Cardiac output monitoring

1. CARDIAC OUTPUT MONITORING
SPEAKER: Dr.SK.AMEENA

2. INDEX
• HISTORY.
• WHAT IS CARDIAC OUTPUT..?
• FACTORS AFFECTING CO.
• INDICATIONS OF CO MONITORING DIAGNOSTIC
• THERAPEAUTIC
• PROPERTIES OF AN IDEAL CO MONITOR.
• BENEFITS OF CONTINOUS CO MONITORING.
• METHODS OF CO MONITORING CLINICAL ASSESSMENT
INVASIVE METHODS
MINIMALLY INVASIVE
NON INVASIVE

3.  Methods of CO monitoring are broadly classified as follows :
(1) CLINICAL ASSESMENT OF CO: By Blood pressure measurments.
By Skin perfusion.
(2) INVASIVE :
o Intermittent bolus pulmonary artery thermodilution.
o Continuous pulmonary artery thermodilution.
(3) MINIMALLY INVASIVE :
o Lithium dilution CO (LiDCO),
o Pulse contour analysis CO (PiCCO )
(4) NON INVASIVE :
o Oesophageal Doppler
o Transesophageal Echo[TEE] monitor.
o Partial gas rebreathing system.
o Thoracic bioimpedance and bioreactance,
o Photoelectric plethysmography.

4.  HISTORY:
• Although, Harvey discovered the circulation 300 years ago it
was only in 1970’s that the cardiac output was monitored.
• It was Dr.Swan Ganz and colleagues who discovered that by
putting pulmonary artery at the bedside cardiac output can be
monitored.
• The ideal monitoring method should be non-invasive,
accurate, reliable, continuous, operator independent and
thus reproducible, compatible, cost effective and conclusive
in both adults and paediatric patients. But, at present no
such single technique meets this criteria.
• Thermodilution through a pulmonary artery catheter (PAC)
has been used extensively in adult cardiac surgery to
measure CO and it has become a standard clinical monitor
to which other methods of measuring CO may be
compared.

5. WHAT IS CARDIAC OUTPUT…??
• Cardiac output (Q) is the volume of blood being pumped by
the heart in the time interval of one minute.
• CO may be measured in many ways, for example dm3/min
(1dm3 equals 1000cm3 or 1litre).
• CO is furthermore the combined sum of output from the right
ventricle and the output from the left ventricle during the
systolic phase of the heart.
• On an average the resting cardiac output would be 5.6 L/min
for a human male and 4.9 L/min for a female.
CO(Q) = Stroke Volume × Heart rate.

6. • When CO increases in a healthy individual, most of the
increase can be attributed to an increase in heart rate
(HR).
• Change of posture, increased sympathetic nervous system
activity and decreased parasympathetic nervous system
activity can also increase cardiac output.
• HR can vary between 60 and 180 beats per minute, while
stroke volume (SV) can vary between 70 and 120 ml.
• So, the summation is that cardiac output is the amount of
blood pumped by the heart to the peripheral circulation each
minute and it is the measurement that reflects the status
of the entire circulatory system and not just the heart,
because it is governed by the autoregulation from the
tissues.

7. • However, different sized individuals have different cardiac
outputs, so the preferred measure is cardiac index (CI),
calculated by dividing cardiac output by body surface area
(BSA); that is CI=CO/BSA.
• If patient’s height and weight are known, their body surface
area (in m2) can be obtained using the Dubois surface chart.
• The normal cardiac index is 2.8-3.6 L/minute/m2.
• CO can thus determined by alteration in heart rate or
rhythm, preload, contractility and afterload.
• Moreover it gives important information about tissue
perfusion and oxygen delivery.

8.  FACTORS AFFECTING CARDIAC OUTPUT :
The blood volume available for ejection – the venous return
or preload.
The resistance to ejection – the afterload.
The strength of ventricular myocardial contractility.
The heart rate and the rhythm.

9.  INDICATIONS FOR CARDIAC OUTPUT MONITORING:
Diagnostic:
i). Assessment of myocardial function following a cardiac
event likely to produce a low output state (e.g) myocardial
infarction.
ii). Assessment of cardiac function where there may be a
high output state e.g. in septic shock.
iii).Measurement of pulmonary and systemic vascular
resistances; oxygen delivery and consumption.

10.  Therapeutic:
i) Monitoring the effects of medical interventions
on cardiac output,
Ex: colloid or inotropic therapy and the effect of drugs on
vascular resistances.
Ex: To reduce systemic vascular resistance in septic
shock.
(ii) Measurement of the efficacy of oxygen delivery
manipulations.

11.  PROPERTIES OF AN IDEAL CO MONITOR:
• An ideal CO monitor should be :
Accurate
minimally or non-invasive,
Continuous.
cost effective.
reproducible.
reliable during various physiological states and
Have fast response time.
Easy data interpretation
Bedside information management
 Used in neonates to adults.
• But, at present no such single technique meets this
criteria.

12. BENEFITS OF CONTINUOUS CARDIAC OUTPUT
MONITORING
True monitor = early warning of deterioration
Weight of scientific evidence for improved outcome.
Optimum fluid management.
 Rational drug administration (e.g. inotropes).
 Optimizing patient – ventilator interaction.
Reduced work of health care staff.

13. Methods of CO monitoring are broadly classified as
follows :
(1) CLINICAL ASSESMENT OF CO: By Blood pressure measurments.
Skin perfusion.
(2) INVASIVE :
o Intermittent bolus pulmonary artery thermodilution.
o Continuous pulmonary artery thermodilution.
(3) MINIMALLY INVASIVE :
o Lithium dilution CO (LiDCO),
o Pulse contour analysis CO (PiCCO and FloTrac),
(4) NON INVASIVE :
o Esophageal Doppler
o Transesophageal echocardiography(TEE)
o Partial gas rebreathing,
o Thoracic bioimpedance and bioreactance,
o Photoelectric plethysmography.

14.  CLINCAL ASSESMENT OF CARDIACOTPUT:
• Cardiac output is a measure of flow and not pressure.
• Blood pressure measurements have been shown to
correlate poorly with changes in cardiac output.
• However, narrowing of the pulse pressure may be
associated with decreased stroke volume.
• Hypotension resulting from a low cardiac output is often an
ominous sign.
• Skin perfusion is a clinically useful sign.

15. • Following cutaneous pressure on a digit for 5 sec,
reperfusion of the capillary bed should occur within 2
sec.
• Progressive prolongation of the capillary refill time is
seen with reducing cardiac output; the skin becomes
progressively cold, pale and mottled.
• Although capillary refill is a reproducible sign,
interpretation may be altered by ambient or patient
temperature.
• Conversely, high cardiac output states may manifest
as warm peripheries and bounding peripheral pulses.
• The difficulty and inaccuracy of quantifying cardiac
output using clinical parameters has been a
significant driving force in the development of so
many different monitoring techniques.

16. • There are various methods of CO monitoring based on Ficks
principle, thermodilution, Doppler, pulse contour analysis and
bioimpedance.
• Each method has its own merits and demerits .

17.  Pulmonary Artery Catheter History:
• The first introduction of a catheter into a human pulmonary
artery was in 1929 by Forsmann.
• He inserted a urinary catheter into his own cubital vein
and into his right heart.
• In 1954 a catheter was developed by Lategola and Rann and
used in dogs.
• In 1970 that Dr.Swan was on an outing with his family and
noticed how easy it was for a sailboat to move even in the
slightest breeze.
• Up until this point no one had been able to float the catheter
into the pulmonary artery.
• Dr. Swan then invented the balloon tipped catheter.
Around the same time Dr.Ganz was working on thermodilution
methods to calculate cardiac output.
• So the pulmonary artery catheter was named Swan Ganz.

18.  INVASIVE –PAC THERMODILUTION :
• Continous CO is a modification of PAC with copper filament in
the catheter that remains in the right ventricle.
• There is intermittent heating of blood in the right heart by the
filament and the resultant signal is captured by thermistor near
the tip of the catheter.
• Average value of CO measured over time is displayed on the
monitor.
• Main advantages of CCO over conventional PAC are
avoidance of repeated boluses thus reducing the infection
risk and operator errors.

19. • For the past 30 years, the pulmonary artery catheter (PAC)
has been the main-stay [GOLD STANDARD] of
haemodynamic monitoring for the critically ill and
cardiovascular unstable.
• It was initially used to measure intracardiac pressures.
• The application of the Fick principle provides an accurate
and reproducible measure of cardiac output.
• The Fick principle – based upon the conservation of mass,
states that the amount of a substance taken up by an
organ per unit time is equal to the arterial minus venous
concentration of the substance multiplied by blood flow.

21. • Development of dye dilution and thermodilution techniques
increased the clinical usefulness of the PAC.
 DYE DILUTION TECHNIQUE : [HAMILTON]
• A known quantity of dye (normally indocyanine green or Evan
blue) is injected into the pulmonary artery and timed arterial
samples are analysed using a photo-electric spectrometer.
• cardiac output caliculation :

22.  THERMODILUTION TECHNIQUE :
• This is an intermittent technique widely accepted in clinical
settings - a method based on a principle similar to indicator
dilution, but it uses heat rather than colour as an
indicator.
• This method uses a special thermistor – tipped catheter (Swan-
Ganz catheter) inserted from a central vein into the
pulmonary artery.
• A cold solution of D/W or normal saline (temperature 0 oC) 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 pulmonary artery where the temp
is measured by a thermistor placed in the pulmonary
artery catheter.
.

23. • 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.

24.  The cardiac output can be derived from the modified
Stewart-Hamilton conservation of heat equation.

25. • The degree of change is inversively proportional to cardiac
output.
• Temperature change is minimal if there is a high blood flow but
temperature change high if blood flow is low.
• Thermodilution technique remain the most common approach in
use today and is considered as the golden standard approach
to cardiac output monitoring , although it involves many risks, such
as pneumothorax, dysrythmias, perforation of the heart
chamber, tamponade and valve damage).
• Factors that may effect this technique are:
 shunts,
 tricuspid regurgitation,
 cardiac arrythmias,
 abnormal respiratory patterns and
 low cardiac output

26.  MEASURMENTS THAT CAN BE DONE FROM PA
CATHETER:
• CVP.
• Rt ventricular pressure.
• PCWP.
• Cardiac output.
• Cardiac index.
• Stroke volume
• Stroke volume index= stroke volume/BSA[ml/m2]
• SVR
• PVR

27.  COMPLICATIONS DUE TO PAC:
Dysarrythmias.
PA/RA/RV rupture.
Kink or coiling of catheter.
Infection.
Balloon rupture.
Thrombus.
Air Embolus.
Pneumothorax.
Phrenic nerve block.
Horner’s syndrome.

29. MINIMALLY INVASIVE :
1. Lithium dilution CO (LiDCO) monitoring.
2. Pulse contour analysis CO (PiCCO ).

30. LITHIUM DILUTION CARDIAC OUTPUT
:[LiDCO]
• A bolus of isotonic lithium chloride (LiCl) solution is
injected via the venous line.
• The usual dose for an adult is 0.3 mmol . Arterial
concentration is measured by withdrawing blood across
a selective lithium electrode at a rate of 4 mL/min.
• Cardiac output is calculated based on the lithium dose
and the area according to the concentration–time
circulation .

32.  The LiDCO haemodynamic monitor calculates a number
of derived parameters :
• Body surface area
• Systolic pressure variation
• Pulse pressure variation
• Cardiac index
• Stoke volume variation
• Stroke volume index
• Systemic vascular resistance
• Systemic vascular resistance index

33. ADVANTAGES OF LiDCO :
Provides an obsolute cardiac output value.
Requires no additional invasive catheters to insert into
the patient.
It is safe.
Is simple and qucik to set up.
Is not temperature dependent.

34.  INDICATIONS FOR ITS USE…..
Acute heart failure
 sepsis
Drug intoxication
Acute renal failure
Severe hypovolemia
Management of high risk patients
Pts with a history of cardiac disease
Fluid shifts
Medical emergencies.

35. • This technique is contraindicated in patients on Li therapy
and with high doses of NMB’S b/c these drugs can
cross react with lithium sensor or electrode.
• Its accuracy is affected by :
Aortic regurgitation.
 Intra aortic balloon pump (IABP)
Damped arterial line
 post aortic surgery
Arrhythmia and
 Intra or extracardiac shunts.

36. PULSE CONTOUR ANALYSIS-PiCCO
• INTRODUCTION: The PiCCO system (PULSION medical
system, Munich, Germany) was the first pulse contour device
introduced and was replaced with PiCCO2 in 2007.
• PiCCO – Pulse index Contour Cardiac Output. It enables
assessment of the patient’s haemodynamic status to guide fluid or
vasoactive drug therapy.
• PiCCO uses a combination of two techniques for advanced
haemodynamic and volumetric monitoring.
 Transpulmonary thermodilution. [STEWART HAMILTON METHOD]
 Pulse contour analysis.
• PiCCO is a relatively invasive method as it requires both
arterial and venous cannulation.

37. • The PiCCO system continously estimates the stroke
volume from the arterial waveform, using an arterial
catheter.
• Cardiac output is then estimated from the stroke
volume and heart rate.
• Provides continuous beat by beat parameters which
are obtained from the shape of the arterial
pressure wave form.
• The area under the arterial curve during systole,
minus diastolic area is assumed to be proportional
to the stroke volume. This means that the stroke
volume and thus the cardiac output can be
measured beat to beat.

38. • Continuous CO readings are achieved using the area under the
systolic part of the curve, a calibration factor (cal) derived from the
thermodilution , the heart rate (HR) and the individual’s aortic
compliance [which is termed C (p)].
• PiCCO requires the insertion of a central venous catheter and a
thermodilution arterial line. The arterial line can be placed in the axillary,
brachial, femoral or radial artery,
• PiCCO not only gives information about cardiac output but can give
measurements to assess preload, contractility, afterload and extravascular
lung water

39. PROCEDURE :
• PiCCO requires the insertion of a central venous
catheter and an arterial line. The arterial line can be
placed in the axillary, brachial, femoral or radial artery.
Femoral artery is the preferred site.
• Indicator solution injected via central venous cannula and
blood temperature changes are detected by a thermistor tip
catheter placed in the artery.
• Thus, 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.

41. • In addition, thermodilution curve can be used to measure
intrathoracic blood volume (ITBV), global end diastolic
volume (GEDV) and extravascular lung water (EVLW).
• GEDV and ITBV are a measure of cardiac preload and
EVLW (interstitial, intracellular or intra alveolar) is a mean
to quantify pulmonary edema.
• It also measures SVV/PPV which is marker of fluid
responsiveness.

42. PiCCO ARTERIAL CATHETER

43. PARAMETERS MEASURED FROM PiCCO DEVICE
 Thermodilution Parameters:
• CO – Cardiac Output
• CI – Cardiac Index
• Preload
• GEDI – Global end diastolic index
• ITBVI – Intra thoracic blood volume index
• Pulmonary oedema
• ELWI –Extravascular lung water index
• PVPI - Pulmonary vascular permeability index
• Contractility
• CFI - Cardiac function index
• GEF - Global ejection fraction

44.  Pulse contour Parameters :
• Flow
• PCC -Pulse contour cardiac output
• ABP - Arterial blood pressure
• HR - Heart rate
• SV - Stroke volume
• Volume responsiveness
• SVV - Stroke volume variation: <10%
• PPV - Pulse pressure variation
• Afterload
• SVRI - Systemic vascular resistance index
• Contractility
• Index of left ventricular contractility

45. INDICATIONS FOR USE OF PiCCO :
• Shock: cardiogenic, hypovolaemic, septic
• Sepsis
• Trauma
• Pulmonary oedema
• Acute lung injury
• Burns
• Any condition that requires assessment of haemodynamic
function .

46. CONTRAINDICATIONS FOR USE OF PiCCO :
• Atrial/ventricular arrythmias
• Intra-aortic balloon pump
• Aortic aneurysm
• Extra corporeal circuit
• Pneumonectomy
• Massive pulmonary embolism
• Intra cardiac shunt

47. Its accuracy may be affected by :
 vascular compliance.
aortic impedence and
peripheral arterial resistance.
 presence of air bubble, clots.
 Inadequate indicator may also affect the accuracy.
Valvular regurgitation, aortic aneurysm, significant
arrhythmia.
 Rapidly changing temperature may also affect its
accuracy.

48. NON INVASIVE :
o Oesophageal doppler
oTransesophageal Echo monitoring[TEE]
oPartial gas rebreathing,
oThoracic bio impedance and bio reactance.
o Endotracheal cardiac output monitor
(ECOM),
oPhotoelectric plethysmography.

49.  OESOPHAGEAL DOPPLER:
• This technique was first described in 1971.
• A flexible probe with a Doppler transducer at the tip of
the probe is inserted into the oesophagus and the
transducer is positioned facing the descending aorta.
• Cardiac output can be estimated using Doppler ultrasound
to determine the flow of blood through the aorta.
• The volume of blood passing through the aortic valve over
a given cardiac cycle is the stroke volume.
• Multiplying the stroke volume by Heart rate gives CO.

50. • The ODM displays a waveform as shown in the figure, the
area under the waveform, generated by the descending
aortic blood flow, is defined as stroke distance.
• The stroke volume is calculated from the measured
stroke distance.
• Hemodynamic variables including stroke volume, cardiac
output and cardiac index to be calculated.

51. • Velocity is shown on the Y-axis and time along the x-axis.
• The waveform (triangle) displays: Peak velocity, stroke distance,
mean acceleration, and flow time measurements.
• Stroke distance is the area of the triangular waveform and is related
to stroke volume.
• Peak velocity and mean acceleration are markers of contractility.

52. • The velocity–time integral (VTI) is calculated from the area
under the velocity–time curve and used as the stroke
distance.
• An estimate of aortic cross-sectional area (CSA) is taken
either from a nomogram (height, weight, and age) or utilizing
M-mode ultrasound.
• Cardiac output is then calculated using the equation:
CO = CSA VTI HR.

54.  ADVANTAGES OF ODM :
• The ODM probe can be placed in the oesophagus within
a few minutes.
• It is easy to use; however training is recommended.
• The risks and complications associated with ODM probe
placement seem to be low, because of the small size of the
probe (diameter of ―5 mm).

55.  LIMITATIONS OF ODM SYSTEM:
• Need for frequent probe repositioning.
• Decreased accuracy during aortic manipulation, and the
calibration procedures.
• Oesophageal Doppler measurement assumes a fixed relationship of
blood flow in descending aorta only.This relation ship is based on
healthy individuals. However, this may be altered in ICU patients.
• May not be reliable in situations such as severe aortic stenosis,
because of turbulent blood flow.
• Although the ODM is capable of continuous CO monitoring the
probe needs to be checked regularly because of possible probe
movement.
• Movement of the probe results in loss of signal, which is
reflected in an unsharp triangular waveform and a decrease in volume of
the distinctive Doppler sound.

56.  Oesophageal Doppler is contraindicated
in patients with oesophageal disease
and severe bleeding disorders.

57.  TRANSOESOPHAGEAL ECHOCARDIOGRAPHIC
MONITORING(TEE):
• TEE has now been widely used monitor in perioperative
setting.
• It is an important tool for the assessment of cardiac
structures, filling status and cardiac contractility.
• Moreover, aortic pathology can also be detected by TEE.
• Echocardiography may be used for the measurement of CO
by measuring flow through the heart valves.
• Measurements may be performed at the level of the
pulmonary artery, the mitral valve or the aorta valve.
• Using TEE, cardiac output measurement is the result of
calculating stroke volume, which can be multiplied by heart
rate.

58. • In order to assess stroke volume, it is necessary to measure
flow velocity and determine the cross-sectional area
• While the calculations are time-consuming at present, the
degree of accuracy has been promising.
• It is a useful tool in hemodynamically unstable patient under
mechanical ventilation.
• However, a skilled operator is required, limited availability
and cost factor are major limitations for Its use.
• Standard TEE probe cannot be kept in the patient for too
long.
• Hemodynamic TEE is a disposable thinner TEE probe
which can be left in situ for several days.

59.  THORACIC ELECTRICAL BIOIMPEDENCE:
• In the 1960s, the National Aeronautical and Space
Administration (NASA) and William Kubicek developed
impedance cardiography, using the thoracic electrical
Bioimpedance.
• This technique can be used in conscious as well as
unconscious patients.
• This technique employs four pairs of electrodes. Two pairs
are applied to the neck base on opposite sides and two
pairs are placed at the level of the xiphoid junction.
• With these electrodes, low-level electricity conducted by body
fluid is transmitted.

60. • Another set of two electrodes is used to monitor a single ECG signal.
• This electricity is harmless and not felt by the patient.
• The first derivative dZ/dt of the impedance waveform is related
linearly to aortic blood flow.
• Changes in impedance correlate with stroke volume, calculated using the
following formula
SV = ρ * L2/Z02 * (dZ/dt)max × T
• where SV=stroke volume
• ρ=resistivity of blood (Ω/cm)
• L= mean distance between the inner electrodes (the thoracic length)
• Zo=basal thoracic impedance
• (dZ/dt)max=the maximum value of the first derivative during systole
(Ohms/second)
• T=ventricular ejection time (sec).
• Cardiac output is calculated from the stroke volume
and heart rate and the equation is : CO = SVxHR.

61. • Bioimpedance and its connection to the body surface.
• An alternating current is passed through the chest, where the
change in impedance is related to stroke volume.

62. • Cardiac output is calculated from the stroke volume and
heart rate and the equation is : CO = SVxHR.
• Cardiac output is easier to measure by impedance
cardiography than by thermodilution with a pulmonary artery
catheter, can be applied quickly, and does not pose a risk of
infection, blood loss or other complications associated with
arterial catheters.
• This method can be used to calculate various other
parameters, such as cardiac index, stroke volume, end-
diastolic index and other hemodynamic parameters
including systemic vascular resistance.

63. Thoracic bioimpedance is used:
• in post-cardiac surgery Patients,
• In acute emergency trauma patients and
• In pulmonary hypertension patients ,
• In critical illness and
• post-coronary artery bypass.

64.  Limitations :
 Operator error can occur with the incorrect placement of the
sensors. and insufficient knowledge of the impedance waveform.
 To avoid cannulation sites or wounds, commonly seen in patients
in the ICU, the position of the electrodes sometimes needs
adjustment.
Advancing age, peri-operative fluid shifts, pulmonary oedema,
myocardial ischaemia and electrical interference may cause
errors.
 Moreover, because the left ventricular ejection time is determined
using the interval between QRS complexes on the ECG,
arrhythmias can interfere with CO measurement.

65.  BIO REACTANCE:
• Bioreactance - CO measurement is based on analysis of relative
phase shifts of a high-frequency current, applied across the
thorax.
• These relative phase shifts are related to changes in intrathoracic
blood volume, so that the peak rate of change in the relative
phase shift is related to the peak aortic flow during a heartbeat.
• The NICOM system currently the only commercially available
system based on bio reactance, detects this relative phase shift by
detecting a high frequency current that is applied and recorded
from the left and right side of the thorax, by four dual electrodes
placed at the corners of the thorax.

66. • Because bioreactance is not based on changes
in amplitude- like bioimpedance, it is less limited
by factors such as body size, pleural and
pulmonary fluid and patient movement.
• Just like bioimpedance, bioreactance uses ECG
signals to determine left ventricular ejection time.

67. • The NICOM system and its connection to the body
surface.

68. Advantages :
• The NICOM monitor is easy to use and interpret and can be
used in conscious as well as unconscious patients.
• Bioreactance technology 100 times superior to bioimpedance
and therefore makes it, more eligible CO monitor in the ICU.
Limitations :
• Theoretically, as is the case with bioimpedance, bioreactance
will overestimate the CO in patients with aortic insufficiency.
• Moreover, measurements can be altered by nurses
interventions, movements of the patient or loss of patch
adhesive which occur over time.

69.  PARTIAL GAS REBREATHING SYSTEM:
• Cardiac output can be estimated by using the Fick principle
with carbon dioxide as the marker gas.
• A new monitor called NICO is based on the application
of the Fick 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 oxymeter.
• VCO2 is calculated from minute ventilation and its carbon
dioxide content.
• The arterial CO2 content (CaCO2) is estimated from end-tidal
carbon dioxide.

70. • 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.
• We assume that cardiac output remains unchanged under
normal (N) and rebreathing conditions (R).
• Because the diffusion rate of carbon dioxide is 22 times more
rapid than that oxygen, it is assumed that no difference in venous
CO2 (CvCO2) will occur, whether under normal or rebreathing
conditions.
• Then the equation becomes :
CO=ΔVCO2/ΔCaCO2.

71.  HOW IT WORKS….??
• A rebreathing apparatus is attached to the patient’s tracheal
tube and serial measurements are taken every 3 min.
• The computer cycles every three minutes from the non-
rebreathing mode to a 50-second period of rebreathing by
adding an additional dead space.
• The NICO monitor measures the carbon dioxide production (VCO2)
with an integrated CO2/flow sensor, by calculating the difference
between expired and inspired CO2 concentration.
• At steady state, the amount of CO2 entering the lungs via the
pulmonary artery is proportional to the cardiac output and equals the
amount exiting the lungs via expiration and pulmonary veins.
• During 50 s of rebreathing, the amount entering does not change,
but the amount eliminated by expiration decreases and the ETco2
increases in proportion to the cardiac output.

72.  Advantages :
• The NICO system is easy to use without any previous
experience.
 Limitations :
• This technique is restricted to the mechanically ventilated
patient.
• The partial CO2 rebreathing technique involves 50 seconds of
dead space ventilation in the rebreathing period, which causes a
transient rise in arterial CO2 tension (PaCO2), up to 4 mmHg.
• This may make the partial CO2 rebreathing technique unsuitable
for patients with severe hypercapnia, raised intracranial pressure
or pulmonary hypertension.
• Because of the non-rebreathing and rebreathing cycle, the partial
rebreathing technique has a relatively long response time,
making the NICO system a less suitable monitor for continuous
cardiac output monitoring as compared with the other non-invasive
CO monitors