[Effects of different respiratory rates on PaCO2 and plasma potassium concentration]

To observe the effects of different respiration rates on the CO2 partial pressure and plasma potassium concentration during general anesthesia.

Forty-two surgical patients under general anesthesia with normal preoperative arterial CO2 partial pressure and plasma potassium concentration were randomized into 2 groups. After intubation, all patients were placed on the mechanical ventilator (Ohmeda 210 anesthesia machine) with tidal volume 10 ml/kg and oxygen flow rate 1.5 L/min. Initial respiratory rate of group I patients was set to 12 times per minute for 30 minutes, and then was adjusted according to calculation with 0.3 x PaCO2. Respiratory rate of group II patients was set to 8 times per minute. Blood samples were collected for arterial blood gas analysis preoperatively, and 30 and 60 minutes after intubation. Linear regression was used to analyze the relationship between PaCO2 and plasma with K+ concentration and Student's t test was used to test the differences at different time points. An overall alpha of 0.05 was considered significant for the outcome variables.

In two groups, PaCO2 (mmHg) was related with K+ concentration (mmol/L) (P < 0.05) at all time points. 30 minutes after intubation, PaCO2 and K+ concentration significantly decreased (P < 0.05) and recovered to preoperative level after the respiratory rate being adjusted [(to 8.85 +/- 0.73) times/min] for another 30 minutes in group I. No significant difference in respect to PaCO2 and K+ concentration was found between any two time points in group II.

Traditional ventilation with VT 10 ml/kg and RR 12-15 times/min will probably produce hyperventilation and hypokalemia.