In Reply:—
Drs. Singh Heir and Gottumukkala are right: If a surgeon asks for an “extreme Trendelenburg” position, one cannot use the head-up position as a strategy to counteract hypoxemia, unless in an emergency!
The negative effects of the Trendelenburg position have been investigated by others.1–3Any conclusion taken from our data on the Trendelenburg position is merely speculative, provided we set out to investigate the beach chair position and positive end-expiratory pressure (PEEP).4Nevertheless, our data allow some speculations on the aspect brought up by Drs. Singh Heir and Gottumukkala in their letter.
In a sedated and paralyzed patient, pleural pressures displace the relaxed diaphragm downward, hence increasing end-expiratory lung volume, while intraabdominal pressure (IAP) acts as a counter pressure on the diaphragm. When the bowels are allowed to slide under gravity in the head-up position, bladder pressure increases, as we have shown (from 17.87 ± 5.45 to 23.92 ± 4.35 cm H2O; P < 0.01). The increase of bladder pressure in head-up position may thus correspond to a reduction of pressure pushing the diaphragm upward. When we estimated pleural pressure as Ppl = (PEEP + PEEPi) × Ew/(El + Ew)5in the supine and beach chair positions with and without PEEP, and considered changes of IAP due to the beach chair position (ΔIAP) to obtain an indicative “push-down pressure” (Ppl +ΔIAP), we found that this was positively correlated with end-expiratory lung volume (R 2= 0.857, P < 0.001, m = 0.67, multiple linear regression analysis). We did not measure IAP changes occurring in the extreme Trendelenburg position. However, we would anticipate that an increase in the forces shifting the diaphragm upward would ensue following the same physical rule. In this condition, PEEP (hence Ppl) may be used as a counterpressure that opposes diaphragm upward shift, thus preserving lung volume.
Recruitment maneuvers proposed by Drs. Singh Heir and Gottumukkala to improve oxygenation have been discussed in our article.4They are possibly even more important in the extreme Trendelenburg position than in the supine position to open up the lung. However, their effects are short-lived,6whereas PEEP is known to maintain lung units open, once recruited.
*Fondazione Policlinico Ospedale Maggiore, Mangiagalli e Regina Elena–IRCCS, Milano, Italy. franco.valenza@unimi.it