Mechanical ventilation basics

Positive-pressure ventilation replaces or supports the pressure gradient normally generated by respiratory muscles. The clinician chooses a mode, tidal volume or inspiratory pressure, rate, inspiratory time or flow, inspired oxygen and PEEP. The machine then delivers breaths through a circuit and airway whose resistance and compliance can change abruptly. The essential skill is to distinguish what was set from what was delivered and from the physiological result.

Tidal volume should be related to predicted body weight because adult lung size follows height and biological sex more closely than adipose mass. The 2019 international surgical-patient consensus recommends an initial 6–8 mL/kg predicted body weight and PEEP 5 cmH2O, then individualising PEEP and the rest of the strategy. This is a perioperative starting point, not a universal prescription for ARDS, one-lung ventilation, children, pregnancy or every person with obesity. Recruitment manoeuvres can cause hypotension or overdistension and should respond to a clinical indication rather than a fixed ritual.

In volume-controlled ventilation, the set volume and flow pattern are delivered unless a safety pressure limit interrupts inspiration; peak pressure therefore changes when resistance or compliance changes. In pressure-controlled ventilation, inspiratory pressure and time are set; tidal volume falls when compliance worsens or resistance rises and increases if mechanics improve. Neither mode is inherently protective. Protection comes from monitoring delivered volume, pressure, gas exchange and haemodynamics and adjusting to the patient.

Respiratory mechanics explain common alarms. Peak inspiratory pressure is measured while gas is flowing and includes resistance from the tube, airways and circuit plus the elastic pressure needed to distend lung and chest wall. Plateau pressure is measured during a no-flow inspiratory pause and approximates alveolar elastic pressure. A widening peak-to-plateau difference suggests increased resistance such as a kink, secretions or bronchospasm. When peak and plateau rise together, reduced respiratory-system compliance, endobronchial intubation, pneumoperitoneum, positioning or pneumothorax is more likely. Driving pressure is plateau pressure minus total PEEP and should be interpreted as a trend, not as an isolated diagnosis.

Minute ventilation is respiratory rate multiplied by tidal volume, but only alveolar ventilation clears carbon dioxide; dead-space ventilation does not. Increasing rate or volume usually lowers PaCO2 when production and dead space are stable. End-tidal CO2 trends ventilation breath by breath, confirms airway patency and can signal disconnection or obstruction, but the PaCO2-to-ETCO2 gradient widens with dead space, low cardiac output and ventilation-perfusion mismatch. Obtain a blood gas when an exact arterial value will change management.

PEEP holds airway pressure above atmospheric at end expiration. It can preserve recruited lung and improve oxygenation, but excessive PEEP may overdistend already open regions, increase dead space, impede venous return and reduce cardiac output. Inspired oxygen is raised immediately during instability, then reduced to the lowest concentration that provides adequate oxygenation once the cause is treated. Saturation alone cannot distinguish atelectasis, hypoventilation, low cardiac output, airway obstruction or equipment failure.

Waveforms and loops turn alarms into physiology. Failure of expiratory flow to return to zero before the next breath suggests incomplete exhalation and intrinsic PEEP, often from high rate, short expiratory time or airflow obstruction. A scooped expiratory flow-volume limb supports airflow obstruction. A new leak changes inspired and expired volume and may come from cuff, connector or circuit. Always examine the patient and manually ventilate when needed; a waveform interpretation must never delay correction of oxygenation or a tension pneumothorax.

The Association monitoring guideline requires airway pressure, tidal volume and respiratory-rate monitoring during mechanical ventilation, with audible patient-specific alarms. Peak, plateau, mean and end-expiratory pressure waveforms provide breath-by-breath information. Capnography remains essential with a tracheal tube or supraglottic airway and should continue through transfers and emergence. These standards make ventilation a continuously verified intervention rather than a set-and-forget machine function.