The midwall stress-velocity relationship during manipulation of afterload
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Published on: April 8, 2013
Insights
Midwall shortening indexes may prevent overestimation of cardiac contractility in children. This study suggests these indexes can explain the pseudo-hypercontractile state observed in pediatric patients.
Area of Science:
- Pediatric Cardiology
- Cardiac Physiology
- Echocardiography
Background:
- Midwall stress-velocity relationships are used to assess cardiac contractility but can show unexplained hypercontractility in some children.
- Existing methods may overestimate contractility, particularly at low afterload conditions.
Purpose of the Study:
- To investigate if midwall shortening indexes can prevent the overestimation of cardiac contractility at low afterload.
- To explore the relationship between midwall velocity of circumferential fiber shortening and end-systolic wall stress.
Main Methods:
- Studied 12 piglets (5-6 weeks old) with manipulated afterload.
- Afterload was altered using balloon occlusion of the descending aorta and sodium nitroprusside infusion.
- Left ventricular function was assessed using multiple echocardiographic variables.
Main Results:
- The regression line for midwall velocity of circumferential fiber shortening differed from the endocardial stress-velocity relationship.
- The midwall regression line was nearly horizontal (afterload-independent) above 30 g/cm2 end-systolic wall stress.
- Below this threshold, the midwall slope was steeper, showing increased velocity of fiber shortening at low afterload.
Conclusions:
- Increased midwall velocity of fiber shortening at low afterload mirrors the endocardial stress-velocity relationship.
- This phenomenon may explain the pseudo-hypercontractile state observed in some pediatric patients.
- Midwall shortening indexes show promise in accurately assessing cardiac contractility across varying afterload conditions.
Abstract:
Despite providing a physiological correction, measurements of contractility using the midwall stress-velocity relationship still show evidence of an unexplained hypercontractile state in some children. We investigated if by using midwall shortening indexes, the known overestimation of contractility at low afterload could be prevented. In 12 piglets (5 or 6 weeks old), afterload was manipulated by balloon occlusion of the descending aorta and infusion of sodium nitroprusside up to 5 mg/kg/min, and left ventricular function was measured using multiple variables. The regression line between the echocardiographically derived midwall velocity of circumferential fiber shortening and end systolic wall stress differed from the regression line of the endocardial stress-velocity relationship. Although the midwall regression line was almost horizontal (or afterload independent) for end systolic wall stress values of more than 30 g/cm2, the slope was still steeper below a certain point of afterload. The increased midwall velocity of fiber shortening at low afterload is comparable to the endocardial stress-velocity relationship and could account for the pseudo-hypercontractile state found in some children.
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