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Distribution of a neutral cardioplegic vehicle during the development of ischemic myocardial contracture
M R Zile1, W A Neill, W H Gaasch
1Department of Medicine (Cardiology), Tufts University School of Medicine, Boston, MA.
Insights
During cardiac arrest, crystalloid solution delivery to the heart remains effective even with ischemic contracture. This ensures myocardial protection by reaching all regions, including the subendocardium, during prolonged ischemia.
Area of Science:
- Cardiovascular Physiology
- Cardiac Surgery
- Myocardial Protection
Background:
- Uniform delivery of cardioplegic solutions is crucial for myocardial protection during prolonged cardiac arrest.
- Understanding the distribution of crystalloid solutions under ischemic conditions is essential for optimizing cardiac arrest protocols.
Purpose of the Study:
- To investigate the regional and transmural distribution of a neutral crystalloid (dextran-saline) solution during normothermic ischemia in isolated dog hearts.
- To assess the impact of ischemic contracture on crystalloid solution delivery to the left ventricle (LV) and its subendocardium.
Main Methods:
- 18 isolated blood-perfused dog hearts underwent normothermic (37°C) ischemia.
- Crystalloid solution (100 ml) was infused at 100 mmHg during ischemia, with distribution assessed using the radioactive microsphere technique.
- Hearts were reperfused after 60 or 90 minutes of ischemia; LV end-diastolic pressure and myocardial adenosine triphosphate (ATP) levels were measured.
Main Results:
- In baseline conditions, the LV received 67% of coronary blood flow with a subendocardial to subepicardial ratio of 1.33.
- During the first 45 minutes of ischemia, crystalloid distribution mirrored baseline coronary blood flow, with no change in LV end-diastolic pressure but a moderate ATP fall.
- After 75 minutes of ischemia, despite ischemic contracture (LV end-diastolic pressure >20 mmHg) and severe ATP depletion, LV crystalloid delivery increased to 82%, and the subendocardial to subepicardial ratio rose to 1.75, even in hearts with severe contracture (>60 mmHg).
Conclusions:
- The presence of ischemic contracture does not impede crystalloid solution delivery to the left ventricle, including the subendocardium.
- This study demonstrates that crystalloid solutions can effectively reach critical myocardial regions even under severe ischemic conditions, supporting their role in myocardial protection.
Abstract:
During prolonged ischemic cardiac arrest successful myocardial protection depends upon uniform delivery of cardioplegic solutions to all regions of the heart. Accordingly, we studied the regional and transmural distribution of a neutral crystalloid (dextran-saline) solution during normothermic (37 degrees C) ischemia in 18 isolated blood-perfused dog hearts (isovolumic left ventricle). In the baseline state, coronary perfusion pressure was 100 mmHg. At the onset of ischemia and every 15 min throughout ischemia, we infused 100 ml of crystalloid solution (37 degrees C) at a perfusion pressure of 100 mmHg and the distribution of crystalloid solution was assessed (radioactive microsphere technique). The hearts were reperfused after 60 min (n = 9) or 90 mins (n = 9) of ischemia. In the baseline pre-arrest state the left ventricle (LV) received 67 +/- 1.0% of the total coronary blood flow; the LV subendocardial to subepicardial flow ratio was 1.33 +/- 0.18, the LV end diastolic pressure was 7.5 +/- 0.4 mmHg, and mean transmural myocardial adenosine triphosphate (ATP) was 16.4 +/- 1.1 microM/g DW. At the onset and throughout the first 45 mins of ischemia (n = 9), regional and transmural distribution of the crystalloid solution was similar to that of coronary blood flow during the baseline state; there was no change in LV end diastolic pressure, but there was a moderate fall in ATP content (7.26 +/- 1.6 micron/g DW). After 75 mins of ischemia (n = 9), despite the development of ischemic contracture (LV end diastolic pressure exceeded 20 mmHg in all 9 hearts) and marked ATP depletion (2.76 +/- 0.5 microM/g DW), there was an increase in crystalloid solution delivery to the LV as a whole and the subendocardium in particular (the LV received 82 +/- 2.0% and the subendocardial to subepicardial flow ratio was 1.75 +/- 0.1). Even in a subgroup with severe contracture during ischemic arrest (LV end diastolic pressure greater than 60 mmHg, n = 4) there was no reduction in crystalloid solution delivery. Thus, the presence of ischemic contracture does not preclude delivery of crystalloid solution to the LV subendocardium.