Overview
- Cardiac output (CO) curve
- describes a state of cardiac function
- a plot of the relationship between CO and right atrial pressure
- ↑ venous return → ↑ right atrial pressure → ↑ end-diastolic volume (EDV), end-diastolic fiber length → ↑ CO
- ↓ venous return → ↓ right atrial pressure → ↓ EDV, end-diastolic fiber length → ↓ CO
- Vascular return curve
- describes a state of vascular function
- a plot of the inverse relationship between venous return and right atrial pressure
- ↓ right atrial pressure → ↑ ΔP (systemic arteries, right atrium) → ↑ venous return
- ↑ right atrial pressure → ↓ ΔP (systemic arteries, right atrium) → ↓ venous return
- Mean systemic pressure
- also known as mean circulatory pressure
- the x-intercept of the vascular function curve
- shows pressure that would be measured throughout cardiovascular system if the heart were stopped
- stopping the heart guarantees that pressure is equal throughout the vasculature
Inotropic Effects
- Inotropy describes the state of cardiac contractility
- Positive inotropy (↑ contractility)
- positive inotropic agent → ↑ cardiac contractility, ↑ stroke volume, and ↑ CO
- e.g., sympathetic nervous system activity, and digitalis
- CO curve shifts upward
- new steady state or equilibrium → ↓ right atrial pressure (EDV)
- ↓ right atrial pressure → more blood is ejected from the heart on each beat as a consequence of increased contractility and increased stroke volume
- positive inotropic agent → ↑ cardiac contractility, ↑ stroke volume, and ↑ CO
- Negative Inotropy (↓ contractility)
- negative inotropic agent → ↓ cardiac contractility, ↓ stroke volume, ↓ CO
- e.g., congestive heart failure and narcotic overdose
- CO curve shifts downward
- new steady state, or equilibrium → ↓ CO, ↑ right atrial pressure (EDV)
- ↑ right atrial pressure → less blood is ejected from the heart on each beat as a consequence of decreased contractility and decreased stroke volume
- negative inotropic agent → ↓ cardiac contractility, ↓ stroke volume, ↓ CO
- Effects of changes in blood volume
- increases in blood volume
- ↑ blood volume (e.g., transfusion) → ↓ venous compliance → ↑ mean systemic pressure
- ↓ venous compliance → blood is shifted from veins to arteries
- venous return curve shifts to the right
- new steady state or equilibrium → ↑ CO and ↑ right atrial pressure (EDV)
- ↑ blood volume (e.g., transfusion) → ↓ venous compliance → ↑ mean systemic pressure
- decreases in blood volume
- ↓ blood volume (e.g., hemorrhage) → ↑ venous compliance → ↓ mean systemic pressure
- ↑ venous compliance → blood is shifted from arteries to veins
- venous return curve shifts to left
- new steady state or equilibrium → ↓ CO, ↓ right atrial pressure (EDV)
- ↓ blood volume (e.g., hemorrhage) → ↑ venous compliance → ↓ mean systemic pressure
- increases in blood volume
- Effects of changes in total peripheral resistance (TPR)
- increase in TPR
- vasoconstriction of arterioles → ↑ TPR
- e.g., hemorrhage
- ↑ TPR → ↑ mean arterial pressure (MAP) → ↑ afterload → ↓ CO
- cardiac function curve shifts downward
- ↑ TPR → ↓ venous return
- venous return curve rotates counterclockwise
- vasoconstriction of arterioles → ↑ TPR
- decrease in TPR
- vasodilation of arterioles → ↓ TPR
- e.g., exercise
- ↓ TPR → ↓ MAP → ↓ afterload → ↑ CO
- cardiac function curve shifts upward
- ↓ TPR → ↑ venous return
- venous return curve rotates clockwise
- vasodilation of arterioles → ↓ TPR
- increase in TPR