Cardiac Output Measurement

Learning Objectives

  • After reading this chapter, you will be able to:

    1. Define:

    • Cardiac Output (CO): The amount of blood the heart pumps in one minute.

    • Cardiac Index (CI): Standardized measurement of CO relative to body surface area.

    • Stroke Volume (SV): The amount of blood ejected from the left ventricle with each contraction.

    • Venous Return (VR): The amount of blood returning to the right atrium each minute.

    1. Describe:

    • The method of calculation for CO, CI, SV, and VR.

    • Common reference ranges and the effects of sympathetic nervous stimulation on cardiac output.

    1. Describe the effects of:

    • Metabolism on blood flow regulation through organs.

    • Reduced oxygen availability on blood flow through organs.

    1. Explain:

    • The effect of blood loss (hypovolemia) on circulatory function.

    1. Describe the effect of:

    • Mechanical ventilation on CO and blood flow.

    1. Explain the significance of indicators of cardiac output.

    2. List the factors that regulate:

    • Cardiac preload,

    • Afterload,

    • Cardiac contractility.

    1. Calculate:

    • Systemic vascular resistance (SVR) and pulmonary vascular resistance (PVR).

    1. Describe the technique for obtaining CO through invasive methods.

    2. Describe noninvasive methods for evaluating cardiac performance.

Cardiac Output (CO)

  • Definition: The amount of blood the heart pumps in one minute.

  • Formula: CO is the product of Stroke Volume (SV) and Heart Rate (HR).

    • ext{CO} = ext{SV} imes ext{HR}

  • Normal Ranges:

    • CO: 4 to 8 L/min at rest.

    • Factors affecting CO:

    • Age,

    • Sex,

    • Body size,

    • Blood viscosity (hematocrit),

    • Tissue demand for oxygen.

  • Stroke Volume:

    • Definition: The amount of blood ejected from the left ventricle with each contraction.

    • Normal Stroke Volume: Ranges from 60 to 130 mL.

  • Sympathetic Nervous System Stimulation:

    • Increases CO significantly—heart can pump 20 to 25 L/min.

  • Athletic Performance:

    • A trained athlete’s heart can enlarge and pump up to 35 L/min.

  • Congestive Heart Failure:

    • Condition where a diseased or damaged heart cannot pump all the blood returned to it.

Venous Return (VR)

  • Definition: The amount of blood returning to the right atrium each minute.

  • Normal Range: Typically matches CO in a healthy individual.

  • Effects of Vascular Changes:

    • Peripheral Vasodilation: Increases venous return.

    • Peripheral Vasoconstriction: Decreases venous return.

  • Relationship between VR and CO:

    • In a healthy heart, increased venous return correlates with increased CO.

  • Reservoir Function of the Venous System:

    • Approximately two-thirds of total blood volume resides in the venous system.

  • Response to Blood Volume Changes:

    • When blood volume decreases in vital organs, veins and spleen constrict to maintain venous return and pressures.

    • Up to 20% to 25% of total blood volume can be lost without altering circulatory function or pressures.

Measures of Cardiac Output and Pump Function

Cardiac Index (CI)

  • Definition: Describes flow output, normalized to body surface area (BSA).

  • Formula:

    • ext{CI} = rac{ ext{CO}}{ ext{BSA}}

  • Normal Range: 2.5 to 4.0 L/min/m².

  • Importance: Provides a standardized interpretation of cardiac function.

Cardiac Work

  • Definition: The energy the ventricles use to eject blood against aortic or pulmonary pressures (resistance).

  • Correlation: It correlates with the amount of oxygen needed by the heart.

  • Left Ventricle: Typically requires much higher cardiac work than the right ventricle.

Cardiac Work Index (CWI)

  • Definition: Measures work per minute per square meter for each ventricle.

  • Formulas:

    • Left Cardiac Work Index (LCWI):
      ext{LCWI} = ext{CI} imes ext{MAP} imes 0.0136 ext{ (normal range: 3.4 to 4.2 kg/min/m²)}

    • Right Cardiac Work Index (RCWI):
      ext{RCWI} = ext{CI} imes ext{MAP} imes 0.0136 ext{ (normal range: 0.4 to 0.66 kg/min/m²)}

  • Conversion Factor: 0.0136 is for changing pressure to work.

Ventricular Stroke Work (VSW)

  • Definition: Measure of myocardial work per contraction.

  • Formula:

    • Left Ventricular Stroke Work Index (LVSWI):
      ext{LVSWI} = ext{SVI} imes ext{MAP} imes 0.0136 ext{ (normal range: 50 to 60 g/min/m²/beat)}

    • Right Ventricular Stroke Work Index (RVSWI):
      ext{RVSWI} = ext{SVI} imes ext{MPAP} imes 0.0136 ext{ (normal range: 50 to 60 g/min/m²/beat)}

Determinants of Pump Function

Overview

  • Cardiac Output (CO) is determined by both Heart Rate (HR) and Stroke Volume (SV).

    • SV Factors: Preload, Afterload, and Contractility.

    • Heart Rate (HR): Usually not a major factor in control of CO, but extreme abnormalities can alter it (e.g., bradycardia or tachycardia).

Preload

  • Definition: Created by end-diastolic volume.

  • Effect: The greater the stretch on the myocardium prior to contraction, the greater the resulting contraction.

  • Issues: Low preload leads to drop in SV and CO, which may occur with hypovolemia. Conversely, excessive stretch may also reduce SV.

Ventricular Function Curves

  • Graphs relating heart output (vertical axis) with end-diastolic volumes (horizontal axis).

  • Purpose: Illustrate changes in CO associated with varying levels of preload.

  • General Trend: Increase in preload tends to increase SV and CO until a physiological limit is reached.

Ventricular Compliance

  • Definition: The stiffer the ventricle, the greater the preload needed to achieve adequate SV.

  • Causes of Reduced Compliance:

    • Myocardial infarction

    • Shock

    • Pericardial effusions

    • PEEP (Positive End Expiratory Pressure)

    • Positive inotropic drugs.

Factors Affecting Venous Return, Preload, and CO

  • Circulating blood volume

  • Distribution of blood volume

  • Atrial contraction (adds 30% to subsequent ventricular SV).

Effect of Mechanical Ventilation

  • Spontaneous Inspiration: Lowers intrapleural pressures, improving venous return and CO.

  • Positive Pressure Breaths: Increases intrapleural pressures, thereby reducing venous return and CO, with alterations influenced by lung and chest wall compliance.

Afterload

  • Components:

    • Ventricular wall stress

    • Peripheral vascular resistance.

  • Ventricular Wall Stress:

    • Directly related to ventricular tension (calculated as ventricular pressure × radius) and inversely related to wall thickness.

  • Effects of Increased Afterload:

    • Elevated tension increases oxygen and energy demands for contraction.

  • Effects of Decreased Afterload:

    • Positive intrathoracic pressure aids ventricle compression, easing resistance to ventricular emptying but may oppose right ventricular filling.

Peripheral Resistance

  • Components of afterload determined by:

    • Elasticity (compliance) of vessels,

    • Size (radius) of vessels,

    • Blood viscosity,

    • Driving pressure.

Calculating Systemic and Pulmonary Vascular Resistance (SVR and PVR)

  • Systemic Vascular Resistance (SVR): Increases with peripheral vasoconstriction, present during hypertension and vasoconstrictor use.

  • Pulmonary Vascular Resistance (PVR): Increases with pulmonary vasoconstriction, particularly in conditions like hypoxemia and acidosis.

Contractility

  • Definition: Strength of myocardial contraction, determined by:

    • Initial muscle length change caused by stretch (preload).

    • Inotropic state of the heart at any stretch level.

  • Influences on Contractility:

    • Sympathetic nerve stimulation (increases contractility via norepinephrine).

    • Vagal stimulation (decreases contractility).

  • Inotropic Drugs:

    • Positive inotropic effect enhances contraction strength.

    • Negative inotropic effect lowers contraction strength but may decrease myocardial oxygen demand.

Coronary Perfusion Pressure (CPP)

  • Normal Range: 60 to 80 mm Hg.

  • Formula:

    • ext{CPP} = ext{Diastolic arterial blood pressure} - ext{PAWP}

Rate-Pressure Product (RPP)

  • Used for assessing contractility; reflects cardiac work at rest and during exercise stages.

  • Formula:

    • ext{RPP} = ext{HR} imes ext{systolic blood pressure}

  • High Values: Greater than 12,000 indicate increased myocardial work and oxygen demand.

Methods of Measuring Cardiac Output: Invasive Methods

Fick Method

  • Basis: CO can be calculated if oxygen consumption, arterial oxygen content, and mixed venous oxygen content are known.

  • Process:

    • Step 1: Calculate expected oxygen consumption (normal range: 120 to 160 mL/min/m²).

    • Step 2: Calculate arterial and mixed venous oxygen contents (normal range: 3.0 to 5.5 mL/dL).

    • Step 3: Calculate CO.

Pulmonary Artery Thermodilution Cardiac Output (TDCO)

  • Definition: Most common invasive technique.

  • Requirements: Involves placement of a pulmonary artery catheter; computer required for calculation.

  • Injection Technique:

    • Sterile dextrose in water or saline cold (at least 2°C colder than blood) is injected into the proximal port of the PA catheter.

  • Cooling Detection: A thermistor bead detects cooling behind the balloon in the pulmonary artery, recording a temperature-time curve proportional to CO.

  • Assumptions:

    1. Complete mixing of blood and indicator.

    2. No loss of indicator between injection and detection.

    3. Constant blood flow.

  • Errors: Primarily arise from violations of these assumptions.

Acceptable Variations in Thermodilution Technique

  • Achievable results with different injectants (iced, refrigerated, or room temperature).

  • Best accuracy from evenly spaced injections throughout the respiratory cycle.

  • A dedicated injection gun and temperature measuring system enhance consistency and results' reproducibility.

Transpulmonary Indicator Dilution Cardiac Output

  • Definition: Uses indicator dilution like the PA catheter but injects the indicator into a central vein, measuring through arterial line in a large artery.

  • Techniques:

    • Transpulmonary thermodilution

    • Transpulmonary lithium dilution.

PiCCO and VolumeView

  • Mechanism: Involves injection of a cold fluid bolus into a central vein; blood temperature change is measured via a thermistor located in a large artery.

  • Temperature Curve: A thermodilution curve compared to PA thermodilution shows delayed peak temperature.

LiDCO Method

  • Process: Injects lithium chloride through a peripheral venous line; concentration-time curve recorded via blood drawn past a lithium sensor on arterial line.

  • Outcome: Primary and secondary lithium dilution curves are created to calculate CO.

Continuous Cardiac Output Monitoring (CCO)

  • Application: Useful for hemodynamically unstable patients requiring frequent measurement.

  • Current Techniques:

    • Continuous PA thermal dilution.

    • Arterial pulse contour analysis.

Transesophageal Doppler Monitoring

  • Method: Uses Doppler ultrasonic probes on tubes/catheters in various body locations for CO measurement (indirect measurement).

Continuous Measurement via Transthoracic Electrical Bioimpedance (TEB)

  • Process: Measures voltage changes caused by applied low-frequency current.

  • Modification: Transthoracic bioreactance reduces errors related to electrode placement, body size, external noise.

Periodic Noninvasive Measurement of Cardiac Performance

  • Techniques:

    • Transthoracic echocardiography (TTE): Accurately predicts fluid responsiveness in critically ill patients.

    • Transesophageal echocardiography (TEE): Provides clear images of cardiac chambers and mitral valve.