Effects of atrial pacing on regional myocardial gas tensions with critical coronary stenosis

Changes in myocardial carbon dioxide (Pm(CO₂) and oxygen tension (Pm(O₂) measured by mass spectrometry have been shown to reflect quantitatively progressive degrees of regional myocardial ischemia associated with stepwise reduction in coronary blood flow. The present study utilized mass spectrometry to assess the severity of regional myocardial ischemia developing during atrial pacing in the presence of a flow limiting proximal critical coronary artery stenosis. Myocardial blood flow (MBF) to subepicardial and subendocardial layers was measured by the radioactive microsphere technique. Application of a 'critical stenosis' resulted in a 6 mmHg decrease in Pm(O₂) and a 17 mmHg increase in Pm(CO₂) in the region of the myocardium supplied by the stenosed vessel. The addition of atrial pacing resulted in a 3 mmHg further decrease in Pm(O₂) and a 40 mmHg further increase in Pm(CO₂). In the region of myocardium supplied by the critically stenosed vessel MBF increased in the subepicardial layer, but decreased or remained unchanged in the subendocardial layer. The failure of myocardial blood flow to increase in deeper myocardial layers in response to the increased myocardial oxygen demand of atrial pacing would provide a mechanism for the development of subendocardial ischemia in the presence of a critical coronary stenosis.