extra cardio.docx

Quick notes on Venous Return, Cardiac Output, Stroke Volume and Ejection Fraction: Venous Return (VR)- the flow of blood back to the heart Cardiac Output (CO)- the amount of blood the heart pumps from each ventricle per minute (L/min) CO= HR x SV Stroke Volume (SV)- amount of blood ejected per beat...

Quick notes on Venous Return, Cardiac Output, Stroke Volume and Ejection Fraction: Venous Return (VR)- the flow of blood back to the heart Cardiac Output (CO)- the amount of blood the heart pumps from each ventricle per minute (L/min) CO= HR x SV Stroke Volume (SV)- amount of blood ejected per beat by the left ventricle into the aorta (or from right ventricle into the pulmonary artery) [A more precise definition for SV would be the one used in echocardiography- the difference between the ventricular end-diastolic volume(EDV) – the end-systolic volume(ESV)] EDV- filled volume of the ventricle prior to contraction ESV- residual volume of blood remaining in the ventricle after ejection SV= EDV - ESV Preload An increase in VR to the heart increases the filled volume (EDV) of the ventricle, which stretches the muscle fibres → increasing preload → increased force of ventricular contraction→ the heart can eject the additional blood that was returned to it ↑EDV results in ↑SV Afterload Afterload is related to the pressure that the ventricle must generate in order to eject blood into the aorta. Changes in the afterload affect the ability of the ventricle to eject blood and thereby alter ESV and SV. ↑afterload decreases SV and causes ↑ESV Inotropy (Contractility) Changes in ventricular inotropy alter the rate of ventricular pressure development, thereby affecting ESV and SV. ↑inotropy (e.g. sympathetic activation of the heart) ↑SV ↓ESV Effects of changes in EDV and ESV are not independent. For example, ↑ESV results in a compensatory ↑EDV. And if SV is increased by ↑EDV, this can lead to a small ↑ESV because of the influence of ↑afterload on ESV caused by an increase in aortic pressure. Therefore, while the primary effect of a change in preload, afterload or inotropy may be on either EDV or ESV, secondary changes can occur that can partially compensate for the initial change in SV Ejection Fraction (EF)- the proportion of diastolic volume ejected during ventricular contraction Normal range of left ventricular ejection fraction: 55-70% $EF = \frac{\text{SV}}{\text{EDV}}$ or $\frac{EDV - ESV}{\text{EDV}}$ Stroke volume, as mentioned is affected by preload and afterload, whereas ejection fraction is relatively preload independent but is affected by changes in afterload. However, both are affected by contractility.