Cardio-respiratory changes with increased intra-bladder pressure in prone position during anesthesia.

Twenty non-obese patients, 13-29 years of age, operated on for scoliosis were examined for cardio-respiratory changes that occur during positioning on bolsters, and the effect on the cardio-respiratory system of raised intra-abdominal pressure was evaluated. Hemodynamic and respiratory responses were measured when the position was changed from supine to prone and back to supine during anesthesia. We measured the intra-bladder pressure using a transurethral catheter (IBP) as an index of the intra-abdominal pressure. When the position was changed from supine to prone, the cardiac index (CI) decreased by 10-30% and the systemic vascular resistance index increased by 8-14%. IBP rose significantly ( P < 0.001), but it remained below 5 mmHg. These changes continued to be mild until the patient was returned to the supine position. Pa(O)(2), A-aD(O)(2)and Qs/Qt remained unchanged. CI decreased significantly ( P < 0.001) when IBP was increased to 10 mmHg by abdominal compression, but was not affected when IBP was increased to only 5 mmHg. It was concluded that mild abdominal compression in the prone position during anesthesia has little effect on the cardio-respiratory system in lean young subjects.