Effects of atrial pacing on right ventricular contractility after coronary artery surgery

Increasing heart rate enhances the strength of contraction of cardiac fibers. This has been demonstrated in vitro and recently for the left ventricle. To study this phenomenon on the right ventricle, the effects of increasing heart rate by atrial pacing on right ventricular (RV) contractility were observed after coronary artery surgery in 20 patients. Right ventricular ejection fraction (EF) and stroke volume (SV) were measured by use of a rapid-response thermistor pulmonary artery catheter. Right ventricular volumes were calculated from EF and SV. Right ventricular contractility was assessed by the slope of the end-systolic pressure-volume relationship (ESPVR) before and after increasing RV preload by means of military antishock trousers (MAST) inflation. The dP/dtmax/end-diastolic volume index (EDVI) ratio, which is independent of ventricular preload, was also used as an inotropic index. Results are expressed as mean +/- SEM. Right ventricular preload, as reflected by RVEDVI, was increased by MAST inflation (99 +/- 6 mL/m2 v 106 +/- 7 mL/m2, P < 0.01), but returned to control values when inflation of MAST was combined with pacing (100 +/- 6 mL/m2). The slope of the RV ESPVR significantly increased when heart rate was increased (0.22 +/- 0.03 mmHg/mL/m2 before pacing v 0.77 +/- 0.07 mmHg/mL/m2 during pacing, P < 0.05). The dP/dtmax/EDVI ratio was also increased by pacing (2.32 +/- 0.4 mmHg/min/mL/m2 before pacing v 3.15 +/- 0.5 mmHg/min/mL/m2 during pacing, P < 0.01). Moreover, cardiac index was increased by pacing alone (2.45 +/- 0.2 L/min/m2 v 2.78 +/- 0.2 L/min/m2, P < 0.01), and significantly more when MAST were inflated (2.94 +/- 0.2 mL/m2, P < 0.05 v pacing alone). It is concluded that increasing heart rate by atrial pacing increases RV inotropic status after coronary artery surgery.