Non Invasive Stroke Volume is the amount of blood (in ml) pumped by the left ventricle each heartbeat. Stroke index is the stroke volume divided by Body Surface Area (BSA), to normalize it for body size. Normal rage of Stroke volume is 60 – 130 ml and normal range of Stroke index is 35 – 65 ml/m2.
There are three primary factors that determine stroke volume. These are preload, contractility, and afterload.
Preload refers to the amount of blood in the ventricle at the end of diastolic filling, which is also known as “End Diastolic Volume” (EDV). EDV is affected by several parameters including amount of circulating blood, venomotor tone (venous resistance), diastolic filling time (which is determined by heart rate), and posture. The more blood available and the longer the ventricle has to fill, the greater EDV.
Myocardial contractility is determined by the intrinsic force-velocity relationship of the myocardial muscle fibers. The contractile state of the heart may be affected by neural (parasympathetic or sympathetic nerve stimulation) and/or humoral (substances in the blood stream secreted by the body systems (e.g., epinephrine)), or pharmacological agents, such as positive inotropes like digitalis).
Afterload refers to the amount of resistance to blood flow in the arterial system. The overall resistance is determined by the diameter of the aortic valve, distensibility of the aorta, and degree of constriction/dilation of the arterial system. When other factors are held constant, the greater the resistance, the less volume will be ejected with each heartbeat.
Stroke Volume role in the physiology of delivered O2
There are two primary reasons Stroke volume may be low: hypovolemia (low blood volume, which results in low preload) and left ventricular dysfunction (poor myocardial contractility). Consequently, changes in Stroke volume can be early indicators of changes in blood volume and myocardial contractility. These changes will occur earlier than changes in cardiac output.
The usage of Stroke volume is limited as the measurement of Stroke volume is complicated and expensive.
NICaS can measure Stroke volume by only attaching 2 sensors to patient’s two limbs.
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This enables clinicians, at all levels of healthcare, to utilize Stroke Volume as an early indicator of changes in blood volume and myocardial contractility.