Physicians caring for obese ventilated patients need to take into account the physiologic effects of obesity on lung function, according to Jason Poston, MD.
“When we think about lung physiology, [obesity] is a really important contributor to some abnormalities … and of significant clinical importance to the patients,” he told attendees at the American Thoracic Society's annual meeting, held in Philadelphia in May.
Dr. Poston, who is an assistant professor of medicine in the department of pulmonary and critical care medicine at the University of Chicago, offered a review of the basic physiologic differences associated with obesity, as well as some tips for optimal ventilator management.
Regarding lung volumes, obesity has differing effects, Dr. Poston said. For example, residual volume in obese patients does tend to decrease with increasing weight, but by a relatively small percentage. “The residual volume is relatively well preserved, particularly when you compare to the expiratory reserve volume,” he said.
Obesity has more of an effect, however, on the expiratory reserve volume, which Dr. Poston said tends to fall precipitously at very small increases in body mass index (BMI). “The mean falls out of the normal range at a BMI of about 27 or 28, which I think most of us would argue is the best we could hope for in the vast majority of our patients,” he said. For functional residual capacity, the mean doesn't drop below the expected normal until BMIs are much higher, up to approximately 40, he said.
Variables like vital capacity and total lung capacity decrease but are also relatively preserved in obese patients, Dr. Poston said, “because these are volitional and with strong muscles, you can overcome and move that weight in your chest and in your belly, pushing up on your diaphragm.” He noted that vital capacity and total lung capacity values generally begin to fall outside the normal range only at extreme BMIs above 40. Diffusing capacity is also preserved because obesity does not cause an alveolar process in the lungs themselves, he said.
The main thing to remember is that lung mechanics and volumes can vary significantly in patients with the same degree of obesity, he said. When lung volumes are reduced, it often appears as if airway resistance is elevated, which Dr. Poston said is probably due to small airway closure in dependent regions of the lungs.
In obesity, the pleural pressure is higher than the atmospheric pressure, Dr. Poston said. Lung volumes in obese patients are decreased, and the respiratory system compliance is reduced due to reduced lung compliance, which occurs because respiration is at low lung volumes and there is often cyclic airway closure with each breath.
“What does it mean for your patients? For any given breath, you need to generate a greater pressure when you are obese than when you are of normal weight. So it's harder to breathe, and breathing takes more energy,” Dr. Poston said.
The respiratory muscles in lean patients normally consume 1% to 5% of total oxygen consumption, referred to as the VO2resp, Dr. Poston said. In many obese patients, there is a disproportionate increase when they try to increase their minute ventilation, similar to patients with emphysema. This makes both populations susceptible to respiratory failure with increased demand, Dr. Poston said: “It makes them much more short of breath when they walk across the room or up a flight of stairs.”
For obese patients, this shortness of breath and impaired lung function can also be present at rest, Dr. Poston noted. He discussed a study published in the American Journal of Critical Care Medicine in 1999 that looked at the impact of morbid obesity on VO2resp at rest. The authors measured the oxygen consumption of 18 obese and 8 normal-weight patients before and after intubation for elective surgery and found that obese patients used 22% of VO2 for breathing at rest, “which means they're spending a lot more of their metabolic energy just keeping the air moving in and out,” Dr. Poston said.
These physiologic principles have myriad implications for mechanically ventilated obese patients, the first being airway management difficulties. “As the tube goes in, there's an increased risk that you won't get it in and then you'll have a complication during that procedure, including desaturation or aspiration,” Dr. Poston said. Many obese patients will also have comorbid cardiovascular disease or pulmonary hypertension, further raising the risk of a poor outcome, he noted.
Another problem with managing obese ventilated patients is that often physicians are less certain about diagnosis. Obesity has multiple potential causes, and diagnostic tools are more difficult to use in the obese. “The CT scanners, the ultrasounds, and our physical exam, all of these things are limited due to the patient's obesity, so a lot of times we do have to have a shotgun approach,” Dr. Poston said. That in turn may affect the appropriateness of therapies and can increase the likelihood of unnecessary therapies, which may lead to differences in outcomes, he said.
Risks associated with obesity in mechanically ventilated patients include misdiagnosis, missed diagnosis, procedural complications, venous thromboembolism due to immobility both before and during the ICU stay, aspiration because of higher abdominal pressures, and skin breakdown, Dr. Poston said.
In all ventilated obese patients, Dr. Poston recommended using upright positioning and maintaining adequate positive end-expiratory pressure (PEEP), frequently 8 to 15 cm H2O, to prevent atelectasis and keep oxygen saturation at 88% to 95% on a nontoxic fraction of expired oxygen. Clinicians may also have to accept a higher PEEP or lower oxygen saturation in obese patients, he said, and may need to be more comfortable taking them off the ventilator with these values.
Dr. Poston also stressed that the thorax does not change size as patients gain weight. “Your lungs get bigger when you get taller, not wider,” he said. Because many ventilation protocols are based on ideal body weight, it's important to take BMI into consideration for management, he noted.
In acute lung injury, for example, the goal is to achieve 4 to 6 mL/kg of ideal predicted body weight, Dr. Poston said, and maintain a plateau pressure less than 30 cm H2O. “The problem, of course, is that all the adipose tissue in the morbidly obese patient … contributes to the pleural pressure, which will contribute to the plateau pressure,” he said. Therefore, an increase in plateau pressure may simply indicate obesity, not alveolar overdistension, Dr. Poston said.
He recommended putting patients in an upright position to minimize that effect and reducing tidal volume to achieve lung protective strategy if possible. He also cited a 2008 New England Journal of Medicine article that looked at the potential role of esophageal pressure monitoring to titrate plateau pressures.
However, Dr. Poston said, a plateau pressure of 35 to 40 H2O may simply be unavoidable in some obese patients. “It makes us a little uncomfortable, and we don't know exactly whether that's the right thing to do, but from a practical standpoint it's often all we can do,” he said.
When looking to extubate obese patients, keep in mind that the effects of sedative and analgesic medications may persist. “Many of these medications are lipophilic, so the abundant fat stores store [them] up and then will rerelease these medicines into the circulation,” Dr. Poston said. Many obese patients also have obesity hyperventilation syndrome or obstructive apnea, so a small amount of sedative can cause significant upper-airway obstruction, he noted.
Before attempting extubation, minimize sedatives and analgesics, Dr. Poston recommended. “We do that in everyone, but I think we redouble our efforts in the obese patients,” he said. Also, he noted, be mindful of positioning, which becomes critically important, and mobilize patients early and often to decrease atelectasis.
“Everyone's afraid to move [these patients], but when they move they get a lot of benefit, including hemodynamic benefit. You stand them up, you get the weight of their abdomen off their chest, oxygenation improves, venous return improves, blood pressure improves,” he said.
Dr. Poston also reminded the audience not to overlook chronic conditions. “Sometimes we're so overwhelmed with the management of an obese patient on a ventilator that we forget that they may also have cardiovascular disease and be accumulating extra fluid,” he said. “Sometimes when we extubate people we pat ourselves on the back and walk out of the room and forget to write the order for their standing nocturnal positive-pressure ventilation for their sleep-disordered breathing.”
Clinicians may want to consider noninvasive ventilation in newly extubated patients, Dr. Poston said. In a 2006 trial in the European Respiratory Journal, 62 consecutive obese patients given noninvasive ventilation via a nasal mask immediately after extubation had lower rates of respiratory failure and a shorter length of stay than 62 controls. Referral to noninvasive ventilation “can help you sort of weather this storm,” he said. (For more on noninvasive ventilation, see the story below.)
Overall, Dr. Poston advised attendees to be bold but vigilant for failure when liberating obese patients from mechanical ventilation. “It sometimes takes an act of faith,” he said. “Oftentimes you do have to say, ‘We can't just watch this patient forever. They're never going to be perfect. Let's pull the tube and see what we can do.’”