Zeroing consists of 2 steps.
First, the air-fluid interface is opened to atmospheric pressure.Then the monitorís zeroing function key or button is pressed. Zeroing in this fashion has several purposes. Zeroing electronically
establishes for the monitor atmospheric pressure as the atmospheric zero reference point. Zeroing establishes the interface level as the hydrostatic zero reference point. Zeroing also eliminates zerodrift. Zero-drift is the potential, but usually minimal, transducer ofset or distortion occurring over time.Two key practice objectives are related to referencing and zeroing: accuracy and consistency. In order to ensure accuracy, leveling andzeroing must be done whenever the relationship between the airfluid interface and the reference point is changed.
The reason is hydrostatic pressure. For every 1 cm the air-fluid interface is aboveor below the actual level of the left atrium, 0.74 mm Hg of hydrostatic pressure is subtracted or added to the measured pressure. If the airfluid interface is placed 10 cm below the phlebostatic axis, the measured pressure will be an overestimation of the actual hemodynamic pressure by 7.4 mm Hg. This example illustrates that though accurate referencing and zeroing
are important for all hemodynamic monitoring, they become even more crucial when small hemodynamic pressures, such as pulmonaryartery wedge pressure (6-12 mm Hg) and central venous pressure (2-6 mm Hg) are being monitored. In these instances, small offsets from the phlebostatic axis and the zero reference point can cause large errors and may
prompt inappropriate treatment. In order to ensure consistency,once the external reference point has been selected, it should be marked on the patient for easy identification. Consistency allows correct determination of trends in a patientís hemodynamic status.
Whether the same body position must be used for consecutive measurements may be debatable. Although several studies on pulmonary artery pressure monitoring indicated no significant changes in pulmonary artery pressures related to certain bed positions, according to a recent research abstract,3ABP values obtained with various bed positions
differed even when the phlebostatic axis was used as the external reference point. The statistical significance of differences in ABP values for monitoring systems referenced to the phlebostatic axis was not addressed in the abstract.
Dynamic Response Testing. In order to determine if a hemodynamic monitoring system can adequately reproduce a patientís cardiovascular pressures, the dynamic response characteristics of the catheter system must be tested. Only when system accuracy,also termed fidelity, has been confirmed, can the analog waveform be accepted as an accurate
reflection of a patientís status. The dynamic response of a hemodynamic monitoring system is defined by its natural frequency and the damping coefficient. The natural frequency indicates how fast the pressure monitoring system vibrates when shock excited by a signal such as the arterial pressure pulse or the pressure signal caused by a fast-flush test. The damping coefficient of a monitoring system is a measure of how quickly the oscillations of a shockexcited system dampen and eventually come to rest.
Dynamic response testing is a 3-step procedure: determining natural frequency, determining the amplitude ratio of 2 consecutive fast-flush oscillations (as an indirect way of determining the damping coefficient), and determining the dynamic response characteristics of the monitoring system (eg, optimal, adequate, underdamped, overdamped, or unacceptable). These steps aresummarized in Figure 1. Dynamic response testing may seem complicated at first. Clinical experience has shown, however, that with proper training, dynamic
response testing can be performedin less than 2 minutes. A few simple observations provide almost as much information and may serve as a close estimate. (1)If the period between 2 oscillations of the fast-flush test is less
than 1.2 mm, then natural frequency will be at least 21, and the system will most likely be adequate.
If the period between 2 oscillations is 1 mm or less, then the natural frequency is 25 or higher, and the system will almostalways function properly with any degree of damping. As a rule-ofthumb, the higher the natural frequency
(or the smaller the period), the better is the dynamic response of the monitoring system.
(2) The system is overdamped if the fast-flush produces sluggish or no oscillations. If the square wave shows undulations or if ringing occurs after the release of the fast-flush device, the system is most likely underdamped.
Overdamping and underdamping both indicate that the dynamic response characteristics of the monitoring system are
unsatisfactory. Simple visual evaluation of the arterial waveform, the square waveform, and oscillations generated
by the fast-flush has been suggested as a suitable method for determining the dynamic response characteristics of a
monitoring system. However, the square waveform and fast-flush oscillations may look adequate when the natural frequency and amplitude ratio of the system actually make the system inadequate (underdamped, overdamped,
or unacceptable). Or, the arterial waveform may look distorted because of physiological variations, yet the dynamic response characteristics of the system assure a faithful recording of the arterial pressure.When accuracy is needed for clinical decision making, the ideal method of determining if the measured ABP, or any other directly monitored
hemodynamic parameter, is true is to determine the dynamic response characteristics of the monitoring
system.Examples of the importance of formally assessing the response characteristics are discussed in more detail later.