Oxygen extraction from the heart is near maximal at baseline and increases even further with exercise. Therefore, the coronary sinus has the lowest oxygen content in the body both at rest and during exercise.
According to Fick's principle, oxygen consumption is the product of the blood flow to an area of the body and the oxygen extraction from the blood (Illustration A). Myocardial oxygen extraction in the heart is greater than any other tissue. At rest, myocardial tissue extracts 75-80% of the oxygen, while active myocardium extracts up to 90%. Therefore, the main way to increase oxygen consumption in the heart is to increase coronary blood flow. Importantly, due to the structure of the heart, most blood flow occurs during diastole, accounting for ~5% of cardiac output. Oxygen saturation levels are as follows: SVC 75%, IVC 80%, coronary sinus 30%, pulmonary artery 80%, and pulmonary vein 100%.
Achar et al. review the diagnosis of acute coronary syndrome, which encompasses unstable angina, ST-segment elevation (STEMI) and non-ST-segment elevation myocardial infarction (NSTEMI). Diagnosis requires EKG and a careful history and physical exam. Common EKG changes include T-wave tenting/inversion, ST-segment elevation/depression and pathologic Q waves. Troponin T or I is generally the most sensitive determinant of acute coronary syndrome.
Deussen et al. discuss mechanisms of metabolic coronary flow regulation. Multiple mechanisms maintain coronary venous PO2 at a constant level under various physiological conditions, yet flow must be able to increase significantly during exercise and during times of increased demand. Such processes involve metabolic, neuronal and endothelium-dependent mechanisms that rely on metabolites such as adenosine, prostaglandins, nitric oxide and protons.
Illustration A shows Fick's principle, where CO is cardiac output, VO2 is oxygen extraction, Ca is arterial oxygen concentration, and Cv is venous oxygen concentration.
Answer 1: The average oxygen saturation level of the SVC is 75%.
Answer 2: The average oxygen saturation level of the IVC is 80%.
Answer 4: The average oxygen saturation level of the pulmonary artery is 80%.
Answer 5: The average oxygen saturation level of the pulmonary vein is 100%.
Achar SA, Kundu S, Norcross WA. Diagnosis of acute coronary syndrome. Am Fam Physician. 2005 Jul 1;72(1):119-26.
PMID:16035692 (Link to Abstract)
Deussen A, Ohanyan V, Jannasch A, Yin L, Chilian W. Mechanisms of metabolic coronary flow regulation. J Mol Cell Cardiol. 2012 Apr;52(4):794-801.
PMID:22004900 (Link to Abstract)
USMLE World Step 1 QBank Question #2009. Copyright © USMLEWorld, LLC 2012