Page 310 - Clinical Application of Mechanical Ventilation

276 Chapter 10

INTRODUCTION

Evolving technology in hemodynamic monitoring has been a useful adjunct in the
management of patients with cardiovascular instability. This monitoring technology
was initially developed in the 1970s using invasive methods. In recent years, moni-
toring technology has undergone substantial changes to include less-invasive and
noninvasive techniques. Hemodynamic monitoring is not intended for every patient
who requires mechanical ventilation. For many critically ill patients, hemodynamic
data can add valuable information to the overall management strategy.
In the most basic sense, hemodynamic monitoring is the measurement of the
force (pressure) exerted by the blood in the vessels or heart chambers during
systole and diastole.
In addition to systolic and diastolic pressures in both the systemic and pulmonary
circulations, hemodynamic monitoring equipment also measures cardiac output
and mixed venous oxygen saturation. These and other direct measurements gath-
ered through hemodynamic monitoring can be used to calculate other values for
different clinical applications.

INVASIVE HEMODYNAMIC MONITORING

Invasive hemodynamic monitoring requires the use of the central venous and pul-
hemodynamic monitoring:
Measurement of the blood pres- monary artery catheters. The central venous catheter measures the central venous
sure in the vessels or heart cham- pressure (right ventricular preload), and the pulmonary artery catheter measures
bers during contraction (systole)
and relaxation (diastole). the pulmonary artery pressure (right ventricular afterload) and the pulmonary
capillary wedge pressure (left ventricular preload). Impedance cardiography is a
noninvasive method to measure and calculate selected hemodynamic parameters.
central venous pressure (CVP):
Pressure measured in the vena
cava or right atrium. It reflects Technical Background
the status of blood volume in
the systemic circulation. Right
ventricular preload.
Measurement of hemodynamic pressures is based on the principle that liquids
are noncompressible and that pressures at any given point within a liquid are
preload: The end-diastolic stretch transmitted equally. When a closed system is filled with liquid, the pressure exerted
of the muscle fiber. at one point can be measured accurately at any other point on the same level. For
example, if a catheter is placed into the radial artery facing the flow of blood and
then connected directly to a tubing that is filled with liquid, the pressure exerted
afterload: The resistance of
the blood vessels into which the by the blood at the tip of the catheter will be accurately transmitted to the liquid-
ventricle is pumping blood.
filled tubing. This pressure signal can then be changed to an electronic signal by
a transducer and amplified and displayed on a monitor as both a waveform and
digital display.
Hemodynamic monitoring is generally done by using a combination of arterial
catheter, central venous catheter, and pulmonary artery catheter. One or more of
these catheters are introduced into the blood vessel, advanced to a suitable location,
and then connected to a monitor at the patient’s bedside. The display on the monitor

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