• Cardiac output (CO) is the volume of blood pumped by the heart per unit time is measured in liters/minute. In the absence of any intracardiac shunt, both right left ventricular output are roughly equal.
• Continuous CO monitoring provides dynamic information that can facilitate rapid adjustments in therapy.

• Fick's method. Originally described by Adolph Fick in 1870.
  • Technique: CO is measured by dividing the body's oxygen consumption (VO₂) by the difference in arterial mixed venous blood oxygen content (CaO₂–CvO₂). CO = VO₂/(CaO₂–CvO₂). O₂ content of arterial or venous blood is determined by adding the O₂ bound to hemoglobin (Hb) with that dissolved in blood: [Hb × 1.36 × O₂ saturation] + [PaO₂ × 0.003].
  • Equipment: VO₂ can be measured via spirometry or a closed rebreathing circuit. The PaO₂ is attained from an arterial blood gas (ideally pulmonary vein) the PvO₂ from the pulmonary artery.
  • Pros
    • The original reference stard by which all others are evaluated
    • Good accuracy in low CO states because the arterial to venous O2 content difference is reliably augmented
  • Cons
    • Technically difficult to attain VO₂ values. To that extent, it can be estimated from an individual's height weight (Fick's determination) using a value of 125 mL O₂/min/m².
    • Fick's method is not reliable in patients with varying O₂ dems, impaired lung gas exchange, unstable hemodynamics because the O₂ content consumption are rapidly changing.
• Pulmonary artery catheter thermodilution method (PAC TD)
  • Technique: Based on the modified Steward–Hamilton indicator diffusion equation. A known amount of cold solution at a known temperature is injected at one port measured at a downstream port; the change in temperature is calculated extrapolated into a CO measurement. In low CO states, lower blood flow results in greater temperature change (more time to equilibrate with the body's warmer temperature), whereas in high CO states, the blood moves more quickly there is less time for a change in temperature.
  • Equipment: Heat is used as an indicator instead of lithium or other dyes (e.g., indocyanine green). A known amount of cold solution is injected in the proximal port that lies in the right atrium, the change in blood temperature is detected by a thermistor at the catheter tip located in the pulmonary artery.
  • Pros
    • Considered the clinical gold stard due to extensive experience with this technique
    • Allows quick successive measurements
    • No blood sampling is required
  • Cons
    • Several measurements are needed to calculate an average since the pulmonary blood flow changes during the respiratory cycle
    • Many factors including temperature volume of the injectate, concomitant rapid intravenous fluid administration, presence of pulmonic or tricuspid regurgitation can affect the accuracy
    • A right-to-left intracardiac shunt will enable some of the indicator to bypass the thermistor overestimate the cardiac output
    • A left-to-right intracardiac shunt will result in recirculation of dye resulting in underestimation of the cardiac output
    • There is an inherent 10–20% error in measurement
• Pulmonary artery catheter continuous cardiac output (PAC CCO)
  • Technique: Continuous detection of temperature changes from the proximal to distal port. Utilizes dissipation of heat instead of cold injectate used in the bolus thermodilution method. The CO value is averaged over 4–6 minutes, although a STAT mode is available to display it every sixty seconds.
  • Equipment: An embedded heat filament that rests between the right atrium ventricle emits small thermal pulses every 30–60 seconds.
  • Pros
    • Good correlation with bolus thermodilution method
    • Provides hs-free continuous data
  • Cons: Prone to error with sudden changes in blood flow or concomitant rapid infusions of cold or warm fluids.
• Partial CO₂ rebreathing (NiCO₂)
  • Technique: Utilizes the differential Fick's principle; carbon dioxide elimination is measured in lieu of measuring VO₂. CO is based on breath-by-breath measurements of CO₂ elimination.
  • Equipment: The NiCO₂ system is noninvasive has a built in rebreathing valve loop. Infrared sensors flow meters measure calculate CO₂ elimination after a brief period of partial rebreathing.
  • Pros: CO₂ elimination is easier to measure accurately than O₂ due to its high diffusion rate.
  • Cons
    • Requires mechanical ventilation
    • Not accurate in the presence of intrapulmonary shunts (however this is present in most critically ill patients)
• Transpulmonary thermodilution (TPCO)
  • Technique: Measures the temperature change in a central artery after injection of cold saline into a central vein.
  • Equipment: A known amount of cold saline is injected in a central line. After passing through the right heart, pulmonary circulation left heart, the final temperature change is measured by a thermistor-tipped catheter placed in a central artery (axillary or femoral).
  • Pros
    • Less invasive than PAC
    • Validity is comparable to more invasive techniques
    • Simultaneous injection of indocyanine green (which stays in the intravascular space) cold saline (which equilibrates with the extravascular space) can provide measurements of the total thoracic intravascular volume extravascular fluid volume
  • Cons
    • Requires invasive intravascular lines
    • Concomitant rapid intravenous fluid administration can affect accuracy
• Pulse dye densitometry
  • Technique: Dye is injected intravenously, the arterial concentration is measured. Calculation of CO is based on a concentration versus time curve.
  • Equipment: Indocyanine green dye distributes exclusively in the intravascular space. Noninvasive arterial concentration is measured with a fingertip sensor. The same principle of pulse oximetry is utilized (the ratio of the pulse absorbance signals is obtained at two wavelengths).
  • Pros: Noninvasive (indirect sampling)
  • Cons: The accuracy of this technique has not yet been validated.
• Doppler ultrasound (esophageal Doppler probe)
  • Technique: Aortic blood flow is calculated by multiplying blood flow velocity over time with the cross-sectional area of the aorta.
  • Equipment: An esophageal probe is placed at the level of the descending aorta, approximately 35 cm from the incisors. The probe position is optimized by slow penetration rotation to obtain a clear Doppler signal blood flow velocity in the aorta. Aortic diameter is obtained either from a built in nomogram linked to patient demographics or using an M-Mode echo transducer built into the probe.
  • Pros: The shape of the velocity versus time wave can also be used to estimate preload, afterload, contractility.
  • Cons
    • Poorly tolerated in awake, unintubated patients
    • Accuracy is dependent on nonturbulent blood flow, angle of the Doppler beam aorta, the relative distribution of blood flow to the upper part of the body
• Transesophageal echocardiography (TEE)
  • Technique: Based on measuring the area blood flow velocity at a particular location in the heart. The left ventricular outflow tract (LVOT) is the most commonly used site due to its nearly circular shape. LVOT area is calculated using the mid-esophageal long axis or transgastric long axis view. Velocity time integral (VTI) is obtained by placing pulse wave Doppler sample volume at the same location in the LVOT: Stroke volume = LVOT area × VTI.
  • Equipment: Echocardiography
  • Pros
    • Avoids blood distribution assumptions made in the esophageal Doppler method
    • 3D echocardiograms can calculate the CO by direct stroke volume (SV) measurements
  • Cons
    • Transesophageal echocardiography is invasive; alternatively, the transthoracic approach can be utilized, but the image quality may be suboptimal
    • Technically more deming requires special expertise
• Pulse contour analysis-based methods
  • LiDCO system
    • Technique: Based on pulse power analysis, not pulse morphology. The arterial pressure wave is transferred to a volume wave the net power is calculated (proportional to the net flow/SV).
    • Equipment: External calibration is performed using the lithium dilution method. Isotonic lithium chloride is injected into a peripheral vein its concentration is measured with an electrode attached to a peripheral arterial line.
    • Pros: Less invasive. Measures CO independent of sampling site.
    • Cons: The external calibration needs to be repeated after significant changes in hemodynamics or arterial compliance. However, the newer LiDCO plus system is capable of self-calibration.
  • PiCCO system
    • Technique: Based on BP waveform morphology calculates CO by using an algorithm described by Wesseling colleagues.
    • Equipment: Peripheral arterial catheterization provides the pressure waveform. It requires external calibration by the transpulmonary thermodilution method as described above.
    • Pros: Less invasive. Accuracy validated in a number of studies.
    • Cons: Frequent recalibration is required in the presence of intravascular volume changes or hemodynamic instability.
  • FloTrac/Vigileo system
    • Technique: Works on the principle that the pulse pressure is directly proportional to the SV inversely proportional to aortic compliance. Uses arterial pulse analysis in conjunction with patient's height, weight, age, gender. The system recalibrates itself frequently.
    • Equipment: Stard peripheral arterial catheter attached to a special high-fidelity pressure transducer monitor. Arterial site not important but the quality of pulse tracing is relevant.
    • Pros: No external calibration is required. Derives percent stroke volume variation (SVV) that has been shown to be a reliable indicator of intravascular volume status. Good correlation with PAC-based thermodilution method.
    • Cons: Unlike PAC, right heart filling pressure mixed venous O₂ saturation level not provided.
• Bioimpedance cardiography
  • Technique: Based on changes to electrical resistance in the thoracic cavity associated with varying amounts of aortic blood volume during systole diastole. Higher thoracic blood volume reduces impedance. SV is calculated using a built-in algorithm.
  • Equipment: Four pairs of electrodes apply high-frequency, low-amplitude alternating current sense impedance changes related to pulsatile blood flow.
  • Pros
    • Completely noninvasive
    • Can also measure intrathoracic fluid volume LV ejetion time
  • Cons
    • Accuracy is questionable in the setting of significant ambient electrical noise body motion
    • A new device (NICOM, Cheetah Medical) that uses phase shifts of the received signal relative to the applied signal resulting from blood volume induced bioreactance changes in the thorax, may be less susceptible to external noise

• CO is determined by preload, afterload, contractility; therefore, the etiology of low CO state could be traced to three fundamental causes:
  • Pump failure from ischemic or nonischemic causes
  • Hypovolemia
  • Low systemic vascular resistance (SVR)

• Intraoperative hypotension is a common occurrence. Since MAP = CO X SVR, measuring CO will help in the treatment of persistent hypotension unresponsive to simple empirical treatment.
• Knowing CO is also helpful in monitoring the response of inotropic/vasoactive drugs.

• Fick's equation: CO = VO₂/(CaO₂–CvO₂) where VO₂ = O₂ consumption (CaO₂–CvO₂) = arterial mixed venous oxygen content difference.
• CO = VCO₂/(CvCO₂–CaCO₂) where VCO₂ = CO₂ production (CvCO₂–CaCO₂) = venous arterial CO₂ concentration difference.
• VTI = Velocity time integral measured by Doppler ultrasound = stroke distance.
• Stroke volume = stroke distance X cross-sectional area at the sampling site.

• Cardiac Output, High
• Cardiac Output

Amrik Singh , MD