Identify whether each item would increase or decrease stroke volume: – Increase stroke volume – Decrease stroke volume – Increasing EDV – Dehydration – Sympathetic stimulation – Stress – Increased mitral valve regurgitation – Increased HR – Increasing afterload – Significant hemorrhage – Increasing preload – Increased renal reabsorption of water and sodium – Increased activity of the muscular and respiratory pumps – Decreased venous return – Increasing contractility – Acute exercise – Reset
Increases Stroke Volume:
- Increasing EDV
- Sympathetic stimulation
- Stress
- Increasing preload
- Increased renal reabsorption of water and sodium
- Increased activity of the muscular and respiratory pumps
- Increasing contractility
- Acute exercise
Decreases Stroke Volume:
- Dehydration
- Increased mitral valve regurgitation
- Increased HR (very high)
- Increasing afterload
- Significant hemorrhage
- Decreased venous return
Explanation:
Stroke volume (SV) is the amount of blood ejected by the left ventricle in one contraction. It is determined by three primary factors: preload, contractility, and afterload.
- Preload refers to the end-diastolic volume (EDV), or the volume of blood in the ventricles at the end of diastole. An increase in EDV or increased preload stretches myocardial fibers, enhancing contraction via the Frank-Starling mechanism—thus increasing SV. Factors that increase preload include renal sodium and water reabsorption, muscle and respiratory pump activity, and acute exercise.
- Contractility is the strength of the heart’s contraction at a given preload. It increases with sympathetic stimulation and stress (via epinephrine and norepinephrine), both of which enhance calcium availability in cardiac muscle, thereby increasing SV.
- Afterload is the resistance the heart must overcome to eject blood. An increased afterload (e.g., from hypertension) raises the workload on the heart and tends to decrease SV.
Negative factors include dehydration and hemorrhage, both of which reduce venous return and lower preload, thus decreasing SV. Increased HR, if excessive, reduces diastolic filling time and therefore decreases SV. Mitral valve regurgitation causes blood to leak backward, lowering forward stroke volume.
In summary, stroke volume rises with improved filling and contractility, but falls when blood volume drops, resistance increases, or filling time is too short.
