It has been widely accepted that determination of cardiac output
(CO) is a useful and often lifesaving adjunct in the resuscitation
of critically ill patients. Monitoring of these patients typically occurs
within the intensive care unit (ICU), operating room, cardiac catheterization
lab, or Emergency Department. Cardiac output determination has conventionally
been obtained by thermodilution or dye dilution measurements that
are invasive and associated with potential risk to the patient.
Complication rates up to 7.2 percent using the invasive pulmonary
artery (PA) catheter have been reported.1–3 There
is wide variability in CO measurements using a PA catheter as well.
Stetz and Miller identified error rates between 4 and 10 percent
with triplicate CO measurements, while single CO measurements had
a wider variability, ranging between 7 and 17 percent.4 Other
disadvantages to use of the PA catheter include risk of sepsis,
intermittent cardiac output determinations, expense, and restricted
use to “monitored” facilities within a hospital
(i.e., ICU or operating room).
Noninvasive devices that measure cardiac output have been slowly
gaining acceptance in the medical community and have shown very
good agreement with the “gold standard” thermodilution
technique. Transesophageal Doppler (TED) and thoracic electrical
bioimpedance (TEB) are two modalities that can measure CO noninvasively
and continuously. Furthermore, they can measure other hemodynamic
variables such as preload, contractility, and systemic vascular
resistance in real time. This provides the physician with the ability
to follow trends as well as the response to interventions such as
the institution of vasopressors or following the administration
of a fluid bolus. The measurements obtained require little training,
are highly reproducible, and pose little risk to the patient. Another
advantage lies in their ability to identify hemodynamic compromise before
it becomes clinically apparent and when therapy may be most beneficial.
Most Emergency Departments utilize noninvasive devices to measure
blood pressure, heart rate, oxygen saturation, and, on rare occasions,
central venous pressure. These noninvasive devices may allow the
Emergency Physician to monitor the hemodynamic status of a patient
more closely and to institute therapy earlier.
In 1842, Christian Doppler identified that the velocity of a
moving object is proportional to the shift in reflected frequency
of an optic wave of known frequency. This principle has been adapted to
sound waves and is now the basis for Doppler devices that measure
the velocity of blood flow and related hemodynamic variables continuously.
The first use of Doppler to measure the velocity of red blood cells
in humans or animals occurred in 1969.5,6 Cardiac output
determination was first conducted via the suprasternal approach,
but this was cumbersome, and it was difficult to obtain data continuously.7,8 The
device currently in use involves Doppler measurements through a
transesophageal approach, directly measuring the blood velocity
in the descending aorta. The esophageal Doppler monitor (EDM; CardioQ,
Deltex Medical Inc.) is one such device that will display continuous
hemodynamic data (cardiac output, peak velocity, and corrected flow ...