CPR Lecture 11 — Cardiac Output

Abbreviations

CO — Cardiac Output

CVP — Central Venous Pressure

HR — Heart Rate

MAP — Mean Arterial Pressure

MCFP — Mean Circulatory Filling Pressure

MSFP — Mean Systemic Filling Pressure

RAP — Right Atrial Pressure

SVR / TPR — Systemic Vascular / Total Peripheral Resistance

SV — Stroke Volume | VR — Venous Return

Basic Concepts

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Mean Circulatory Filling Pressure (MCFP)

Average equilibrated pressure throughout the entire circulatory system when blood flow is stopped
Measured by stopping flow and letting pressures equilibrate
Function of: total fluid volume in the system + vascular tone

Mean Systemic Filling Pressure (MSFP)

Average equilibrated pressure throughout the systemic circulation (excludes pulmonary) when blood flow is stopped
Normal: ~7 mmHg
Creates the pressure gradient driving blood into the RA
Affected by: blood volume + venous resistance

Venous Return (VR)

VR = flow of blood into the right atrium (RA). Driven by the pressure gradient between MSFP and RAP.

VR = ΔP / R = (MSFP − RAP) / Resistance

Normal: MSFP = 7 mmHg, RAP = 2 mmHg → gradient = 5 mmHg drives ~5 L/min into RA

If RAP rises independently, the gradient narrows → VR falls.

Central Venous Pressure (CVP)

Pressure in the vena cava — synonymous with RAP
Estimates central blood volume and preload
Critical determinant of cardiovascular function

The Closed Loop — CO = VR in Steady State

The cardiovascular system is a closed loop
Whatever exits the LV (CO) must equal what enters the RA (VR) over time
In resting steady state: CO = VR ≈ 5 L/min
CO = HR × SV

Factors Affecting Venous Return

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① Blood Volume

↑ Blood volume → ↑ MSFP → ↑ VR
Central venous volume regulated by kidneys (total blood volume)
Also affected by mechanical factors

② Venomotor Tone (Compliance)

Venoconstriction (↓ compliance) → blood displaced from peripheral to central veins → ↑ VR
Venodilation (↑ compliance) → blood pools peripherally → ↓ VR
↑ Sympathetic → venoconstriction → ↑ VR → ↑ EDV

③ Arteriolar Tone (TPR)

Arteriolar vasoconstriction "holds" blood on arterial side → ↓ venous volume → ↓ VR
↑ TPR → ↓ VR | ↓ TPR → ↑ VR
Minor effect on MSFP (2/3 of blood is venous)

Mechanical Factors Affecting CVP

Body Posture

Upright → venous pooling in lower extremities → ↓ VR

Muscle Pump

Skeletal muscle contraction squeezes peripheral veins → blood displaced centrally → ↑ VR

Respiratory Pump

Inspiration → negative intrathoracic pressure → ↑ blood flow into thoracic veins → ↑ VR

Vascular Function Curve (VFC)

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What It Shows

The VFC plots Venous Return (L/min) on the Y-axis against Right Atrial Pressure, RAP (mmHg) on the X-axis. As RAP increases, the gradient driving VR into the RA decreases → VR falls.

Normal Vascular Function Curve — VR is highest when RAP is low. VR = 0 when RAP = MSFP (no gradient). Flat portion when RAP < 0 (veins collapse).

How the VFC Shifts

↑ Blood Volume

Shifts VFC up and right

MSFP ↑

↑ Venomotor Tone (venoconstriction)

Shifts VFC up and right

MSFP ↑

↑ TPR (arteriolar constriction)

↓ slope of VFC

MSFP unchanged

↓ Blood Volume (hemorrhage)

Shifts VFC down and left

MSFP ↓

↓ Venomotor Tone (venodilation)

Shifts VFC down and left

MSFP ↓

↓ TPR (arteriolar dilation)

↑ slope of VFC

MSFP unchanged

Key rule: Blood volume and venomotor tone change the MSFP (and shift the curve). Arteriolar tone only changes the slope — no effect on MSFP because 2/3 of blood is already in the venous system.

Cardiac Function (Performance) Curve

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What It Shows

The cardiac function curve (CFC) plots Cardiac Output (L/min) vs. Right Atrial Pressure / RAP (mmHg). It illustrates the Frank-Starling Mechanism — as RAP ↑ (preload ↑), EDV ↑, ventricular stretch ↑, and CO ↑. At high RAP the curve flattens (maximum stretch).

Cardiac Function Curve — increasing RAP raises CO (Frank-Starling): ↑ RAP → ↑ EDV → ↑ CO. ↑ contractility shifts curve up-left; ↓ contractility (heart failure) shifts curve down-right.

Contractility Effects

↑ Contractility (positive inotrope) → ↓ ESV → ↑ SV → CFC shifts up and left
↓ Contractility (negative inotrope / heart failure) → ↑ ESV → ↓ SV → CFC shifts down and right

Positive inotropes: sympathetic NS, β₁ agonists, digoxin. Negative: uncompensated heart failure.

Afterload Effects

Afterload ≡ aortic pressure ≡ determined by TPR
↑ Afterload → more resistance against ejection → ↓ SV → CFC shifts down
↓ Afterload → easier ejection → ↑ SV → CFC shifts up

CO = HR × SV — Summary of Determinants

Heart Rate

↑ Sympathetic → ↑ HR
↑ Parasympathetic → ↓ HR
CO is affected more by HR than SV (exercise: HR ↑100%, SV <50%)

Stroke Volume (SV = EDV − ESV)

↑ Preload (EDV) → ↑ SV
↑ Afterload → ↓ SV (↑ ESV)
↑ Contractility → ↓ ESV → ↑ SV

Physiological Variation in CO

↑ Metabolic rate → ↑ CO
Exercise → ↑ CO
Pregnancy → ↑ CO (↑ blood volume)
Sleep → ↓ CO
Age → ↓ CO

Guyton Cross Plot — Steady State

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What It Is

The Guyton Cross Plot superimposes the CFC and VFC on the same graph (both sharing RAP on the x-axis and CO/VR on the y-axis). The intersection point A is where CO = VR — this is the steady-state operating point.

Normal steady state: RAP = +2 mmHg, CO = VR = 5 L/min
Any physiological or pathological change shifts one or both curves → new intersection = new steady state

Guyton Cross Plot — CFC (yellow) and VFC (teal) intersect at steady-state point A (purple). Any shift in either curve moves the operating point.

Clinical Scenarios — Guyton Cross Plot Shifts

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How to Interpret a Scenario

Identify which curve(s) are shifted: CFC (cardiac side) vs. VFC (vascular side) vs. both
Find the new intersection point → read off new CO and RAP
Key rule: CFC shifts do NOT change VFC (and vice versa) unless both sides are affected

↑ Contractility (e.g., Digoxin)

CFC shifts up and left
VFC unchanged
New steady state: ↑ CO, ↓ RAP

More blood ejected → RA empties better → RAP falls.

↓ Contractility (Heart Failure, decompensated)

CFC shifts down and right
VFC unchanged
New steady state: ↓ CO, ↑ RAP

Heart fails to pump out → blood backs up → RA congests → ↑ RAP.

Hemorrhage (↓ Blood Volume)

VFC shifts down and left (↓ MSFP)
CFC initially unchanged
New steady state: ↓ CO, ↓ RAP
Compensation: Baroreceptor reflex → ↑ sympathetic → ↑ contractility + ↑ TPR + venoconstriction → equilibrium shifts toward B (CO partially restored, RAP further ↓)

Exercise

Sympathetic → ↑ contractility → CFC shifts up and left
Sympathetic → venoconstriction → VFC shifts up and right
TPR does not change significantly (skeletal muscle vasodilation balanced by constriction elsewhere)
New steady state: ↑ CO, RAP relatively maintained

↑ TPR (Vasopressor / Arteriolar Constriction)

CFC shifts down (↑ afterload → ↓ SV)
VFC slope decreases (blood "held" on arterial side → ↓ VR)
No change in MSFP
New steady state: ↓ CO, ↓ VR, no change in RAP

↓ TPR (Arteriolar Vasodilation)

CFC shifts up (↓ afterload → ↑ SV)
VFC slope increases (blood moves from arterial to venous side → ↑ VR)
No change in MSFP
New steady state: ↑ CO, ↑ VR, no change in RAP

Heart Failure — Compensated vs. Decompensated

Decompensated

↓ CO → ↓ MAP → baroreceptor reflex not yet corrected. CFC down/right. A→B: ↓ CO, ↑ RAP.

Compensated

↑ Sympathetic → ↑ blood volume (via RAS) + venoconstriction → VFC shifts up/right. New point C: CO almost restored, but RAP is even higher. ↑ RAP → pulmonary/peripheral edema risk.

Once cardiac performance declines further, no additional CO improvement occurs with more blood volume — this is decompensated HF.

Guyton Cross Plot — Quick Reference

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Scenario	CFC Shift	VFC Shift	New CO	New RAP	New VR
↑ Contractility (Digoxin, SNS)	Up / Left	None	↑	↓	↑
↓ Contractility (Heart Failure)	Down / Right	None	↓	↑	↓
Fluid infusion (↑ blood volume)	None	Up / Right; ↑ MSFP	↑	↑	↑
Hemorrhage (↓ blood volume)	None (initially)	Down / Left; ↓ MSFP	↓	↓	↓
Venoconstriction (↑ venomotor tone)	None	Up / Right; ↑ MSFP	↑	↑	↑
↑ TPR (vasopressor)	Down (↑ afterload)	Slope ↓; MSFP unchanged	↓	≈ unchanged	↓
↓ TPR (vasodilation)	Up (↓ afterload)	Slope ↑; MSFP unchanged	↑	≈ unchanged	↑
Exercise	Up / Left (↑ contractility)	Up / Right (venoconstriction)	↑↑	≈ maintained	↑↑
Compensated Heart Failure	Down / Right (↓ contractility)	Up / Right (↑ vol, venoconstriction)	≈ near normal	↑↑	≈ near normal