When the Heart Tricks the Ventilator

This patient with Guillain–Barré syndrome had been on mechanical ventilation for more than five days. When transitioned to pressure support mode, the ventilator waveforms showed patient-triggered breaths, and clinically the patient appeared comfortable, without accessory muscle use or tachypnea. However, the patient’s muscle strength remained at grade 1, and in neuromuscular weakness, such low muscle power would typically result in marked work of breathing on pressure support unless very high levels of assistance are provided.

This mismatch raised suspicion for a non-patient signal triggering the breaths, known as false triggering. False triggers may result from strong cardiac oscillations, air leaks, or excessively sensitive trigger settings. In this case, the patient had a flow trigger sensitivity of 1 L/min. When the trigger sensitivity threshold was increased, the patient-triggered breaths disappeared, and the ventilator delivered only mandatory breaths at the backup rate, making the cardiac oscillations clearly visible on the waveforms.

Cardiac oscillations are a well-recognized cause of false triggering.

An expiratory hold manoeuvre can help confirm this.

• If the breath is truly patient-triggered, there will be large pressure deflections below baseline during the hold.

• If the triggering is due to cardiac oscillations, only small, rhythmic pressure undulations will be observed.

In this case, the expiratory hold revealed small oscillations without large negative pressure swings, confirming that the false triggers were caused by cardiac pulsations rather than true patient effort.