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Heart-rate effects on canine left-ventricular end-diastolic compliance measured by two-dimensional ultrasound
Insights
Increasing heart rate (HR) slightly decreases left ventricular (LV) compliance during open-heart surgery. However, this effect is minimal, indicating HR is a minor factor in intraoperative LV compliance changes.
Area of Science:
- Cardiovascular Physiology
- Surgical Research
- Echocardiography
Background:
- Left ventricular (LV) compliance is crucial during open-heart surgery.
- Previous studies on heart rate (HR) effects on LV compliance are limited by technical challenges.
Purpose of the Study:
- To investigate the impact of varying heart rates on left ventricular compliance during cardiopulmonary bypass.
- To clarify the role of heart rate as a determinant of intraoperative LV compliance.
Main Methods:
- Eight dogs underwent cardiopulmonary bypass with induced atrioventricular (AV) block.
- AV sequential pacing varied HR (100, 125, 150 bpm) while LV end-diastolic pressure (LVEDP) was manipulated.
- Left ventricular end-diastolic volume (LVEDV) was measured using 2-D echocardiography and Simpson's rule.
Main Results:
- A small but significant increase in exponential constants (b) was observed with rising HR, indicating progressive stiffening.
- Despite this trend, overall LV pressure-volume relations were not significantly altered across LVEDP intervals.
- Echocardiography showed a trend towards decreased LV compliance with increased HR.
Conclusions:
- While higher heart rates show a trend towards reduced left ventricular end-diastolic compliance, the effect is minor.
- Heart rate is a less significant factor in intraoperative LV compliance changes compared to other determinants.
Abstract:
Despite recent interest in left ventricular (LV) compliance during open-heart surgery, the effect of heart rate (HR) remains unclear due to technical limitations of previous studies. In the presence of Formalin-induced complete atrioventricular (AV) block, AV sequentially paced heart rate was varied between 100, 125, and 150 beats/min in eight dogs on cardiopulmonary bypass. The heart was cradled in an open pericardial well filled with ultrasound-conducting gel. LV end-diastolic pressure (LVEDP) was varied over 0-20 mm Hg by flow regulation in the bypass circuit. Left ventricular end-diastolic volume (LVEDV) was calculated from three orthogonal two-dimensional (2-D) echocardiographic sections using Simpson's rule. Exponential curves derived from the relation LVEDP = ke(bxLVEDV) demonstrated a small, but significant increase in the mean exponential constants (b = 0.034, 0.037, and 0.049 at HR 100, 125, and 150, respectively) reflecting progressive stiffening with increasing heart rate. However, mean overall pressure-volume relations were not significantly altered when analyzed in four separate intervals of LVEDP. It is concluded that although LV volume determinations with 2-D echocardiography demonstrate a steady trend toward decreased end-diastolic LV compliance with increasing rate, this change is so small as to make heart rate a minor determinant of observed intraoperative compliance changes.