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Hemodynamic Monitoring 297

is placed into the esophagus (via the mouth or nose) with its distal end resting at the
midthoracic level. The probe is rotated until it faces the aorta and is able to pick up
the aortic blood flow signal. In three studies, the cardiac output measured by this
technique correlates well with the measurements using the traditional thermodilu-
tion method (DiCorte et al., 2000; Perrino et al., 1998; Mark et al., 1986).
.
Carbon Dioxide Elimination (VCO )
2
#
Carbon dioxide elimination (VCO ) is a technology that can monitor and mea-
carbon dioxide elimination 2
.
(VCO 2 ): A technology to monitor sure cardiac output based on changes in respiratory CO concentration during a
2
and measure cardiac output based brief period of rebreathing. The NICO ® (with cardiac output option) is a cardio-
on changes in respiratory CO 2 2
concentration during a period of pulmonary management system that incorporates different sensors to measure the
rebreathing. flow, airway pressure, and CO concentration. These measurements are used to
2
calculate CO elimination. A Fick partial rebreathing method is used to derive the
2
cardiac output. #
The original Fick method uses the oxygen consumption (VO ) and arterial-
2
mixed venous oxygen content difference (C (a-v) O ) to calculate the cardiac output.
2
#
(C.O. 5 VO / C (a-v) O ). This method for calculating cardiac output requires the
2
2
use of specialized equipment and has never been suitable in the traditional clinical
#
#
setting. The NICO ® uses VCO instead of VO . End-tidal CO from an exhaled
2
2
2
2
breath sample is used instead of using mixed venous and arterial blood samples (for
C (a-v) O ). The NICO® system (Respironics®) can provide continuous cardiac output
2
noninvasively via this method.
IMPEDANCE CARDIOGRAPHY
Impedance cardiography (ICG), also called thoracic electrical bioimpedance
impedance cardiography (ICG):
A noninvasive procedure to mea- (TEB), is a major division of noninvasive technique for hemodynamic monitoring.
sure or trend the hemodynamic ICG is based on a technology originally used by NASA in the 1960s. The introduc-
status of a patient.
tion of the microprocessor and the working knowledge of echocardiography and
magnetic resonance imaging make ICG possible. ICG is a noninvasive procedure
to measure or trend the hemodynamic status of a patient in clinical settings ranging
from critical care to outpatient care. Several noninvasive ICG devices are available
and each offers different technology to measure and calculate the hemodynamic
values.
The IQ system (Wantagh Incorporated, Bristol, MA) uses a patented signal
processing technique to identify the opening and closing of the aortic valve for
the precise measurement of the ventricular ejection time (VET). Another device
incorporates “ensemble averaging” to estimate the VET by using the QRS of the
ECG and the raw dZ/dt (change in impedance/time) waveform (SORBA Medical
Systems, Inc., Brookfield, WI). A third manufacturer of ICG (BioZ System, Cardio-
Dynamics, San Diego, CA) uses digital signal processing and an R-wave detection
system to establish the dZ/dt. ICG has proven to be a simple and accurate method
to measure and monitor a patient’s hemodynamic status (Clancy et al., 1991).

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