Acid–base balance and respiratory mechanics during robotic-assisted surgery: an observational study

To evaluate changes in acid-base equilibrium in plasma and urine according to the Stewart’s approach, in gas exchange and in respiratory mechanics during robotic-assisted surgery. Prospective observational study on patients undergoing robotic-assisted surgery. Acid-base equilibrium in plasma and urine was measured after 10 min from general anesthesia induction (T0), after 2 h from pneumoperitoneum administration (T1) and after 10 min from pneumoperitoneum interruption (T2). In the same timepoints respiratory mechanics, gas exchange and hemodynamics variables were measured. Seventy-three patients were enrolled. The apparent strong ion difference did not change during the whole study. Urine sodium and chloride concentration did not change throughout the study, although sodium absolute excretion decreased. Urinary anion gap increased from 35 [26–42] at T0 to 39 [28–53] mEq L− 1 at T2. At T1, a significant increase in arterial carbon dioxide partial pressure (45 [42–51] vs. 41 [37–45] mmHg) as well as a consequent reduction in arterial pH (7.36 [7.3–7.38] vs. 7.41 [7.37–7.44]) compared to T0 was found. At T2, arterial carbon dioxide partial pressure and arterial pH decreased but never returned to baseline. The behavior of acid-base equilibrium in plasma and urine was independent from fractional sodium excretion. Pneumoperitoneum did not alter plasma strong ion difference or urinary electrolytes concentration, although it induced respiratory acidosis with acidemia. The absence of any compensatory change in urinary electrolytes was probably related to the hemodynamic effect of pneumoperitoneum on the kidneys, resulting in a reduction in absolute excretion of sodium.