Compare and contrast the supply and demand of oxygen for the right and left ventricle.
An integrated answer to supply and demand of oxygen was expected, as a comparison between the right and left ventricles. Many candidates concentrated on differences not similarities. Myocardial oxygen demand was in general poorly described.
About 85 - 90% of oxygen demand is for internal work (major determinants wall tension 30 -
40%, heart rate 15 - 25%, myocardial contractility 10 - 15%, basal metabolism 25%). 10 - 15%
of oxygen demand for external work or pressure volume work, determined by MPAP x CO.
It was expected answers would comment on the phasic nature of coronary blood flow which
differs between left and right and the consequence of this to subendocardial oxygen supply
during systole usually. Coronary blood flow is affected by coronary perfusion pressure
(determined by aortic pressure and RV pressure) & coronary vascular resistance (determined
by autoregulation, metabolic factors, humoral factors, nervous control interacting with local
Generally, coronary blood flow is tightly coupled to oxygen demand/consumption due to high
basal oxygen consumption (8 - 10 ml/ min/100g) and high oxygen extraction ratio (75%). Better
answers noted that oxygen supply can only be increased to cope with increased demand only
by increased coronary blood flow
It is not clear why the examiners decided to go on about the fractional oxygen consumption of different myocardial workload determinants, as it did not lead to a discussion of either similarities or differences. Moreover, it is impossible to pinpoint precisely where they got these numbers from, and may not be entirely accurate (investigators have reported values as high as 70% for external workload, depending on cardiac output).
Anyway: The best way to approach this question is by using a table. In one column, the determinant factorss of myocardial oxygen supply and demand are listed. In the other two, the right and left ventricle are compared in short comments. Wherever the ventricles are similar, the cells are merged and some statement is made which is equally accurate for both of them.
|Determinants||Left ventricle||Right ventricle|
|Determinants of mmyocardial oxygen demand|
|Heart rate||Both ventricles beat at the same rate|
|Preload||Both ventricles usually have the same preload in the normal heart;|
|Contractility (dP/dT)||More contractile||Less contractile|
|Afterload||High afterload, and therefore high oxygen demand (more work done to overcome pressure)||Low afterload, less work done to overcome pressure|
|Cost of electrical conduction||Both ventricles have approximately the same content of conductive tissue, and in any case this does not contribute significantly to the overall oxygen demand|
|Basal cost of cardiac metabolism||Both ventricles contain the same type of tissue and the basal metabolic rate for myocardiom in general is the same (8ml O2/min/100g, at rest)|
|The left ventricle contains more muscle mass, and the net basal consumption is greater||The RV has a lower muscle mass and consumes less total O2 because of this|
|Determinants of myocardial oxygen supply|
|Oxygen content of blood||The same aortic blood supplies both ventricles|
|Coronary perfusion pressure||High afterload, and therefore high subendocardial systolic pressure (thus, lower blood flow during systole)||Low afterload, low subendocardial systolic pressure, and stable blood flow throughout systole and diastole|
|Anatomy||Two major arteries (LAD and LCx)||One major artery (RCA)|
|Metabolic activity||Owing to oits greater mass, the LV is more metabolically active than the RV||RV has lower net metabolic activity|
|Both ventricles have a high oxygen extraction ratio, and the only way to satisfy increased demand is by increasing blood flow.|
|Autonomic control||Coronary arterial vascular resistance is under autonomic control, which is similar for both ventricles|
Duncker, Dirk J., and Robert J. Bache. "Regulation of coronary blood flow during exercise." Physiological reviews 88.3 (2008): 1009-1086.
MJ Kern MJ and MJ Lim, “Chapter 24: Evaluation of Myocardial and Coronary Blood Flow and Metabolism,” in Grossman & Baim's Cardiac Catheterization, Angiography, and Interventions, pp. 505–544, 8th edition (2013)
Spieckermann, P. G., and H. M. Piper. "Oxygen demand of calcium-tolerant adult cardiac myocytes." Basic research in cardiology 80 (1985): 71.
Engelman, Richard M., et al. "The metabolic consequences of blood and crystalloid cardioplegia." Circulation 64.2 Pt 2 (1981): II67-74.
Mayr, A., W. Pajk, and W. Hasibeder. "Oxygen Supply and Consumption in Tissues." Sepsis and Organ Dysfunction. Springer, Milano, 2000. 43-48.
Neubauer, Stefan. "The failing heart—an engine out of fuel." New England Journal of Medicine 356.11 (2007): 1140-1151.
Suga, Hiroyuki, et al. "Determinants of Myocardial Oxygen Demand: Total Mechanical Energy and Contractility." Microcirculation in Circulatory Disorders. Springer, Tokyo, 1988. 529-534.
Rooke, G. Alec, and E. O. Feigl. "Work as a correlate of canine left ventricular oxygen consumption, and the problem of catecholamine oxygen wasting." Circulation Research 50.2 (1982): 273-286.