RATE-ADAPTIVE CARDIAC PACING USING RIGHT-VENTRICULAR VENOUS OXYGEN-SATURATION - QUANTIFICATION OF CHRONOTROPIC BEHAVIOR DURING DAILY ACTIVITIES AND MAXIMAL EXERCISE

Central venous oxygen saturation (SVO2) closely reflects cardiac outpu t and tissue oxygen consumption. In the absence of an adequate chronot ropic response during exercise, SVO2 will decrease and the extent of d esaturation may be used as a parameter for rate adaptive cardiac pacin g. Eight patients with sinoatrial disease received a DDDR pacemaker ca pable of DDDR pacing by sensing either SVO2 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 a t 50% and 90% of age predicted maximum, respectively. Chronotropic beh avior of the two sensors were compared in the DDD mode with measuremen t of sensor responses, during everyday activities (walking, stair clim bing, postural changes, and physiological stresses) and at each quarti le of workload during a continuous treadmill exercise test. During wal king at 2.5 mph, both sensors showed no significant difference in dela y time (both react within 15 sees) or half-time (SVO2 = 36 +/- 22 sec and activity 24 +/-: 3 sec; P = NS), although SVO2 driven pacing achie ved 90% target rate response slower than activity sensing (124 +/- 16 sec vs 77 +/- 10 sec; P < 0.02). SVO2 pacing was associated with a mor e physiological rate response during walking upslope (68 +/- 12 beats/ min vs 57 +/- 10 beats/min; P < 0.05), ascending stairs (59 +/- 10 bea ts/min vs 31 +/- 6 beats/min; P < 0.05), and standing (34 +/- 7 beats/ min vs 9 +/- 2 beats/min; P < 0.05). The SVO2 sensor significantly ove rpaced 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 exp ected for the remainder of exercise. ''Underpacing'' was observed with the activity sensor at the higher workload. In conclusion, the SVO2 s ensor demonstrated a more physiological response to activities of dail y living compared with the activity sensor. Using a quantitative metho d, the speed of onset of rate response of the SVO2 sensor was comparab le to activity sensing, and was more proportional in rate response. Si gnificant overpacing occurs at the beginning of exercise during SVO2 d riven pacing, which may be improved with the use of a curvilinear algo rithm.