[Hemodynamic effects of changes in the heart rate]

The effects of the variations in heart rate (HR) within the range of 80 and 180 beats/min were studied in open-chest dogs with complete AV block. Left ventricular systolic pressure (LVSP) and the maximal velocities of development (+P) and fall (-P), left ventricular end diastolic pressure (LVEDP), cardiac output (CO) and duration of ejection (DE) were recorded and analyzed. Contractility was assessed by the force developed by an isometric segment of the left ventricular wall (DT) and the maximal velocities of rise (+T) and fall (-T). Stroke volume (SV), stroke work (SW), stroke power (SP), +P/-P and +T/-T ratios, the time constant of LV pressure fall during isovolumic relaxation (T tau), and mean rate of ejection of left ventricle (MRE) were calculated from the precedent data. LVSP, LVEDP and -P did not show any statistically significant difference, while +P fell by 10 +/- 5% and 16 +/- 4% at 160 and 180 beats/min, respectively. +P/-P ratios were 1.25 +/- 0.07 at 80 beats/min; 1.08 +/- 0.06 at 140 beats/min (P less than 0.01); 1.03 +/- 0.05 at 160 beats/min (P less than 0.01) and 1.02 +/- 0.06 at 180 beats/min (P less than 0.01). CO remained unchanged and due to this SV fell significantly when HR increased. DT, +T, -T, +T/-T and T (tau) were unchanged. DE fell significantly at 120, 140, 160 and 180 beats/min with respect to the initial HR. SW fell significantly at every HR studied. MRE was 147 +/- 13 ml/sec at 80 beats/min and 100 +/- 8 and 93 +/- 7 ml/sec at 160 and 180 beats/min, respectively (P less than 0.01 with respect to control). SP was 161.7 +/- 19.3 g X m X sec-1 at 80 beats/min, and 122.3 +/- 13.2 and 103.8 +/- 9.2 g X m X sec-1 at 160 and 180 beats/min, respectively (P less than 0.01 with respect to control). We may come to the following conclusions: 1) When HR changed in our experimental conditions, CO remained unchanged and due to this the SV fell significantly, showing the difficulty of the heart pump to regulate the CO by itself. 2) The process of relaxation is not affected within the used HR range. 3) In our experiments the inotropic effect due to increments of heart rate ("Bowditch staircase phenomenon") was not detected. However, it might become evident in preparations with more depressed contractility.