Mechanical Ventilation in Obese Patients

Obesity (and more specifically central obesity) causes cranial displacement of the diaphragm by increased tissue mass in the abdomen. Part of the increased intra-abdominal pressure (IAP) is transmitted into the thorax causing a rise in the pleural pressure and compression on the alveoli. This causes a decrease in the resting lung volume after normal expiration. The functional residual capacity (FRC) is estimated to decrease by 5–15% per 5 kg/m2 increase in BMI [1]. The decrease in FRC will be greater in supine position as more cranial displacement of the diaphragm and further increased with the loss of respiratory muscle tone by the use of sedatives and muscle relaxants in the ICU.

The reduction of end-expiratory lung volume results in airway closure and formation of atelectasis which in turns lead to gas exchange impairment and respiratory mechanics alteration. Therefore, the main objective in ventilating an obese patient is to prevent lung collapse and improve the FRC [2].

In obese patients, the chest wall compliance is decreased and associated with an increase in the intraabdominal pressure (IAP). The target plateau pressure (Pplat) in an obese patient with ARDS should still be below 30 cm H2O when clinically feasible. However, it can be adjusted to account for the decreased chest wall compliance and the increased pleural pressure. Similarly, the PEEP should be adjusted to overcome the increased pleural pressure and the resultant alveolar collapse.

The preferred way of selecting inspiratory pressure and PEEP in an obese patient is by using the esophageal pressure as a proxy for the pleural pressure. However, if an esophageal manometry is not available, we can approximate the pleural pressure by measuring bladder pressure (IAP) and adjust the Pplat (or the PEEP) using the following formula [3]:

Target Pplat (adjusted, cmH2O) = target Pplat + (IAP−13)/2

In case an esophageal manometry is available, we adjust plateau pressure and PEEP based on the end-inspiratory transpulmonary pressure (PL EI) and the end-expiratory transpulmonary pressure (PL EE), respectively. Inspiratory pressure (or volume) can be adjusted to keep PL EI ≤ 20 cm H2O and the PEEP can be adjusted to keep the PL EE at 0-10 cm H2O [4].

PL EI = Pplat - Peso ≤ 20 cm H2O

PL EE = PEEP- Peso = 0-10 cm H2O

More recently, the concept of mechanical power (MP) has been introduced and linked to decreased risk of VILI and mortality in critically ill patients. Mechanical power refers to the energy transferred towards the respiratory system, by the mechanical ventilation forces. Neto et al, in an analysis of 8207 data stored in the databases, showed that there is a consistent increase in the risk of death with MP higher than 17.0 J/min [5]. It is suggested to adjust the ventilator parameters to keep MP less than 17.0 J/min (unsure about the lowest value and unknown if obese patients can tolerate higher values). More studies are needed before routine implementation in critical care practices is recommended. The mechanical power can be computed in a simplified formula as the following:

MP (J/min) = 0.098 ∙ RR ∙ VT ∙ (Ppeak − ΔP/2)

where RR is the respiratory rate (per min), VT is the tidal volume (L), and Ppeak and ∆P are the peak and driving pressures (cmH2O), respectively. I will dedicate a special blog to explain more the mechanical power in the near future.

REFERENCES

1. Jones RL, Nzekwu MM. The effects of body mass index on lung volumes. Chest. 2006 Sep; 130(3):827-33.

2. Pépin JL, Timsit JF, Tamisier R, et al. Prevention and care of respiratory failure in obese patients. Lancet Respir Med. 2016;4(5):407–18.

3. Ball, L., Pelosi, P. How I ventilate an obese patient. Crit Care 23, 176 (2019). https://doi.org/10.1186/s13054-019-2466-x

4. Grassi, L., Kacmarek, R., Berra, L. Ventilatory Mechanics in the Patient with Obesity. ANESTHESIOLOGY, (May 2020) V 132 • NO 5

5. Serpa Neto A, Deliberato RO, Johnson AEW, et al. Mechanical power of ventilation is associated with mortality in critically ill patients: an analysis of patients in two observational cohorts. Intensive Care Med. 2018;44(11):1914–22.