What are the three major determinants of myocardial oxygen consumption? How are they measured clinically?
Answer:
The three major determinants of myocardial oxygen consumption are myocardial wall tension, contractility, and HR.
Myocardial wall tension of the LV is estimated by the following:
The Laplace equation states that the myocardial wall tension (T) is analogous to the diameter of the ventricle (R, radius) and the intracavitary pressure (P) and inversely proportional to the myocardial wall thickness (Th):
Diastolic wall tension is related to the diastolic radius (R) of the ventricle, which is considered a surrogate of preload or left ventricular end-diastolic volume (LVEDV); the latter is estimated by the PAOP (because neither LVEDP nor left atrial pressure can be directly measured).
Afterload is approximated via the systolic BP, which equals systolic ventricular pressure if there is no aortic stenosis.
Contractility is evaluated by:
Invasive techniques. Maximal velocity of contraction (Vmax), dP/dt (pressure time indices of ventricle: how fast [dt] the intraventricular pressure [dP] rises), or left ventricular end-systolic pressure/volume ratio
Noninvasive technique. Pre-ejection period/left ventricular ejection time, and global and regional ventricular wall motion by echocardiography (Fig. 5.3)
Reference(s):
- Kaplan JA, Augoustides JGT, Manecke GR, et al, eds. Kaplan’s Cardiac Anesthesia for Cardiac and Noncardiac Surgery. 7th ed. Philadelphia, PA: Elsevier; 2017:179-205.