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Effects of systemic hypertension on ischemic and nonischemic regional left ventricular function in awake, unsedated
Insights
Systemic hypertension acutely improves myocardial blood flow during canine myocardial infarction. However, sustained hypertension for 24 hours worsens ventricular dysfunction in ischemic segments.
Area of Science:
- Cardiovascular Physiology
- Myocardial Ischemia Research
- Hypertension Studies
Background:
- Hypertension and coronary artery disease frequently coexist.
- The impact of elevated aortic pressure on ischemic left ventricular dysfunction remains unclear.
Purpose of the Study:
- To investigate the effects of increased aortic pressure on segmental left ventricular function during myocardial ischemia in a canine model.
- To assess alterations in regional myocardial oxygen supply and demand under hypertensive conditions during ischemia.
Main Methods:
- Utilized piezoelectric crystals to measure segmental wall thickness in awake dogs.
- Induced myocardial ischemia via left anterior descending artery occlusion.
- Administered phenylephrine (PE) or aortic constriction to elevate blood pressure.
- Measured regional myocardial blood flow (RMBF) and intramyocardial PCO2.
Main Results:
- Acute hypertension (6 hours) increased endocardial and midwall RMBF and decreased intramyocardial CO2 accumulation during ischemia.
- Transiently depressed segmental systolic thickening in ischemic segments during acute hypertension.
- Sustained hypertension (24 hours) exacerbated end-diastolic wall thinning in paradoxically moving segments, with no recovery to baseline.
Conclusions:
- Acute systemic hypertension does not appear to adversely affect myocardial oxygen supply/demand balance during evolving canine myocardial infarction.
- Prolonged mild hypertension (24 hours) can worsen diastolic dysfunction in severely ischemic myocardial segments, impairing their recovery.
Abstract:
Hypertension and atherosclerotic coronary arterial obstruction frequently coexist in patients. However, the effect of increased aortic pressure on ischemic segmental dysfunction is not well understood. We studied the effects of aortic pressure increases on segmental left ventricular function during myocardial ischemia. Eighty-two dogs instrumented with three to six pairs of pulse-transit piezoelectric crystals were studied in an awake, unsedated state to measure segmental wall thickness. A pneumatic balloon occluder was positioned around the proximal left anterior descending artery (LAD). Thirty-three dogs underwent LAD occlusion and served as normotensive controls (group A). Group B dogs (n = 23) received a 6-hour infusion of phenylephrine (PE) beginning 5 minutes after LAD occlusion to increase aortic diastolic arterial pressure to 120-130 mm Hg; aortic pressure was then allowed to return to normal for the subsequent 18 hours. The eight dogs in group C received a 6-hour infusion of PE, but no coronary arterial occlusion was produced. In group D (n = 12), distal constriction of the thoracic aorta was maintained for 24 hours after LAD occlusion. Regional myocardial blood flow (RMBF) was measured with radioactive microspheres in six conscious dogs and both RMBF and intramyocardial PCO2 were measured in seven open-chest dogs to assess alterations in regional myocardial oxygen supply and demand. Segments of myocardium were arbitrarily grouped according to the amount of net systolic thickening (NET) present 5 minutes after LAD occlusion and before increasing aortic pressure: group 1 retained 67-100+% of control NET, group 2 0-67%, and group 3 less than 0% (paradoxic motion). In dogs receiving PE plus LAD occlusion and in dogs with aortic constriction and LAD occlusion, NET was transiently depressed in groups 1 and 2 compared with the normotensive cohort; 24 hours after occlusion, NET in groups 1, 2 and 3 did not differ significantly from that in the normotensive dogs. Systemic hypertension resulted in a significant increase in endocardial and midwall RMBF and, in seven open-chest dogs, decreased the intramyocardial accumulation of carbon dioxide after LAD occlusion. Increased aortic pressure in dogs without coronary occlusion produced reversible decreases in end-diastolic wall thickness, NET and LV dP/dt. Thus, the production of systemic hypertension with diastolic pressures of 110-120 mm Hg acutely or for 6 hours during evolving canine myocardial infarction does not appear to exert an important deleterious effect on myocardial oxygen supply and demand. However, 24 hours of mildly increased aortic pressure accentuates end-diastolic wall thinning in segments with paradoxic systolic motion and results in a failure of their return to control values at this period.