always protect the lung from derecruitment. Reinius et al investigated 30 patients (BMI 45±4) undergoing gastric bypass surgery and documented that an intra-operative PEEP of 10 alone did not reduce atelectasis or improve oxygenation. A recruitment maneuver with 55cmH2O for 10 seconds without PEEP also had no sustained effect. Only the recruitment maneuver followed by PEEP was effective. These results were confirmed by another group that demonstrated a vital capacity maneuver followed by PEEP 10 to effectively prevent lung atelelectasis and deterioration of oxygenation during and after bariatric surgery.

For obese patients the best lung recruitment may be achieved with the vital capacity maneuver (inspiratory pressure of 40cmH2O x 15 seconds) repeated every 10 minutes and combined with a PEEP level of 10cmH2O.

Hypercapneic Respiratory Failure in the Obese Patient

Ventilatory Failure: Other conditions that may cause hypoventilation should be considered during patient evaluation.

Identify mechanical limitations (kyphoscoliosis or chronic lung disease), neurologic conditions (neuropathies or diaphragm paralysis), central control problems (eg, hypothyroidism) leading to a different diagnosis.

Check airway mechanics.

Empiric bronchodilators

Consider steroids.

Weaning and Liberation from Mechanical Ventilation in the Obese Patient

All efforts should be directed at avoiding atelectasis and hypoventilation. Treatment includes adequate analgesia while attempting to avoid oversedation, early mobilization, vigorous pulmonary toilet, and upright positioning. The importance of the simple maneuver of upright or reverse Trendelenburg positioning has been shown to improve respiratory system compliance, closing volume, and, most significantly, oxygenation.

Consider NIV after extubation.

Properly position patients.

Minimize sedative use.

Mobilization

Avoid inappropriate feeding.

Consideration for tracheostomy

Some studies have noted increased ventilator days in obese ICU patients, while others have failed to find an association. However, many of the physiologic changes associated with obesity can affect weaning from mechanical ventilation. Increased blood volume, systolic or diastolic dysfunction, increased chest wall loading, decreased respiratory muscle endurance, elevated intra-abdominal pressure (IAP), upper airway narrowing, and potential alternation in central ventilatory drive may impair the ability to breathe spontaneously.

Diminished level of arousal may play a greater role in obese patients, who already may have significant narrowing of the upper airway and low lung volumes. The use of an endotracheal tube may blunt the reflexes of the upper airway and sedation may diminish tone in pharyngeal dilator muscles. All of these features may result in upper airway obstruction after extubation.

Problems of using steroids in stridulous patients

Post-extubation data

El Solh and colleagues compared extubation outcomes in 62 consecutive patients with a BMI 35 or higher who were treated immediately after extubation with NIV. Use of NIV was associated with a statistically significant 16% reduction in post-extubation respiratory failure. Rescue NIV given after the development of respiratory failure in the historical controls enabled a minority of these patients to avoid reintubation, with a resultant trend toward a decreased need for reintubation.

Obese patients are at higher risk for prolonged weaning and chronic ventilator dependence, and the post-extubation course may be complicated by the presence of sleep apnea. Usual assessments demonstrate elevated PEEP needs, marginal oxygen saturations, and shallow breathing patterns. Clinicians should utilize upright positioning, sedation-sparing strategies, and post-extubation noninvasive ventilation to combat these obstacles.

The rate of reintubation post-extubation in severely obese patients has been reported at 8% to 14% among patients undergoing mechanical ventilation for more than 48 hours.

NIV has been considered a promising therapy to avert respiratory failure after weaning. One trial of patients had severely obese patients undergo NIV for the first 48 hours immediately post-extubation. NIV was administered using bilevel positive airway pressure mode (BiPAP) initially set at 12 and 4 in a spontaneous mode. Pressure settings increased gradually to the patient's tolerance with the aim of achieving a respiratory rate (RR) less than 25 and saO2 greater than 90%. Compared with conventional therapy, the institution of NIV resulted in a 16% absolute risk reduction in the rate of respiratory failure.

NIV after extubation may reduce the incidence of reintubation in the obese patient. Early tracheostomy may provide benefits in this patient group.

The supine position is harmful for obese patients due to the elevation of IAP. This leads to a reduction in lung volume and consequently hypoxemia.

Trendelenburg's position in obese patients is more dangerous than the supine position. Reductions in PaO2 may be realized as a byproduct of increased venous return from the lower limbs to the pulmonary circulation, the vigorous pressure of visceral fat masses on the diaphragm with resultant decreases in FRC and generation of atelectasis. For application of central venous catheters, the obese patient should not be positioned in Trendelenburg's position. If a patient must be placed head down, he or she should be endotracheally intubated and mechanically ventilated, and continuous monitoring is needed.

Similarly, intrinsic PEEP has been measured in the flat, recumbent position. Turning into the lateral decubitus position reduced PEEPi (from 4.2 to 2.0).

For optimal ventilation the best body positions include semi-erect, reverse Trendelenburg, and the lateral decubitus position because the impact of the panniculus on abdominal pressure and diaphragm function are reduced/minimized.

Mechanical loading of the diaphragm by abdominal adipose tissue, viscera, and/or ascites can increase the work of breathing, reducing the likelihood of successful extubation of the patient with marginal respiratory status.

Positioning may play a role in optimizing respiratory mechanics. Burns and colleagues showed the effect of reverse Trendelenburg posture on spontaneous breathing in 19 patients with abdominal distention related to obesity, ascites, or intestinal distention. They found that positioning patients in this posture at an angle of 45 degrees was superior to that seen with the head elevation of 45 degrees without Trendelenburg. This position is futher supported by observations in the OR during anesthesia induction. Patients in the reverse Tredelenburg position had the fastest recovery of gas exchange and longest safe apnea time.

2. Emergency Management

Acute Respiratory Failure in the Obese Patient

  1. Verify adequate oxygenation and ventilation.

  2. Consider NIV if airway reflexes preserved.

  3. Intubation and MV with rescue plan and equipment available

  4. Optimize patient position.

  5. Consider therapies to avoid regurgitation.

  6. Check mechanics immediately after intubation, especially if NMBA employed.

  7. Treat for reversible airway obstruction if resistance elevated and reasonable clinical suspicion.

  8. Give Optimal PEEP.

  9. Early assessment of hemodynamics including early central venous catheterization and echocardiography.

  10. Check abdominal pressure.

3. Diagnosis

CBC evaluates for erythrocytosis.

TSH with free thyroxine

Serum electrolytes

ABG

Echocardiography

Bicarbonate: An elevated serum bicarbonate level due to metabolic compensation of respiratory acidosis is typical in patients with OHS and confirms chronic nature of the hypercapnea.

Brain natriuretic peptide

4. Specific Treatment

Pharmacokinetics

Phamacokinetics are hard to predict in the obese; increased volume of distribution, fat storage, and glomerular filtration rate, combined with alternations in plasma-binding proteins, preclude simple rules of drug dosing. Drugs with a narrow therapeutic window, such as digoxin and theophylline, may confer toxicity when dosed according to total body weight. Reference to published guidelines for individual drugs is recommended; however, one can categorize dosing regimens based on adherence to total body weight, adjusted body weight, ideal body weight, and those that require plasma level monitoring.

Nutritional Support

Obese patients exhibit increased levels of free fatty acids and triglyceride-rich adipose tissues. In critical illness, obese patients cannot use these lipid-energy sources effectively. They catabolize relatively more protein, losing muscle mass, and utilize fat less than patients with normal weight.

SCCM and the American Society for Parenteral and Enteral Nutrition published guidelines for nutrition support in obese, adult, critically ill patients. They recommend enteral nutrition with permissive underfeeding or hypocaloric feeding for this patient collective. For all patients with BMI > 30, enteral nutrition should not exceed 60% to 70% of target energy requirements or 11 to 14 kcal/kg actual body weight/day, with protein of greater than 2.0 g/kg ABW/day for BMI 30 to 40 and 2.5 g/kg or more ABW/day for BMI greater than 40. Contraindications to high-protein nutrition, such as renal failure and hepatic encephalopathy, must be taken into consideration.

Hypocaloric feeding with reduced caloric load, but a high proportion of proteins, may limit hyperglycemia, improve insulin sensitivity, prevent hypercapnea, reduce fluid retention, attenuate hypertriglyceridemia, and minimize muscle catabolism. Weight reduction is a side effect but should never be the primary goal of nutrition during critical illness.

Estimating the metabolic needs of the critically ill obese patient is difficult. Direct calculations using indirect calorimetry may be helpful. There is debate whether ideal body weight or actual weight should be used for metabolic formulas such as the Harris Benedict equation. Some investigators have advocated an obesity-adjusted weight with a 25% correction for excess weight above ideal weight: adjusted body weight = (actual weight - IBW) 0.25 + IBW.

Pathophysiology

Physiologic Effects and Risk in the Obese Patient

Respiratory Effects of Obesity

Reduced lung volumes

Atelectasis

Ventilation-perfusion mismatch

Obstructive sleep apnea

Obesity hypoventilation syndrome

Obstructive airways disease (asthma, mechanical)

Cardiovascular Effects of Obesity

Hypertension

Coronary artery disease

Atrial fibrillation

Congestive heart failure, systolic and diastolic dysfunction

Pulmonary hypertension

Other

Diabetes mellitus

Increased risk gastric reflux, aspiration

Increased risk venous thromboembolism

Renal

Pressure ulcers

Epidemiology

Worldwide, at least 1 billion people are overweight or obese, and more than 300 million are obese. The United States is experiencing an epidemic of overweight and obesity. The prevalence of excess weight is increasing rapidly across the country, and close to 65% of the adult population is either overweight or obese.

In 2007-8, 32.2% of American men and 35.5% of American women were obese. During the same period, 4.2% of men and 7.3% of women met criteria for morbid obesity.

Comparing the periods 1976-1980 with 1999-2000, the prevalence of overweight has increased by 40% and the prevalence of obesity has risen by 110%. In 2008, only one state, Colorado, had an obesity prevalence less than 20%. The magnitude of these numbers and frequent comorbid conditions of obesity guarantee the presence of such patients in the ICU.

Prognosis

Global prognosis for the obese critically ill patient

Given the association of being overweight or obese with increased all-cause mortality among the general population, one might expect obesity to be a bad prognostic factor for ICU outcomes. However, in critically ill patients, recent evidence suggests that obese and extremely obese patients do not have increased ICU or hospital lengths of stay, and may even have lower mortality rates than normal-weight patients. Other studies have found that obese and extremely obese patients may have longer ICU and hospital lengths of stay, duration of mechanical ventilation, and increased mortality.

A recent meta-analysis reported that extremely obese patients had decreased hospital mortality compared with normal-weight patients, but this did not reach statistical significance (RR 0.83, 95% CI 0.66-1.04). Additionally, a recent study reported that mortality in critically ill patients who have a BMI 35 or greater was improved among those who received adequate nutrition (energy and protein).

Although many would expect obesity to be associated with poor ICU outcomes, current evidence fails to establish this association consistently. Studies have shown a negative or equivocal effect on outcomes, while several studies have even found obesity to be protective. A recent meta-analysis of 14 studies involving more than 15,000 obese patients in medical and surgical ICUs showed no significant difference in ICU mortality rate when compared with normal-weight patients. However, ICU length of stay and duration of mechanical ventilation were increased. In this study, a subgroup analysis of moderately obese patients (BMI > 30, < 40) found an increase in ICU survival rate among these obese patients compared with normal-weight patients (RR 0.86; CI 0.81-0.91, P < 0.001).

Special considerations for nursing and allied health professionals.

Maintaining skin integrity is problematic in obese patients. Skinfolds lead to accumulation of moisture, accelerating skin breakdown. Limited mobility, difficulty in nurse-assisted turning, decreased vascularity within adipose tissue, and excessive weight all contribute to pressure ulcer risk. Pressure ulcers that begin in skinfolds can go undetected during early stages unless all such regions are examined thoroughly during turning. The physical demands of turning can lead to suboptimal visualization.

Turning the morbidly obese patient can present a risk for injury to the patient and staff but can be minimized with proper training, staffing, and equipment. Executing turns with clear roles for each participant can ensure that the details of examination and safety are not neglected in the physical stress of the task.

Mobilization and rehabilitation in obese patients may require increased personnel and equipment. Up to four participants may be necessary for turns alone. Over-bed trapeze lifts and specialty beds that can shift to a chair egress mode to shift the burden of supporting weight to the patient may facilitate the gradual return of strength and mobility. Physical therapy staff experienced in the treatment of obese patient are pivotal to help the patient and staff so that transfers and weight bearing can occur safely.

What's the evidence?

El-Solh, AA, Auilina, A, Pindea, L, Dhanvantri, V, Grant, B, Bouquin, P. "Noninvasive ventilation for prevention of post-extubation respiratory failure in obese patients". Eur Respir J. vol. 28. 2006. pp. 588-95.

(Sixty-two consecutive severely obese patients were assigned to NIV immediately post-extubation and compared with 62 historically matched controls.. The primary end-point was the incidence of respiratory failure in the first 48 hours post-extubation. Compared with conventional therapy, the institution of NIV resulted in 16% absolute risk reduction in the rate of respiratory failure in the first 48 hours post-extubation and shorter duration of ICU and hospital LOS.)

Burns, SM, Egloff, MB, Ryan, B. "Effect of body position on spontaneous respiratory rate and tidal volume in patients with obesty, abdominal distention and ascites". Am J Crit Care. vol. 3. 1994. pp. 102-6.

Boyce, JR, Ness, T, Castroman, P. "A preliminary study of the optimal anesthesia positioning for the morbidly obese patient". Obes Surg. vol. 13. 2003. pp. 4.

(Twenty-six morbidly obese patients were randomly assigned to one of three positions for induction of anesthesia: 1) 30 degrees Reverse Trendelenburg; 2) Supine-Horizontal; 3) 30 degrees Back Up Fowler. 30 degrees Reverse Trendelenburg position provided the longest safe apnea period and may be the optimal position for induction.)

Rao, Sl, Kunselman, AR, Schuler, HG. "Laryngoscopy and tracheal intubation in the head-elevated position in obese patients: a randomized, controlled equivalence trial". Anesth Analg. vol. 107. 2008. pp. 1912.

(Eighty-five obese adults undergoing elective surgery randomized to either the blanket method or the table-ramp method showed no significant difference in time to intubation. Prior to induction of anesthesia, obese patients can be positioned with their head elevated above their shoulders on the operating table, on a ramp created by placing blankets under their upper body or by reconfiguring the OR table. For the purpose of direct laryngoscopy and tracheal intubation, these two methods are equivalent.)

El Solh, Jaafa, W. "A comparative study of the complications of surgical tracheostomy in morbidly obese critically ill patients". Crit Care. vol. 11. 2007. pp. R3.

Malhotra, A, Hillman, D. "Obesity and the lung: 3. Obesity, respiration and intensive care". Thorax. vol. 63. 2008. pp. 925.

El-Khatib, MF, Kanazi, G, Baraka, AS. "Noninvasive bilevel positive airway pressure for preoxygenation of the critically ill morbidly obese patient". Can J Anaesth. vol. 54. 2007. pp. 744.

Byhahn, C, Lischke, V, Meininger, D. "Peri-operative complications during percutaneous tracheostomy in obese patients". Anaesthesia. vol. 60. 2005. pp. 12.

Akinnusi, ME, Pindea, LA, El-Solh, AA. "Effect of obesity on intensive care morbidity and mortality: a meta-analysis". Crit Care Med. vol. 36. 2008. pp. 151-8.

Hogue, CE, Steans, JD, Colantuoni, E. "The impact of obesity on outcomes after critical illness: a meta-analysis". Intensive Care Med. vol. 35. 2009. pp. 1152-70.

Dhonneur, G, Abdi, W, Ndoko, SK. "Video-assisted versus conventional tracheal intubation in morbidly obese patients". Obes Surg. 2008.

Frat-J-P, Gissot, V, Ragot, S. "Impact of obesity in mechanically ventilated patients: a prospective study". Intensive Care Med. vol. 34. 2008. pp. 1991-8.

Gong, MN, Bajwa, EK, Thompson, BT, Christiani, DC. "Body mass index is associated with the development of acute respiratory distress syndrome". Thorax. vol. 65. 2010. pp. 44-50.

Collins, JS, Lemmens, HJM, Brodsky, JB. "Laryngoscopy and morbid obesity: a comparison of the "sniff" and "ramped" positions". Obesity Surgery. vol. 14. 2004. pp. 1171-5.

"Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome". N Engl J Med. vol. 341. 2000. pp. 1301-8.

Hansson, PO, Eriksson, H, Welin, L, Svardsudd, K, Wilhelmsen, L. "Smoking and abdominal obesity: risk factors for venous thromboembolism among middle-aged men: “the study of men born in 1913.”". Arch Intern Med. vol. 159. 1999. pp. 1886-90.

(In a random population of 855 men, smoking and abdominal obesity were found to be independent risk factors for venous thromboembolism.)

Dickerson, RN, Boschert, KJ, Kudsk, KA, Brown, RO. "Hypocaloric enteral tube feeding in critically ill obese patients". Nutrition. vol. 18. 2002. pp. 241-6.

Cenden, JC. "Utilization of intensive care resources in bariatric surgery". Obes Surg. vol. 15. 2005. pp. 1247-51.

(In a single center experience, 19% (241/1279) of post-bariatric surgical patients required ICU care. The mortality rate for ICU-utilizing bariatric surgical patients was 2.9%. (In a cohort of 40 critically ill obese patients admitted to the trauma or surgical intensive care unit, hypocaloric feeding resulted in decreased ICU length of stay, decreased duration of antibiotic therapy, and a trend towards decreased days of mechanical ventilation. Nineteen intubated, spontaneously breathing patients with abdominal distention, ascites or obesity demonstrated that reverse Trendelenburg at 45 degrees position resulted in a significantly larger tidal volume and lower respiration rate than the 90 degrees position.)
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