Cardiac Output: Factors Governing and Measurement

Presenter: Dr. Suresh PradhanModerator: Prof. PK Datta

Cardiac Output• is the amount of blood pumped by the heart per

unit of time

• is the product of the stroke volume (SV) and the heart rate (HR)

• is also the quantity of blood that flows through the circulation

• is pivotal in maintaining arterial BP (BP=CO X SVR) and oxygen delivery

• varies widely with the level of activity of the body• following factors directly affect cardiac output

(1) the basic level of body metabolism(2) whether the person is exercising(3) the person’s age(4) the size of the body

• for young, healthy men, resting cardiac output averages about 5.6 L/min

• for women, this value is about 4.9 L/min

• with exercise it may rise to 35 L/min

Cardiac Index• cardiac output is frequently stated in terms of the

cardiac index• is the cardiac output per square meter of body

surface area• average human being who weighs 70 kilograms has

a body surface area of about 1.7 square meters, which means that the normal average cardiac index for adults is about 3 L/min/m2 of body surface area

Variation of Cardiac Index with age

Distribution of Cardiac Output

Factors Governing• determined by four factors• two factors that are intrinsic to the heart

heart rate myocardial contractility

• two factors that are extrinsic to the heart but functionally couple the heart and the vasculature preload afterload

Heart rate• is defined as the number of beats per minute

• is mainly influenced by the autonomic nervoussystem

• increase in heart rate escalate cardiac output aslong as ventricular filling is adequate during diastole

Myocardial Contractility• can be defined as the intrinsic level of contractile

performance that is independent of loading conditions

• intrinsic ability of the heart to adapt to increasing volumes of inflowing blood is called the Frank Starling mechanism of the heart• Or, stated another way: Within physiological limits,

the heart pumps all the blood that returns to it by way of the veins

• Factors that modify contractility are Exercise Adrenergic stimulation Changes in pH Temperature Drugs such as digitalis

Preload• is defined as the ventricular load at the end of

diastole, before contraction has started• first described by Starling, a linear relationship exists

between sarcomere length and myocardial force• in clinical practice, surrogate representatives of left

ventricular volume such as pulmonary wedge pressure or central venous pressure are used to estimate preload• with the development of transesophageal

echocardiography a more direct measure of ventricular volume is available

Afterload• is defined as systolic load on the LV after contraction

has begun• aortic compliance is an additional determinant of

afterload• aortic compliance is the ability of the aorta to give

way to systolic forces from the ventricle• changes in the aortic wall (dilation or stiffness) can

alter aortic compliance and thus afterload

• pathologic conditions that alter afterload are aortic stenosis and chronic hypertension

• both impede ventricular ejection, thereby increasing afterload

• Preload and afterload can be thought of as the wall stress that is present at the end of diastole and during left ventricular ejection, respectively

Effects of Intravenous Anaesthetic agents on CO• CO reduced by Propofol>Thiopentone>Etomidate• secondary to decreased contractility

• Propofol also causes vasodilatation and bradycardia

Effects of Inhalational Anaesthetic agents on CO• CO reduced by

Enflurane>Halothane>Isoflurane/Desflurane>Sevoflurane>N2O• Secondary to • direct myocardial depression• inhibition of the sympathetic nervous system

outflow• altered baroreceptor activity

Measurement of Cardiac Output

• AIM : hemodynamic monitoring and support in the critically ill so as to optimize oxygen delivery to the tissues

• oxygen delivery is determined by Cardiac Output and amount of oxygen carried in the blood

• allows us to assess the blood flow to the tissues, and provides information on how to best support a failing circulation

Why should we measure?three major factors have driven efforts to measure cardiac output in clinical practice

1. the recognition that in many critically ill patients, low cardiac output leads to significant morbidity and mortality

2. the clinical assessment of cardiac output is unreliable/ inaccurate

3. newer techniques for measurement of cardiac output are less invasive than Pulmonary Arterial Catheterization monitoring and thus might provide benefit to many patients without the attendant risks associated

Where should we monitor?• high risk critically ill surgical patients in whom large

fluid shifts are expected along with bleeding and hemodynamic instability

• an important component of goal directed therapy (GDT), i.e., when a monitor is used in conjunction with administration of fluids and vasopressors toachieve set therapeutic endpoints thereby improving patient care and outcome

• Signs of inadequate oxygen delivery acidosis elevated lactate oliguria low superior vena cava oxygen saturation

Measurement1. True non-invasive

2. Low-invasive

3. Invasive

True noninvasivei. Clinical Examinationii. Trans Thoracic Echocardiography (TTE)iii. Bioimpedance Cardiography (ICG)

a. Thoracic ICGb. Whole-body or Regional ICG

iv. Bioreactancev. Applied Fick’s Principle

a. Partial gas rebreathingb. Pulsed dye densitometry

vi. Endotracheal Cardiac Output Monitor (ECOM)vii. Photoelectric plethysmography

Low-invasivei. Trans Esophageal Echocardiography (TEE) /

Esophageal Doppler

ii. Arterial Pulse contour analysis (PiCCO and FloTrac)

iii. Pulse power analysis: Lithium dilution CO (LiDCO)

Invasivei. Pulmonary Artery Catheter (PAC)

(Dye, Thermodilution)

(intermittent/ continuous)

ii. Fick Principle

Factors affecting selection of cardiac output monitoring devices

Features of an ideal Cardiac Output monitor

• safe and accurate• give measurements that are reproducible• quick and easy to use (both in terms of set-up and

interpretation of information)• operator independent (i.e. the skill of the

operator doesn’t affect the information collected)• provide continuous measurement• reliable during various physiological states

TRUE NON-INVASIVE TECHNIQUES

Clinical Examination• clinical signs of cardiac output pertain to the state

of end-organ tissue perfusion• no single clinical sign can be used to make an

accurate assessment of cardiac output• however, if used together they can be useful in

estimation of cardiac output

Clinical signs include:⌐ Skin colour⌐ Capillary Refill Time⌐ Heart Rate⌐ Skin temperature⌐ Core-peripheral

temperature difference

⌐ Urine Output⌐ Mental State

Clinical Examination

• Assess adequacy

• End organ perfusion

• Brain (confusion, altered consciousness)

• Kidney (UO)

• Tissues (lactate)

• Skin (CRT)

• BP correlates poorly

Lactate• produced by anaerobic metabolism• an indicator of tissue hypoperfusion• measured in the laboratory, and most modern

blood gas machines now give a lactate value as part of arterial blood gas analysis• can be used to monitor therapy, as it will fall as

oxygen delivery improves, and as liver perfusion increases

Trans Thoracic Echocardiography (TTE)• Echocardiography is cardiac ultrasound• can be used to estimate Cardiac Output by

direct visualization of the contracting heart in real time• gaining acceptance as one of the safest and

most widely used cardiac output monitors in the critically ill

• be used to assess cardiac output (intermittently)• The USCOMTM device (USCOM, Sydney, Australia)

targets the pulmonary and aortic valves accessed via the parasternal and suprasternal windows in order to assess cardiac output completely non-invasively

• Transthoracic Approach:• non-invasive• quick• allows for measurement of the ventricles,

visualization of the valves, estimation of ejection fraction• disadvantages of

user dependent possible interference from bone, lung and soft

tissues non-continuous

Impedance Cardiography (ICG)

• is a noninvasive technology for measuring total electrical conductivity of the human body and its changes over time to process continuously a number of hemodynamic parameters such as• Stroke Volume (SV)• Heart Rate (HR)• Cardiac Output (CO) • Body water• Total peripheral Resistance (TPR)• Cardiac Power (CP)

• Two basic technologies are currently in use for impedance cardiography:

I. Thoracic ICG, were the sensors are placed on the root of the neck and the lower part of the chest

II. Whole-body or Regional ICG, were four pairs of sensors are used, one pair on each limb

• in Whole-body Impedance Cardiography, peripheral volumetric signal is borne throughout the length of the arterial tree• in Thoracic ICG waveform is generated by multiple

sources including the aorta, lungs, vena cava, and artifacts due to heart movement• the peripheral systolic impedance changes are

more reliable than the thoracic impedance changes for calculating the cardiac Stroke Volume

• first described by Kubicek and colleagues• is based on changes in electrical impedence

occurring with ejection of blood during cardiac systole• disposable electrodes are applied to the skin

continuous current is applied

• bioimpedance CO is computed for each cardiac cycle and continuously displayed as an average value over several heartbeats• bioimpedance measurement leads to calculation of

SV• electrode placement and appropriate contact with

the skin are important sources of error

• SV= ρL2 x VET x max dZ/ dT Zo

2

Where, SV= stroke volume ρ= specific resistivity of bloodL= thoracic lengthZo= basal thoracic impedenceVET= ventricular ejection timeMax dZ/ dT= max change in impedence time during systole upstroke

Bioreactance• newer and more accurate technology (NICOM

device, Cheetah medical, Portland, Oregon)• based on Bioreactance or the Phase Shifts that

occur when an alternating current is passed through the thorax• when an alternating current (AC) is applied to the

thorax, the thoracic pulsatile blood flow through the large arteries causes the amplitude of the applied thoracic voltage to change

• also causes a time delay or Phase Shift between the applied current and the measured voltage• AC current to the thoracic cavity is applied via

four transmitting sensors• detection of the phase shifts is done with an

additional four receiving sensors

• extensive research has shown that the Phase Shiftsare tightly correlated with stroke volume• by accurately and continuously measuring Phase

Shifts this technology is able to determine the stroke volume

Phase Shift

Applied Fick’s PrinciplePartial Gas Rebreathing•Cardiac Output can be estimated by using the

Fick’s principle with carbon dioxide as the marker gas (Berton & Cholley 2002, Mathews & Sigh 2008)•Partial CO2 rebreathing• called NICOTM system (Novametrix Medical

Systems, Wallingford, Conn, United States)

• is based on the application of the Fick’s principle to carbon dioxide, in order to estimate cardiac output non-invasively• the monitor consists of a carbon dioxide sensor, a

disposable airflow sensor and a pulse oximeter• VCO2-CO2 consumption is calculated from minute

ventilation and its carbon dioxide content• the arterial CO2 content (CaCO2) is estimated from

end-tidal carbon dioxide

• The Fick equation for carbon dioxide is:

CO=VCO2/CvCO2-CaCO2

where VCO2, CvCO2, CaCO2 are CO2 consumption, venous CO2 concentration and arterial CO2 concentration respectively

• Major limitation--tracheal intubation with fixed ventilator setting is required

• not very accurate in patients with • severe chest trauma• significant intrapulmonary shunt• high CO states• low minute ventilation

Pulsed dye densitometry• allows intermittent cardiac output measurement

based on transpulmonary dye dilution with transcutaneous signal detection adapted frompulse oximetry (pulsed dye densitometry)• the concentration of indocyanine green is

estimated in the arterial blood flow by optical absorbance measurements after its venous injection•Cardiac Output is calculated from the dye dilution

curve according to the Stewart Hamilton principle

• The results are altered by vasoconstriction interstitial edema movement ambient light artefacts

Endotracheal Cardiac Output Monitor (ECOM)

• ECOM (Con-Med, Irvine, Calif, United States) measures CO using impedance plethysmography• is based on the principle of bioimpedance• current is passed through electrodes attached to

endotracheal tube shaft and cuff• current is passed from electrode on the shaft of

endotracheal tube and change in impedance secondary to aortic blood flow is detected by electrode on the cuff

• an algorithm calculates SV based on impedance changes and CO can be calculated• impedance is affected by aortic blood flow• Limitations:• electrocautery affects its accuracy• coronary blood flow is not calculated• is still adequately not validated in humans• is costly and has not become very popular

Photoelectric Plethysmography

• is a completely non-invasive pulse pressure analysis device that assesses pulse pressure using photoelectric plethysmography in combination with a volume-clamp technique (inflatable finger cuff)• Nexfin HD (BMEYE B.V, Amsterdam, Netherlands)• Cardiac Output is derived by Modelflow method• very few validation studies to state its efficacy

LOW-INVASIVE TECHINQUES

Trans Esophageal Echocardiography (TEE) / Doppler• a widely used monitor in perioperative setting• is an important tool for the assessment of

cardiac structures, filling status and cardiac contractility• aortic pathology can also be detected by TEE• Doppler technique is used to measure CO by

Simpson’s rule measuring SV multiplied by HR

• Measurement can be done at the level of pulmonary artery, mitral or aortic valve• TEE can quantify Cardiac Output more precisely by

measuring both the velocity and the cross-sectional area of blood flow at appropriate locations in the heart or great vessels

i.e. flow = CSA X Velocity SV= flow X ET ( Systolic Ejection time) CO=SV X HR

• Advantages⌐ Minimally invasive⌐ Minimal interference form bone, lungs and soft

tissue (as seen with transthoracic route)⌐ Quickly inserted and analyzed⌐ Little training required⌐ Very few complications

• Disadvantages⌐May require sedation⌐User dependent⌐Interference from surgical instruments eg. NG

tube, Diathermy impairs signal⌐Depends on accurate probe positioning

⌐ Probe may detect other vessels e.g. intracardiac /intrapulmonary

⌐ Contraindicated in Esophageal Surgeries⌐ Assumes a constant percentage of cardiac

output (approx 70%) enters the descending aorta. May therefore be inaccurate in a hypovolaemic patient where flow may be redirected to the cerebral circulation

Arterial Pulse contour analysis

• is based on the principle that SV can be continuously estimated by analyzing the arterial pressure waveform obtained from an arterial line• characteristics of the arterial pressure waveform are

affected byi. interaction between SV and vascular complianceii. aortic impedanceiii. peripheral arterial resistance

• for reliable Cardiac Output measurement using all devices that employ pulse pressure analysistechnology, optimal arterial waveform signal is aprerequisite• severe arrhythmias may reduce the accuracy of

cardiac output measurement• the use of an intra-aortic balloon pump precludes

adequate performance of the device• limited accuracy during periods of hemodynamic

instability, i.e. rapid changes in vascular resistance

PICCO system• the PiCCO system (PULSION medical system,

Munich, Germany) was the first pulse contour device introduced• was replaced with PiCCO2 in 2007• requires both central venous (femoral or internal

jugular) and arterial cannulation (femoral/radial)• Indicator solution injected via central venous

cannula and blood temperature changes are detected by a thermistor tip catheter placed in the artery

• it combines pulse contour analysis with the transpulmonary thermodilution CO to determine hemodynamic variables• it requires manual calibration every 8 h and hourly

during hemodynamic instability

accuracy may be affected by:• vascular compliance• aortic impedence• peripheral arterial resistance• air bubble, clots• inadequate indicator• valvular regurgitation• aortic aneurysm• significant arrhythmia• rapidly changing temperature

FloTrac system• FloTrac (Edwards LifeSciences, Irvine, United States)

is a pulse contour device introduced in 2005• is a minimally invasive method as it requires only an

arterial line (femoral or radial)• the system does not need any external calibration,

is operator independent and easy to use• it is based on the principle that there is a linear

relationship between the pulse pressure and SV

Pressure recording analytical method (PRAM)• PRAM – MostCare® (Vytech, Padova, Italy), which

is based on mathematical assessment of the pressure signal obtained from an arterial line without calibration• similar to other devices that use pulse contour

analysis, the accuracy of PRAM derived cardiac output is affected by• the quality of the pressure signal• by factors that interfere with the ability to detect a

pressure signal

EV1000 / Volume view• a new calibrated pulse wave analysis method

(VolumeView™/EV1000™, Edwards Lifesciences, Irvine, CA, United States)• is based on pulse pressure analysis, which is

calibrated by transpulmonary thermodilution• is currently under trial• its comparison with PICCO2 system in critically ill

patients found comparable results• very few studies are available for its validation

Pulse power analysis• is based on the principle that change of the blood

pressure about the mean is directly related to the SV• various factors affect its accuracy like • compliance of the arterial tree• wave reflection• damping of the transducer• aortic systolic outflow

Lithium dilution CO (LiDCO)

• LiDCO (Cambridge, United Kingdom) system combines pulse contour analysis with lithium indicator dilution for continuous monitoring of SV and SV variation• is a minimally invasive technique first described in

1993• requires a venous (central or peripheral) line and an

arterial catheter

• a bolus of lithium chloride is injected into venous line and arterial concentration is measured by withdrawing blood across disposable lithium sensitive sensor containing an ionophor selectively permeable to Li• CO is calculated based on Li dose and area

according to the concentration time circulation• requires calibration every 8 h and during major

hemodynamic changes

• is contraindicated in patients on Li therapy• calibration is affected by neuromuscular blockers as

quaternary ammonium residue causes electrode to drift• accuracy is affected by • aortic regurgitation• Intra aortic balloon pump• Damped arterial line• Post aortic surgery• Arrhythmia• intra or extracardiac shunts

INVASIVE TECHINQUES

Pulmonary Artery Catheterization

• pulmonary artery catheter (PAC) as a monitor to measure flow and pressure was developed by Dexter and modified later on by Swan et al to measure CO and central filling pressures• is still considered as gold standard monitor to

measure CO since 1970’s• it has been used as a monitoring tool in high risk

surgeries and critical care units

The Swan Ganz Catheter• Most popular design with 5 lumens, 7.5 F catheter,

110 cm long Distal lumen: at tip of catheter, lies in a branch

of the pulmonary artery, connected to a pressure transducer. Yellow port: distal PA port

Balloon lumen: permits introduction of 1.5 ml of air into the balloon at the distal tip. Red port: balloon port

Thermistor lumen: bead situated 4 cm from the tip of the catheter and measures temperature

Proximal lumen: 25 cm from tip, lies in right atrium, measures central venous pressure (CVP) Blue port: proximal to the PA port ; CVP port

Clear port/ white port: 30 cm from the tip and used to inject drug

Use of the Catheter• sampling of mixed venous blood

• measurement of cardiac output using thermodilution

• assessment of CVP

• derivation of other cardiovascular indices, such as the pulmonary vascular resistance, oxygen delivery and uptake

• arrhythmias on insertion • pneumothorax• pulmonary artery rupture• balloon rupture • valve injury• pulmonary infarction • infection• thrombosis leading to embolism• knotting of catheter in right ventricle

Complications

• various technical errors may lead to false readings like ⌐ loss of injectate⌐ variability of temperature⌐ thermistor malfunction⌐ clot over catheter tip⌐ coiling of catheter or timing of injectate > 4

seconds • intracardiac shunts, mechanical ventilation or valvular

dysfunction may lead to incorrect readings

Thermodilution• A cold solution of D/W 5% or normal saline

(temperature 0°C) is injected into the right atrium from a proximal catheter port• this solution causes a decrease in blood

temperature in right heart and flows to the pulmonary artery where the temperature is measured by a thermistor placed in the pulmonary artery catheter

• the thermistor records the change in blood temperature with time and sends this information to an electronic instrument that records and displays a temperature-time curve / thermodilution curve

• The Cardiac Output can be derived from the Modified Stewart-Hamilton conservation of heat equation CO= V1( Tb- T1) K1 K2

ξ ΔTb (t) dtWhere,V1= injectate volume in mlTb= temperature of pulmonary artery bloodT1= injectate temperature °CK1= density factorK2= computation constant taking in account the catheter

deadspace and heat exchange in transit ; both computation constant

Denominator: change in temp and change in time: corresponds to the area under thermodilution curve

• The degree of change is inversely proportional to cardiac output• Temperature change is minimal if there is a high

blood flow but high if flow is low

Accurate measurements of cardiac output depend on:

• rapid and smooth injection• precisely known injectant temperature and volume• correct entry of the calibration factors for the

specific type of PAC into the cardiac output computer• avoidance of measurements during electrocautery

• Continuous thermodilution cardiac output using an electric heating coil to warm the pulmonary artery blood removes errors associated with fluid injectate techniques• But there is the need to average the smaller signal

over a longer time interval

Dye dilution/ Indicator dilution

• Based on the observation that, for a known amount of indicator introduced at one point in the circulation, the same amount of indicator should be detectable at a downstream point• Indocyanine green injected through a Central

venous catheter and its appearance in arterial system detected by a densitometer. • the area under dye indicator curve is related to

Cardiac Output

• CO is calculated using the Stewart-Hamilton equation:

CO = cardiac outputQ = amount of indicator injected∫C dt = the integral of indicator concentration over timeBlood flow is directly proportional to the amount of the indicator delivered

Fick Principle• Adolfo Fick in 1870: the blood flow to an organ per

unit time is calculated by the ratio of consumption of marker by that organ to the difference between arterial and venous content of that marker• Fick used oxygen as the marker

• CvO2 and CaO2 measured by PAC and arterial line in place• Oxygen consumption from the difference between

the oxygen content in inspired and expired gas

Integrative concept for the use of cardiac output monitoring devices

Advantages and disadvantages of methods of cardiac output monitoring

THANK YOU!!!