Are Morbidly Obese Patients Receiving Invasive Mechanical Ventilation at Higher Risk of Death?

Abstract & Commentary

By Betty Tran, MD, MS, Assistant Professor of Medicine, Pulmonary and Critical Care Medicine, Rush University Medical Center, Chicago. Dr. Tran reports no financial relationships relevant to this field of study.

Synopsis: Using a national database, the authors found that morbidly obese patients undergoing invasive mechanical ventilation had a similar risk of in-hospital mortality compared to non-obese individuals, despite having higher rates of invasive mechanical ventilation and tracheostomy.

Source: Kumar G, et al. Outcomes of morbidly obese patients receiving invasive mechanical ventilation: A nationwide analysis. Chest 2013; Jan 24. [Epub ahead of print.]

Given inconsistent results from prior studies that have focused on outcomes in the critically ill, obese population, Kumar and colleagues sought to clarify whether morbidly obese (BMI ≥ 40 kg/m2) adults receiving invasive mechanical ventilation (IMV) had significantly different outcomes compared to their non-obese counterparts. The authors used ICD-9-CM diagnosis and procedures codes from the Nationwide Inpatient Sample (NIS), a publicly available, all-payer database approximating a 20% stratified sample of U.S. community hospitals, to identify 4,070,419 morbidly obese individuals who were hospitalized in the United States between 2004 and 2008. Of these, 119,759 (2.9%) required IMV. Compared to non-obese individuals hospitalized during that time, morbidly obese patients had 1.37 (95% CI, 1.20-1.57) times higher odds of receiving IMV. Morbidly obese patients receiving IMV were also significantly younger, more likely to be female and black, more likely to be admitted for elective reasons, less likely to have comorbid conditions, and more likely to require IMV for respiratory-related reasons (COPD, asthma), with fewer other organ involvement (P < 0.05 for all comparisons).

After multivariable adjustment, the odds of in-hospital mortality in morbidly obese patients receiving IMV were no different than non-obese patients (odds ratio [OR] 0.89; 95% CI, 0.74-1.06). The median hospital length of stay was about 1 day shorter in morbidly obese survivors compared to non-obese survivors (P < 0.001). Relative to non-obese patients receiving IMV, however, there was a stepwise increase in the risk of in-hospital mortality with an increasing number of organ failures in morbidly obese patients, which was significant once at least three organs (other than the respiratory system) were involved. Morbidly obese patients were significantly more likely to undergo tracheostomy (OR 2.19; 95% CI, 1.77-2.69). On the other hand, there were no significant differences in the proportion of morbidly obese patients requiring prolonged mechanical ventilation (> 96 hours), and they were more likely to be discharged to home, with or without home health care (P < 0.05), compared to non-obese patients.

Commentary

We have likely all taken care of morbidly obese patients and are familiar with the accompanying challenges surrounding their care: difficulties with invasive procedures and diagnostic testing (e.g., central line placement, intubation, CT scans), reduced thoraco-abdominal compliance resulting in difficulties with IMV and ventilator weaning, and limited cardiopulmonary reserve. The observations that morbidly obese patients were more likely to require IMV for COPD, asthma, or CHF exacerbations and more likely to undergo tracheostomy attest to some of the challenges in ventilating and gauging volume status in this patient population. Although this study does not show evidence of higher in-hospital mortality or length of stay in this population, as the authors point out, the morbidly obese patient being admitted to the hospital is often younger, more likely to be admitted for elective reasons, and has, perhaps surprisingly, fewer comorbidities. Although these factors were accounted for in their multivariable analyses, the authors correctly acknowledge that this selection bias toward the “healthier” morbidly obese population may not have been completely removed in their adjustments.

The question of whether certain processes of care may confound some of the outcomes presented is an interesting one and should be explored further. Is adherence to protocols such as spontaneous breathing trials, low tidal volume ventilation, and interruption of sedation different when morbidly obese patients are involved? In some instances, we can hypothesize that nonadherence to these accepted practices may bias results toward worse outcomes in morbidly obese patients, but other areas may be murkier. For example, morbidly obese patients may be less likely to undergo daily spontaneous breathing trials or have sedation interrupted because they are perceived to be “higher risk” because of their body habitus, but these patients may be more likely to receive lower tidal volume ventilation because their measured plateau pressures underestimate actual transpulmonary pressures.

Given that currently one in three individuals is obese in the United States, it will be important to continue research efforts on this topic, especially if the trends of increasing percentages of morbidly obese patients who require hospitalization and IMV as seen in this study persist.