Studies on intra-pulmonary gas distribution in the extremely obese. Influence of anaesthesia and artificial ventilation with and without positive end-expiratory pressure.

Intrapulmonary gas distribution was studied in 10 extremely obese patients: (1) during spontaneous breathing awake; (2) during anaesthesia with controlled ventilation and zero end-expiratory pressure (ZEEP), and (3) as under (2) but with a positive end-expiratory pressure of approximately 15 cmH2O (PEEP). Gas distribution was assessed quantitatively by means of a multiple-breath nitrogen wash-out technique and subsequent fractional analysis, which permitted the calculation of nitrogen wash-out delay (NWOD). Gas distribution was also analyzed by means of a single-breath nitrogen wash-out in order to determine the slope of the "alveolar plateau." Gas distribution was within normal limits during spontaneous breathing, judged from multiple-breath as well as single-breath wash-out. With anaesthesia and ZEEP, NWOD was higher, indicating less efficient gas mixing, and the slope of the "alveolar plateau" was twice as steep as during spontaneous breathing. With PEEP, distribution of inspired gas improved (lowered NWOD and flatter slope). Theoretical considerations and clinical experiments led to the conclusion that uneven distribution in the anaesthetized obese is caused both by regional differences in the pulmonary time constants (as in obstructive lung disease) and by airway closure.