‘The patient’s sats were 100% when I pushed the induction drugs. I thought the sats were fine as the pitch of the beeping from the monitor stayed the same, but when I looked back the patient had already started to desaturate!’

The Zoll X Series monitor is carried on our helicopters and road ambulances, and is widely used by retrieval services across Australia. Like many monitors it has a HR/PR tone function which can improve situational awareness, but its functionality is only briefly mentioned in the Operator’s Guide. While the above quote is fictional, it illustrates a potential source of confusion with this function.

Some of our monitors are set to HR/PR Tone ‘On’ and HR/PR Selected Source ‘ECG’ by default. This means when the ECG leads are applied to the patient, the monitor will beep with each heartbeat, however the pitch of the tone stays constant regardless of the oxygen saturations, because the selected source for the HR/PR tone is the ECG.

However, the monitor will use another available source for the HR/PR tone if the selected source is not available. For example, if only the pulse oximeter is attached to the patient, then that will be used as the source for the HR/PR tone. The pitch of the tone will change with the patient’s saturations, and is lower than if ECG is the source even with saturations of 100%.

Clinicians tend to be divided between those who prefer a quiet monitor, and those who prefer the HR/PR tone on as an audible cue to the patient’s oxygen saturations. In the latter case, it is important to ensure that ‘Sats’ and not ‘ECG’ is selected as the source of the HR/PR tone.