LAUSR

We appreciate Dr. Dickstein’s physiologically nuanced point in calling attention to the complete alveolar gas equation (AGE) in West’s Respiratory Physiology primer (1) in response to our review, “Mechanical Ventilation for Acute Respiratory Distress Syndrome during Extracorporeal Life Support” (2). The extra term in the complete AGE does, in fact, account for a portion of the decrease in oxygenation that would be anticipated with the introduction of extracorporeal CO2 removal (ECCO2R). Indeed, at high levels of FIO2, the difference between the calculated partial pressure of alveolar oxygen (PAO2) based on the abbreviated and complete versions of the AGE is more pronounced but of less clinical significance, with PAO2 well above 200 mm Hg at an FIO2 at or above 0.5. However, at lower FIO2, which may be more relevant when ECCO2R is used for patients with hypercapnia with relatively preserved oxygenation, the difference between the PAO2 in the abbreviated and complete AGE is less pronounced. This results in more clinically significant decreases in PAO2 when the respiratory exchange ratio (RER) is decreased in the context of ECCO2R, regardless of which formula is used. For example, at FIO2 of 0.3, the partial pressure of alveolar carbon dioxide (PACO2) 40 mm Hg, and RER of 0.8, the PAO2 in the abbreviated AGE would be 164 mm Hg, and with the complete AGE, it is nearly identical at 166 mm Hg; when RER is decreased to 0.4, the PAO2 would be 114 mm Hg with the abbreviated AGE and 131 mm Hg with the complete AGE, so the PAO2 does still decrease, just to a lesser degree than would be expected from the abbreviated AGE (Figure 1). There remains a notable decrease in PAO2 regardless of which AGE is used, which poses a real risk of leading to hypoxemia. This decrease in PAO2 because of reductions in RER could be overcome with an increase in FIO2, as the author demonstrates in his figure. Also, the concept of passive movement of fresh gas into the alveolus, as a consequence of the difference between inspired volume and expired volume, must be put into context. The extreme example is zero expired gas flow, which amounts to apneic oxygenation and, in principle, no change in PAO2. However, this passive flow requires specific conditions to be maintained. Under normal conditions, it might take place at low RER, but under pathological conditions (e.g., chronic obstructive pulmonary disease or the acute respiratory distress syndrome), with low RER usually associated with a low _ V= _ Q; the risk of alveolar collapse will be much higher, depending on mixed venous gas content and the presence of nonhomogeneous distribution of _ V= _ Q (3), also potentially contributing to hypoxemia. The portion of hypoxemia due to shunt physiology will, notably, not be overcome by increasing FIO2. n