Invasive blood pressure monitoring is widely used in operating room for continuous
beat-to-beat monitoring of arterial blood pressure. Analysis of arterial waveform
gives additional information about systemic vascular resistance, intravascular volume
status, and certain valvular lesions. Proper acquisition of arterial waveform without
artifact is a fundamental prerequisite for its accurate interpretation.
We report the case of a 37-year-old woman with atlantoaxial dislocation scheduled
for occipitocervical fixation under general anesthesia. Her routine blood investigations
were unremarkable. A written informed consent was taken from the patient. General
anesthesia was induced with propofol 100 mg preceded by fentanyl 100 µg. Tracheal
intubation was facilitated with rocuronium 50 mg. Anesthesia was maintained with oxygen,
nitrous oxide (50:50), and sevoflurane (1.0–1.5 minimum alveolar concentration [MAC]).
Right radial artery was cannulated with a 20-G cannula for arterial pressure monitoring.
On connecting it to pressure transducer, a normal arterial pressure waveform was observed.
About half an hour after positioning the patient prone for surgery, an aberrant waveform
was noticed with every alternate beat ([Fig. 1]). Corresponding electrocardiographic and plethysmographic waveforms were normal.
Fast flush test was performed multiple times to achieve normal arterial waveform.
Next, the tubing connecting arterial cannula to the transducer was examined and found
to be softer than usual. Replacing this highly complaint tubing with a stiffer tube
led to the disappearance of the artifact. Excessive compliance of the tubing was the
cause for the initial artifact, which later started showing damped waveform with absent
dicrotic notch.
Fig. 1 An aberrant waveform with alternate beat.
Proper reproduction of arterial waveform on the monitor is dependent on natural frequency
of transducer system and its damping coefficient. Natural frequency of the cannula-tubing-transducer
system determines how fast the system oscillates after a stimulus. Most commercially
available pressure transducer systems have a natural frequency of 200 Hz. This value
decreases by the use of very long, excessively compliant tubing, decrease in tubing
diameter, presence of air bubbles, blood clot, and unnecessary stops in the tubing.
All these factors increase damping of the system and lead to distortion of waveform.[1] In our case, excessively compliant tubing usage showed lower invasive blood pressure
values when compared with values from stiffer tube reading (difference of [systolic
blood pressure of 12 mm Hg, diastolic blood pressure of 8 mm Hg, and mean blood pressure
of 5 mm Hg]). Fast flush test or square wave test is a bedside dynamic response test
that can be employed to determine the natural frequency and damping coefficient of
the transduction system in vivo. While calculation of these values requires measurement
of amplitude and duration between oscillations, simple visual inspection of fast flush
gives required information about damping of the system.[2] Saline pressurized to 300 mm Hg is flushed through the system by fast flush valve.
The test would be successful only if tubing is stiff enough to generate the pressure.
Our case is a reminder to all anesthesiologists who routinely use invasive pressure
monitoring, to be aware of such miniature hindrance, which can majorly influence the
interpretation of the arterial pressure waveform and values, thereby leading to inaccurate
monitoring with the potential risk of patient harm due to under/overtreatment.
