Volume vs pressure control

Core idea: what is “controlled” vs what can change?

• In mechanical ventilation, you can choose to primarily control either volume (tidal volume) or pressure (inspiratory pressure).
• Whichever variable you control, the other one becomes the “result” and can change with lung compliance (stiffness) and airway resistance (narrow/obstructed airways).
• Compliance: how easily the lungs expand (low compliance = stiff lungs, e.g. obesity, pneumoperitoneum, ARDS).
• Resistance: how hard it is to push gas through airways/tube (high resistance = bronchospasm, kinked tube, secretions).

Volume Control Ventilation (VCV): definition and behaviour

• VCV guarantees a set tidal volume (VT) each breath (unless a safety limit stops delivery).
• Airway pressure varies depending on compliance and resistance: if lungs get stiffer or airways narrow, pressures rise to deliver the same VT.
• Typical settings to know: VT (mL), respiratory rate (RR), PEEP, inspiratory flow/flow pattern, FiO2.
• Key advantage: predictable minute ventilation (RR × VT), useful when you want stable CO2 control.
• Key risk: rising peak/plateau pressures if compliance worsens or resistance increases; can increase barotrauma/volutrauma risk if not recognised.

Pressure Control Ventilation (PCV): definition and behaviour

• PCV guarantees a set inspiratory pressure (above PEEP) for a set inspiratory time (Ti).
• Tidal volume varies: if lungs get stiffer or airways narrow, VT falls at the same pressure.
• Typical settings to know: inspiratory pressure (Pinsp), RR, Ti (or I:E ratio), PEEP, FiO2.
• Key advantage: limits peak airway pressure (pressure-capped), often helpful when pressures are high or compliance is poor.
• Key risk: hypoventilation if VT drops (e.g. pneumoperitoneum, head-down tilt, bronchospasm, tube issue) unless you notice and adjust.

Peak pressure vs plateau pressure (why you care)

• Peak airway pressure (Ppeak): highest pressure during inspiration; influenced by resistance and compliance.
• Plateau pressure (Pplat): pressure after an inspiratory pause (no flow); reflects alveolar pressure and compliance more than resistance.
• Simple interpretation: high Ppeak with normal Pplat suggests increased resistance (e.g. bronchospasm, kink/secretions).
• High Pplat suggests reduced compliance (e.g. pneumoperitoneum, obesity, pulmonary oedema, ARDS) or excessive VT/PEEP.

Choosing a mode: common “first time” theatre scenarios

• Routine elective case with normal lungs: VCV or PCV both acceptable; choose what you can monitor and troubleshoot confidently.
• Laparoscopy (pneumoperitoneum ± Trendelenburg): compliance often worsens; in VCV expect pressures to rise, in PCV expect VT to fall—watch whichever is not controlled.
• Obesity: reduced compliance; PCV may help limit pressures, but ensure adequate VT and minute ventilation.
• Bronchospasm/high resistance: VCV may generate high peak pressures; PCV may reduce peak pressure but VT may drop—treat bronchospasm and check the circuit/tube.
• When you need stable CO2 control (e.g. neuro, long cases): VCV can make minute ventilation more predictable, but still monitor pressures.

Safe starting points (adult, intubated) and what to monitor

• Tidal volume: aim 6–8 mL/kg predicted body weight (PBW), not actual weight; consider 6 mL/kg if lung injury risk or high pressures.
• PEEP: commonly 5 cmH2O to start; adjust to oxygenation and lung mechanics (avoid excessive PEEP causing hypotension/overdistension).
• RR: set to achieve appropriate minute ventilation; then adjust based on end-tidal CO2 and clinical context.
• Oxygen: use the lowest FiO2 that maintains safe saturations (balance oxygenation vs absorption atelectasis).
• Always monitor: SpO2, end-tidal CO2, airway pressures (peak and if available plateau), exhaled VT (especially in PCV), minute ventilation, and alarms.

Troubleshooting: sudden high airway pressure (especially in VCV)

• First: look at the patient and the circuit—ensure oxygenation/ventilation and call for help early if unstable.
• Check for: kinked/bitten tube, circuit obstruction, water in HME/filter, closed APL (if manual), blocked catheter mount, patient coughing/bucking, bronchospasm, pneumothorax.
• Differentiate resistance vs compliance: if Ppeak high but Pplat normal → think resistance; if both high → think compliance/overdistension.
• Immediate actions: hand ventilate to feel compliance, suction if needed, deepen anaesthesia/analgesia, give bronchodilator if wheeze, check tube position, consider pneumothorax if sudden deterioration.

Troubleshooting: low tidal volume / low minute ventilation (especially in PCV)

• Check exhaled VT and minute ventilation trends; don’t rely only on set pressure.
• Common causes: reduced compliance (pneumoperitoneum, positioning, atelectasis), increased resistance (bronchospasm, tube/circuit obstruction), leak (cuff leak, circuit disconnect), inadequate Pinsp or too short Ti.
• Actions: check circuit and cuff, suction, treat bronchospasm, recruit/adjust PEEP if appropriate, increase Pinsp in small steps, consider increasing Ti (watch for air-trapping), or switch to VCV if you need guaranteed VT.

Alarms and safety limits (what they mean in practice)

• High pressure alarm: in VCV it may stop inspiration and reduce delivered VT; in PCV it may indicate obstruction or patient-ventilator asynchrony.
• Low minute ventilation/low VT alarm: particularly important in PCV—alerts you to falling VT.
• Disconnect/leak alarm: think circuit disconnect, cuff leak, or large leak around supraglottic airway.
• Set alarm limits thoughtfully at the start; re-check after position changes, pneumoperitoneum, or draping.