M18Setting up the ventilator: modes, first settings and safety limits

Clinical overview

Most obstetric units ventilate rarely and urgently. The machine arrives with a trademarked mode name on the screen, the patient is deteriorating, and the temptation is to accept whatever the previous patient's settings were. That is the moment this module is written for: what to choose, what to type in, and what to check before you walk away.

Two ideas make everything else simpler. First, the proliferating mode names are marketing for a small number of generic behaviours — every mode is defined by what triggers a breath, what the machine controls during the breath, and what ends it. Second, pregnancy does not change the physics of positive pressure, but it changes the starting numbers: predicted body weight is calculated from height and never from the pregnant weight, the baseline PaCO₂ is lower, functional residual capacity is reduced, and oxygen consumption is higher, so desaturation is faster and the margin for a slow setup is smaller.

Core knowledge

Every mode answers three questions

• What starts the breath (trigger)? The patient's effort, or the clock. A mandatory rate guarantees breaths; a triggered breath follows the patient.
• What is controlled during the breath? Volume (you set the tidal volume and the pressure follows from compliance) or pressure (you set the pressure and the volume follows). Neither is inherently safer — what matters is which variable you have chosen to watch and limit.
• What ends the breath (cycling)? A delivered volume, a set inspiratory time, or a fall in flow.

Anything called PRVC, VC+, autoflow, adaptive or volume-support is a control loop layered on those answers: the machine targets a volume while delivering it as a decelerating pressure breath, adjusting between breaths. Useful, but it can silently increase support as the patient tires — so the trend that matters is the pressure the machine needed, not the volume it achieved.

The four modes worth knowing well

A patient in AC who is triggering every breath is not "on full support because she cannot breathe" — she is receiving a full breath each time she asks. Sedation choices should follow that observation rather than the mode label.

Tap to enlarge

Figure M18.1 — Every ventilator mode can be decoded by its trigger, controlled variable and cycling rule: assist-control gives a full set breath after either patient or clock triggering, SIMV interleaves mandatory and spontaneous breaths, pressure support is patient-triggered and flow-cycled without a backup rate, and CPAP provides continuous pressure without inspiratory assistance.

Initial settings in pregnancy

1. Mode: volume assist-control unless a specific reason exists to do otherwise. It is the mode most teams read correctly under pressure.
2. Tidal volume: 6 mL/kg predicted body weight, calculated from height — see M4 for the equation and the worked example. A late-pregnancy weight will overshoot the lung by a wide margin.