THE EFFECTS OF VENTRICULAR PACING ON LEFT-VENTRICULAR GEOMETRY, FUNCTION, MYOCARDIAL OXYGEN-CONSUMPTION, AND EFFICIENCY OF CONTRACTION IN CONSCIOUS DOGS

The effects of ventricular pacing on left ventricular(LV) dynamic geom etry, function, and myocardial oxygen consumption (MVO2) were measured in 12 conscious dogs using sonomicrometry, micromanometry, ultrasonic flow probes, and oximetry catheters during right atrial (A-) and righ t ventricular (V-) pacing at 150 beats/min. Systolic function was quan tified using slopes (M-w) and volume-intercepts (V-w )of linear relati onships between end-diastolic volume (EDV) and stroke work (SW) for da ta obtained during vena caval occlusion. V-pacing shifted SW-EDV relat ionships downward (M-w decreased from 97 +/- 21 to 81 +/- 22 Kerg/mL) and to the right (V-w increased from 14 +/- 11 to 20 +/- 12 mL) in com parison with A-pacing (P < 0.05). These functional changes correlated with altered contractile geometry manifest as early shortening in the septal free wall relative to anterior-posterior dimension (increased m inor axis mid-wall eccentricity at end-diastole and begin-ejection). S teady-state LV power output decreased from 802 +/- 213 mW during A-pac ing to 514 +/- 170 mW during V-pacing (P < 0.05), while MVO2 remained relatively unchanged during V-pacing (10 +/- 3 mL O-2/min vs 11 +/- 3 mL O-2/min during A-pacing, P = NS). As a result, overall LV efficienc y decreased from 0.24 +/- 0.08 during A-pacing to 0.16 +/- 0.06 during Ir-pacing (P < 0.05). These data illustrate the impact of V-pacing on dynamic LV geometry and function, including impaired LV work output a t all physiological levels of preload, Most importantly, the relations hip between LV work output and MVO2 is depressed during V-pacing, emph asizing the interaction between LV mechanics and pump efficiency in in tact subjects. As a result, measures taken to restore normal contracti le geometry might improve LV efficiency and performance when V-pacing is necessary.