Physiology, Stroke Volume

Excerpt

To understand the principles of cardiac stroke volume (SV), it is necessary first to define the concept of cardiac output. Cardiac output (CO) is the blood volume the heart pumps through the systemic circulation over a period measured in liters per minute. Various parameters are utilized to assess cardiac output comprehensively, but 1 of the more conventional approaches involves multiplying the product of the heart rate (HR) and the stroke volume.

1. CO = SV x HR

The definition of stroke volume is the volume of blood pumped out of the heart's left ventricle during each systolic cardiac contraction. The average stroke volume of a 70 kg male is 70 mL. Not all the blood that fills the heart by the end of diastole (end-diastolic volume - EDV) can be ejected from the heart during systole. Thus the volume left in the heart at the end of systole is the end-systolic volume (ESV). The SV volume may be calculated as the difference between the left ventricular end-diastolic and left ventricular end-systolic volumes (ESV).

1. SV = EDV - ESV

Critical care physicians employ several variables when monitoring severely ill hypovolemic patients. Utilizing stroke volume as a hemodynamic variable compared to other commonly used parameters is becoming increasingly popular in assessing cardiac pump function and organ perfusion as it is subject to less influence from compensatory mechanisms. Cardiologists also use stroke volume when assessing cardiac dysfunction in those with congestive heart failure. The computation of left ventricular ejection fraction (LVEF) involves dividing the stroke volume by the end-diastolic volume (EDV) and is considered a central component in assessing both systolic and diastolic heart failure.

1. LVEF = SV/EDV