Cardiac Physiology

Hemodynamic Parameters

• The European Society of Cardiology suggests that an optimal Ea/Ees ratio of 0.7-0.8 indicates efficient coupling between the heart and vascular system, as measured by ventriculo-arterial coupling 1
• Measurements obtained under general anesthesia may not accurately reflect the patient's true hemodynamic state, according to the American Heart Association 2
• The normal range for systemic vascular resistance (SVR) is 800-1200 dynes·sec·cm⁻⁵, with values below 800 dynes·sec·cm⁻⁵ considered low and values above 1200 dynes·sec·cm⁻⁵ considered elevated, representing optimal vascular tone for adequate tissue perfusion, as recommended by the American College of Critical Care Medicine and supported by the American College of Cardiology 3, 4, 5
• Maintaining SVR between 800-1200 dynes·sec·cm⁻⁵ is crucial for optimizing cardiac function, especially in critically ill patients, cardiac donor management, and post-cardiopulmonary bypass patients, as suggested by the Society of Critical Care Medicine and the American Heart Association 3, 6, 7
• The formula for calculating SVR is: SVR = (Mean Arterial Pressure - Right Atrial Pressure) / Cardiac Output, with units of dynes/sec/cm^5, and its values should be interpreted in the context of other hemodynamic parameters and the patient's condition, as recommended by the American College of Cardiology and the American Heart Association 4, 5, 7
• Abnormal SVR values are associated with specific conditions: low SVR (<800 dyn·s·cm⁻⁵) with distributive shock, high SVR with the need for vasodilators, and values <450 dyn·s·cm⁻⁵ with increased mortality, highlighting the importance of SVR monitoring in critically ill patients, as suggested by the Society of Critical Care Medicine and the American College of Critical Care Medicine 6, 7
• SVR represents the "steady" component of left ventricular afterload, primarily determined by arteriolar tone, blood viscosity, and vascular capacitance, and is inversely related to Cardiac Output when other factors remain constant, and directly related to Blood Pressure (MAP = CO × SVR + RAP), as noted by the American Heart Association 4, 7
• In heart failure management, SVR guides vasodilator therapy and helps differentiate types of shock, and elevated estimated SVR has been associated with increased risk of heart failure, major cardiovascular events, and mortality, as recommended by the American Heart Association 7
• The relationship between SVR and other hemodynamic parameters, including Cardiac Output, Blood Pressure, Ejection Fraction, and Pulmonary Vascular Resistance, is complex, as noted by the American Heart Association and the American College of Cardiology 4, 7, 8
• The following table summarizes the functional and structural correlates of SVR:

• Peripheral vascular resistance (PVR) is a major determinant of arterial blood pressure, and baroreceptor-induced adjustments of vascular resistance help maintain cerebral perfusion when changing positions, as stated by the European Society of Cardiology 9, 10
• Autonomic failure and diabetes can affect peripheral vascular resistance, leading to orthostatic hypotension and reduced decrease of peripheral vascular resistance during exercise, respectively, highlighting the importance of considering these conditions in the context of PVR, as reported by the European Society of Cardiology and the European Association for the Study of Diabetes 9, 10

Hemodynamic Monitoring

• The American College of Cardiology and the American Heart Association recommend interpreting SVR values in the context of other hemodynamic parameters and the patient's condition 4, 5, 7
• The Society of Critical Care Medicine and the American Heart Association suggest that maintaining SVR between 800-1200 dynes·sec·cm⁻⁵ is crucial for optimizing cardiac function, especially in critically ill patients, cardiac donor management, and post-cardiopulmonary bypass patients 3, 6, 7

Clinical Applications

• In heart failure management, SVR guides vasodilator therapy and helps differentiate types of shock, and elevated estimated SVR has been associated with increased risk of heart failure, major cardiovascular events, and mortality, as recommended by the American Heart Association 7
• The European Society of Cardiology and the European Association for the Study of Diabetes report that autonomic failure and diabetes can affect peripheral vascular resistance, leading to orthostatic hypotension and reduced decrease of peripheral vascular resistance during exercise, respectively 9, 10