The impact of morbid obesity, pneumoperitoneum, and posture on respiratory system mechanics and oxygenation during laparoscopy

We studied the effect of morbid obesity, 20 mm Hg pneumoperitoneum, and body posture (30° head down and 30° head up) on respiratory system mechanics, oxygenation, and ventilation during laparoscopy. We hypothesized that insufflation of the abdomen with CO₂ during laparoscopy would produce more impairment of respiratory system mechanics and gas exchange in the morbidly obese than in patients of normal weight. The static respiratory system compliance and inspiratory resistance were computed by using a Servo Screen pulmonary monitor. A continuous blood gas monitor was used to monitor real-time Paco₂ and Pao₂, and the ETco₂ was recorded by mass spectrometry. Static compliance was 30% lower and inspiratory resistance 68% higher in morbidly obese supine anesthetized patients compared with normal-weight patients. Whereas body posture (head down and head up) did not induce additional large alterations in respiratory mechanics, pneumoperitoneum caused a significant decrease in static respiratory system compliance and an increase in inspiratory resistance. These changes in the mechanics of breathing were not associated with changes in the alveolar-to-arterial oxygen tension difference, which was larger in morbidly obese patients. Before pneumoperitoneum, morbidly obese patients had a larger ventilatory requirement than the normal-weight patients to maintain normocapnia (6.3 ± 1.4 L/min versus 5.4 ± 1.9 L/min, respectively; P = 0.02). During pneumoperitoneum, morbidly obese, supine, anesthetized patients had less efficient ventilation: a 100-mL increase of tidal volume reduced Paco₂ on average by 5.3 mm Hg in normal-weight patients and by 3.6 mm Hg in morbidly obese patients (P = 0.02). In conclusion, respiratory mechanics during laparoscopy are affected by obesity and pneumoperitoneum but vary little with body position. The Pao₂ was adversely affected only by increased body weight.