QRS-activated ventilation during general anesthesia for extracorporeal shock wave lithotripsy

High frequency ventilation has been claimed to improve the efficiency of extracorporeal shock wave lithotripsy (ESWL) by minimizing the movement of urinary stones during the procedure. A ventilatory mode, QRS-activated ventilation, was developed in which the stones remain motionless during the delivery of shock waves. As the shock wave is triggered to occur approximately 20 milliseconds after the R wave of the QRS complex, the mechanical breath was synchronized to occur approximately 150 ms later. QRS-activated ventilation is used in 16 patients undergoing ESWL under general anesthesia. Tidal volume was set at 3 ml/kg (234 ± 36 ml; mean ± SD) at a rate that equaled the heart rate (71 ± 9 beats/min). The time between the R wave and the initiation of mechanical breath (T1) was 124 ± 25 ms, time of mechanical breath itself (T2) was 431 ± 67 ms, and time between end of T2 and next R wave (T3) was 264 ± 84 ms. End-tidal C0₂ measured by the large breath technique was 28.1 ± 4.8 mmHg. During the clinical use of QRS-activated ventilation and during earlier studies using an EKG simulator and a test lung, the shock wave occurred invariably at end-expiration even at high heart rates.