Mechanical ventilation for the morbidly obese patient

"Mechanical ventilatory support" in the broadest sense is a Level 1 topic from Section 2.1.5 in the second edition of the CICM Syllabus for the Second Part Examination. Though one might argue that the morbidly obese patient belongs in the section dealing with Populations requiring special considerations in Intensive Care. But they can be two things. The only reason the same chapter in Deranged Physiology is not seen in two places is because humans are thermodynamically asymmetric in time, and so to the CICM exam candidate, time appears linear, and process of preparing for this tests appears to obey the laws of causality. Catering to this perspective requires for chapters and questions to be arranged in a logical order , such that none appear twice, nor are they accidentally omitted, and the reader can assure themselves that, as they work through the website from front to back,  they will cover the entire content of the historical papers and syllabus.

Question 9 from the first paper of 2024 was the first appearance of this in the written exam, though no doubt endless morbidly obese bodies had been examined by hot case candidates over the years, leading to a reasonable familiarity with the question. Mechanical ventilation in morbid obesity had previously never appeared in the CICM Fellowship exam, but the growing waistlines of the Western world raised the possibility that at some stage in the near future, it might, and so ultimately this prophesy came true, to delight the scryer of future exam content. Mechanical ventilation for the morbidly obese patient is challenging for a variety of reasons, which can be summarised by saying that there is simply too much patient to ventilate using conventional settings. Intensivist responses to the challenges of obesity can generally be described as "ventilate them harder". 

In summary:

In terms of peer-reviewed reading, the best (free, recent) article on the subject is the comprehensive review by De Jong et al (2017), followed by the more recent De Jong from 2020, and the excellent Cobilinschi et al (2023)  Unfortunately, because of so little published evidence, many authors are only able to make very generic recommendations, remarking for example that HFNP "could be interesting". Fortunately, these papers offer an excellent bibliography for further reading about this subject.

Effects of morbid obesity on mechanical ventilation

The general effects of morbid obesity on ICU care are summarised elsewhere. In summary, there are several problems with their lung mechanics:

• Expiratory reserve volume is decreased
• FEV1 to FVC ratio is increased.
• VC, TLC and FRC are decreased.
• Work of breathing is increased
• The weight of the chest wall contributes to a decreased respiratory compliance
• Airway resistance is increased (until you increase the FRC with PEEP)

There are also problems which are associated with their size:

• There will inevitably be sleep apnoea and CO₂ retention, as well as severe pulmonary hypertension.
• O₂  consumption and CO₂ production is increased (more tissue metabolising), thus ventilatory needs are greater
• They may have dysregulated responses to CO₂

El Sohl et al (2001) found that on average morbidly obese patients are intubated for longer (for almost twice as long as patients of normal size), mainly because their oxygen requirements tend to be higher and they take longer to wean from the ventilator. Because of their increased abdominal and chest wall mass, morbidly obese patients "dedicate a disproportionately high percentage of total VO₂ to conduct respiratory work, even during quiet breathing" (Kress et al, 1999). The amount of additional O₂ / CO₂ flux to be expected is something like 150% of the normal values.

High flow nasal oxygen in morbid obesity

In summary:

• HFNP is no worse than NIV in morbidly obese patients recovering from surgery
• HFNP is certainly better than  conventional oxygen in that group
• HFNP may be a valid choice for pre-oxygenation before surgery 
• Much of the data is extrapolated from cardiothoracic and pre-op bariatric surgery patients 
• There may not be much of an effect on the development of atelectasis, probably because the level of PEEP is so low. 

In post-operative patients, there appears to be some benefit, or at least non-inferiority. Stephan et al (2017)  found that French cardiothoracic surgical patients are not harmed by being on HFNP instead of NIV after their CABGs, their comfort was greater and a statistically insignificant trend towards improved reintubation rates was observed. For the record, they used 50L/min flow, with 50% FiO₂. The NIV settings were fairly gentle (8 of PEEP, 4 of bi-level pressure support), but then again these were Frenchmen and Frenchwomen of relatively average proportions, with BMIs on average around 34. Sahin et al (2018) ran a very similar study in Turkey but compared HFNP to conventional oxygen, and found that HFNP was by far the better choice. All respiratory parameters and self-reported comfort scores were better with the high flow. In contrast, Corley et al (2015) extubated their obese post-CABG patients directly on to HFNP and found that their primary outcome (development of atelectasis) was not affected.

Heinrich et al (2014) reported on the use of HFNP to pre-oxygenate patients before intubation for their bariatric surgery. The high flow was compared to standard oxygen and NIV. All groups ended up getting well-oxygenated (PaO₂ over 300) but the HFNP group had the higher PaO₂, 380 mmHg. This trial did not distinguish its use as a clearly superior therapy, but rather established it along the others as a valid alternative option.

Non-invasive ventilation in morbid obesity

In summary:

• If they are getting NIV for hypercapnea associated with obesity hypoventilation syndrome, NIV is as good as it is in COPD patients.
• A higher PEEP is suggested (perhaps 25% more than for patients of normal weight)
• NIV is less likely to be successful the heavier you are.

There is not a lot of data on the use of NIV in the morbidly obese population with respiratory failure, but what data we do have are encouraging. For example, Carillo et al (2012) compared the use of NIV in morbidly obese hypercapneic patients to the use of NIV in hypercapneic COPD patients and found that "similar efficacy and better outcomes" could be expected.

How much PEEP do you need on NIV? Gursel et al (2011) reported on this in their cohort, and found that a higher PEEP was generally needed, but the numbers are not completely believable - the patients with BMIs less than 35 were ventilated with a PEEP of 5, whereas the morbidly obese patients had a PEEP of 7. It is not completely clear how this reflects on the obese West where PEEP in excess of 10 is routinely used, but one might interpret this as a signal that increasing your "routine" starting PEEP by about 25-30% is appropriate for a hyper-obese patient.

How do you know when they are going to need intubation? Turns out, the heavier they are, the less likely NIV is to succeed,  Duarte et al (2007) determined that NIV is unsuccessful in about a third of these patients, and weight was a major determinant-  those with BMI in excess of 60 were more likely to fail NIV, whereas successful use of NIV was associated with BMIs lower than 50. To simplify the comparison, that corresponds to a weight of 140kg in a 170cm male.

Invasive ventilation in morbid obesity

Because this had appeared in the SAQs, and the rubric is available, we know that a pass-level of understanding here requires some logical structure, and must include some thoughts about the selection of ventilator settings (especially PEEP, I:E and a lung-protective tidal volume) as well as rescue strategies such as prone ventilation. What follows will hopefully satisfy this minimum requirement:

Ventilator adjustments 

• Pressure control mode: the decelerating flow pattern is thought to be ideal, because of the sustained pressure that recruits basal lung units with slow time constants. In terms of evidence, Aldenkortt et al (2012) found no difference between pressure control and volume control, but De Jong (2017) recommended pressure control anyway because of the theoretical benefits of the constant pressure and decelerating flow pattern.
• PEEP: If you are larger, your PEEP requirements appear to be greater. Pirrone et al (2016) found poorer lung compliance with clinician-set PEEPs (10-14 cmH₂O) among patients who were all of horrendous size (BMI >50), and the best PEEP settings were actually around 20 cmH₂O. So how do you individualise these settings?
  • Oesophageal manometry may help to calculate the actual transpulmonary pressure and eliminate the contribution from the chest wall, which means one could bravely use PEEP in the territory of 25-30 cmH₂O. As long as you keep the transpulmonary pressure in the -2 to +2 cmH₂O territory (aiming for zero), there should not be any worsening of the lung injury.
  • For those who do not have oesophageal manometry, De Jong et al recommend setting the PEEP about 8 cmH₂O higher than what the ARDSNET tables would recommend.
• Tidal volume: One should adjust their tidal volume expectations to the patient's predicted body weight (PBW), for which there is unfortunately a variety of different equations. The favoured method seems to be the  ARDSNet method, mostly because it was used in the study that we base our lung protective strategies on. This is:
  • Men: 50.0 + 0.905 × ([height in cm] - 152.4)
  • Women: 45.5 + 0.905 × ([height in cm] - 152.4)
    
    Calculate it yourself, or use this applet:
    
    Sex:  M F
    Height(cm)
    Calculate
    PBW = ___ kg
    VT = ___-___ ml
     
• Respiratory rate: The obese patient likely has a much higher ventilatory requirement than the patient with normal body mass, considering the amount of lean aerobic tissue. The total CO₂ production is likely to be higher at baseline owing to the increased metabolic rate.
• I:E ratio: keep the I-time longer to improve recruitment of long time constant lung units, and to reduce the expiratory time, which creates a desirable auto-PEEP effect. We are, however, extrapolating from non-ARDS data here.
• Tolerating higher peak inspiratory and plateau pressure: One should try to keep the P_plat under 35 cmH₂O, but sometimes this is impossible, considering the PEEP requirements. The peak inspiratory pressure is also going to be higher than usual. Alarms may need to be disabled.
• Position: reverse Trendelenberg is best, as it keeps the abdominal contents off the diaphragm. There is even an RCT to confirm this (Hassan & Baraka, 2021)

Rescue manoeuvres:

• Recruitment: Post-intubation recruitment manoeuvres are recommended to improve lung volumes after the inevitable peri-intubation atelectasis develops; Futier et al (2011) demonstrated that applying a continuous positive airway pressure of 40 cm H₂O for 40 seconds helped maintain oxygenation during sleeve gastrectomy.
• Prone ventilation: De Jong et al (2013) had reported some excellent outcomes from this, demonstrating that it is both feasible and safe (provided some planning is involved). And it was actually more effective than in patients without obesity, as compared to patients of a normal weight.