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Comparison of Available Noninvasive Cardiac Output Monitors
AdvantagesLimitationsTruly noninvasive?
CO2 Fick principle, for example, NICO®
  • Good agreement between NICO® and ultrasound transit time
  • Patient may be breathing spontaneously or on a ventilator
  • Accuracy adversely affected by hyperventilation and V/Q mismatch (preexisting lung disease or postoperative atelectasis)
Yes; however, patient must be intubated
Esophageal Doppler, for example, CardioQ (single use), WAKIe TO (reusable)
  • High validity for monitoring changes in cardiac output in critically ill patients
  • Using esophageal Doppler to optimize fluid management has been shown to improve outcomes
  • Patient must be intubated and sedated
  • Small risk of injury to the esophagus
  • Limited agreement with thermodilution
No, requires placement of probe in the esophagus
Transpulmonary lithium dilution, for example, LiDCO
  • High signal-to-noise ratio
  • Good agreement with thermodilution
  • Low risk of toxicity
  • Not approved for patients <40 kg, pregnant women, or patients taking lithium
  • Accuracy reduced by aortic insufficiency, intra-aortic balloon pump, nondepolarizing NMB
No, requires an intra-arterial line
Arterial pulse contour analysis, for example, Vigileo®
  • Good agreement with other methods of cardiac output determination
  • Continuous measurement of SV and CO
  • Also display SVV (SV variation) with a value >15% indicating fluid responsiveness
  • Easy to use
  • Adversely affected by a poor arterial waveform (arrhythmia, intra-aortic balloon pump)
  • Older devices may not adequately compensate for changes in vascular tone
  • Newer devices have good agreement with thermodilution and esophageal Doppler
No, requires an intra-arterial line
Bioimpedance, for example, Cheetah NICOM®, BioZ Dx®
  • Completely noninvasive
  • Impedance to high-frequency current between electrodes on chest; estimates thoracic blood volume
  • Relative hemodynamic stability required
  • Accuracy reduced by arrhythmias, aortic insufficiency, intra-aortic balloon pump
  • Contraindicated in patients with pacemakers
Yes, adhesive surface electrodes only

  • The indirect Fick equation is defined as the following:

    Image not available.

    where V̇CO2 is the CO2 produced, CvCO2 is the central venous CO2 content, and CaCO2 is the arterial CO2 content. V̇CO2 is determined by the minute ventilation and instantaneous CO2 content. CaCO2 is estimated from the end-tidal CO2.

  • By combining measurements during rebreathing and during normal ventilation, CvCO2 can be eliminated from the equation
  • To improve accuracy, shunt fraction can be estimated from FiO2 and Pao2 from blood gases


  • A Doppler transducer is inserted into the esophagus of an anesthetized intubated patient such that the characteristic waveform of blood flow in the descending aorta is displayed
  • The area under the aortic velocity envelope is calculated as a velocity time integral (VTI) that is referred to as the stroke distance (SD)
  • The monitor utilizes a computer algorithm to determine the stroke volume (SV) and CO by multiplying the SD by the cross-sectional area of the descending aorta (Figure 41-1)

Figure 41-1. Typical Esophageal Doppler Descending Aortic Waveform
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