Rate Adaptive Cardiac Pacing Using Right Ventricular Venous Oxygen Saturation: Quantification of Chronotropic Behavior During Daily Activities and Maximal Exercise

Central venous oxygen saturation (S_vO_z) closely reflects cardiac output and tissue oxygen consumption. In the absence of an adequate chronotropic response during exercise, S_vO₂ will decrease and the extent of desaturation maybe used as a parameter for rate adaptive cardiac pacing. Eight patients with sinoatrial disease received a DDDR pacemaker capable of DDDR pacing by sensing either S_VO₂ or piezoelectric detected body movement. Both sensors were programmed to attain a rate of about 100 beats/min during walking, and with the lower and upper rates set at 50% and 90% of age predicted maximum, respectively. Chronotropic behavior of the two sensors were compared in the DDD mode with measurement of sensor responses, during everyday activities (walking, stair climbing, postural changes, and physiological stresses) and at each quartile of workload during a continuous treadmill exercise test. During walking at 2.5 mph, both sensors showed no significant difference in delay time (both react within 15 sees) or half-time (S_VO₂= 36 ± 12 sec and activity 24 ± 3 sec; P = NS), although S_VO₂ driven pacing achieved 90% target rate response slowerthan activity sensing (124 ± 16 sec vs 77 ± 10 sec; P < 0.02). S_VO₂ pacing was associated with a more physiological rate response during walking upslope (68 ± 12 beats/min vs 57 ± 10 beats/ min; P < 0.05), ascending stairs (59 ± 10 beats/min vs 31 ± 6 beats/min; P < 0.05), and standing (34 ± 7 beats/min vs 9 ± 2 beats/min; P < 0.05). The S_vO₂ sensor significantly overpaced in the first quartile of exercise (51.8 ± 25.6% in excess of heart rate expected from workload), but the rate was within 20% of expected for the remainder of exercise. “Underpacing” was observed with the activity sensor at the higher workload. In conclusion, the S_vO₂ sensor demonstrated a more physiological response to activities of daily living compared with the activity sensor. Using a quantitative method, the speed of onset of rate response of the S_vO₂ sensor was comparable to activity sensing, and was more proportional in rate response. Significant overpacing occurs at the beginning of exercise during S_VO₂ driven pacing, which may be improved with the use of a curvilinear algorithm.