Page 219 - ACCCN's Critical Care Nursing

196 P R I N C I P L E S A N D P R A C T I C E O F C R I T I C A L C A R E

should reflect how much information is required to opti- monitor should display zero (0 mmHg), as this equates
mise the patient’s condition, and how precisely the data to current atmospheric pressure (760 mmHg at sea level).
are to be recorded. As Pinsky argues, a great deal of infor- With the improved quality of transducers, repeated
mation is generated by this form of monitoring, and yet zeroing is not necessary, as once zeroed, the drift from
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little of this is actually used clinically. Consequently, the baseline is minimal. Some critical care units,
monitors are not substitutes for careful examination and however, continue to recalibrate transducer(s) at the
do not replace the clinician. The accuracy of the values beginning of each clinical shift.
obtained and a nurse’s ability to interpret the data and Fast-flush square wave testing, or dynamic response mea-
choose an appropriate intervention directly affect the surement, is a way of checking the dynamic response of
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patient’s condition and outcome. 25
the monitor to signals from the blood vessel. It is also a
PRINCIPLES OF HAEMODYNAMIC check on the accuracy of the subsequent haemodynamic
pressure values. The fast-flush device within the system,
MONITORING when triggered and released, exposes the transducer to
A number of key principles need to be understood in the amount of pressure in the flush solution bag (usually
relation to invasive haemodynamic monitoring of the 300 mmHg). The pressure waveform on the monitor will
critically ill patients. These include haemodynamic accu- show a rapid rise in pressure, which then squares
racy, the ability to trend data and the maintenance of off before the pressure drops back to the baseline (see
minimum standards. These are reviewed below. Figure 9.17).

Haemodynamic Accuracy Interpretation of the square wave testing is essential; the
clinician must observe the speed with which the wave
Accuracy of the value obtained from haemodynamic returns to the baseline as well as the pattern produced.
monitoring is essential, as it directly affects the patient’s One to three rapid oscillations should occur immediately
condition. 26,27 Electronic equipment for this purpose has after the square wave, before the monitored waveform
four components (see Figure 9.16): resumes. The distance between these rapid oscillations
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1. an invasive catheter attached to high-pressure should not exceed 1 mm or 0.04 sec. Absence, or a
tubing reduction, of these rapid oscillations, or a ‘square wave’
2. a transducer to detect physiological activity with rounded corners, indicates that the pressure moni-
3. a flush system toring system is overdamped; in other words its respon-
4. a recording device, incorporating an amplifier to siveness to monitored pressures and waveforms is reduced
increase the size of the signal, to display (see Figure 9.18). An underdamped monitoring system
information. will produce more rapid oscillations after the square wave
than usual.
High-pressure (non-distensible) tubing reduces distor-
tion of the signal produced between the intravascular Data Trends
device and the transducer; the pressure is then converted The ability to trend data via a monitor or a clinical infor-
into electrical energy (a waveform). Fluid (0.9% sodium mation system is essential for critical care practice. Current
chloride) is routinely used to maintain line patency using monitoring systems used in Australia and New Zealand
a continuous pressure system; the pressure of the flush can retain data for a period of time, produce trend graphs,
system fluid bag should be maintained at 300 mmHg, and link to other devices to allow review of data from
which normally delivers a continual flow of 3 mL/h.
locations other than the immediate bedside. The data
Accuracy is dependent on levelling the transducer to the trends can be used to assess the progression of a patient’s
appropriate level (and altering this level with changes in clinical condition and monitor the patient’s response to
patient position as appropriate), then zeroing the trans- treatment.
ducer in the pressure monitoring system to atmospheric
pressure (called calibration) as well as evaluating the Haemodynamic Monitoring Standards
response of the system by fast-flush wave testing. There are stated minimum standards for critical care units
The transducer must be levelled to the reference point of in Australia and New Zealand. 30,31 The standards require
the phlebostatic axis, at the intersection of the 4th inter- that patient monitoring include circulation, respiration
costal space and the midthoracic anterior-posterior diam- and oxygenation, with the following essential equipment
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eter (not the midaxillary line). Error in measurement available for every patient: an ECG that facilitates con-
can occur if the transducer is placed above or below the tinual cardiac monitoring; a mechanical ventilator, pulse
phlebostatic axis. 26,27 Measurements taken when the oximeter; and other equipment available where necessary
patient is in the lateral position are not considered as to measure intra-arterial and pulmonary pressures, cardiac
accurate as those taken when the patient is lying supine output, inspiratory pressure and airway flow, intracranial
or semirecumbent up to an angle of approximately 60 pressures and expired carbon dioxide. 30
degrees. 28
BLOOD PRESSURE MONITORING
Zeroing the transducer system to atmospheric pressure
(calibration of the system) is achieved by turning the Indirect and direct means of monitoring blood pressure
3-way stopcock nearest to the transducer open to the air, are widely used in critical care units. These are outlined
and closing it to the patient and the flush system. The in more detail below.