Mechanical Ventilation

A ventilator delivers breaths, but the mode and settings you understand determine whether those breaths help or harm. Choosing wrong can cause barotrauma, atelectasis, or hemodynamic collapse.

Mechanical ventilation provides positive-pressure breaths through an endotracheal or tracheostomy tube when a patient cannot maintain adequate gas exchange independently. The three core modes tested on NCLEX are Assist-Control (AC), Synchronized Intermittent Mandatory Ventilation (SIMV), and Pressure Support (PS). In AC, the ventilator delivers a set tidal volume (typically 6–8 mL/kg ideal body weight) for every breath the patient triggers or at the set rate — whichever is higher. Risk: hyperventilation and respiratory alkalosis if the patient breathes rapidly. SIMV delivers mandatory breaths at a set rate but allows spontaneous breaths in between at whatever volume the patient generates, making it useful for weaning. PS augments the patient's own spontaneous breaths with a preset pressure boost; the patient must initiate every breath (no mandatory rate is set), so PS is used during weaning or with an intact respiratory drive. Key settings the nurse monitors include tidal volume, respiratory rate, FiO2 (started high then titrated down to keep SpO2 ≥ 94%, or ≥ 88–92% for CO2 retainers), and PEEP (positive end-expiratory pressure, typically 5 cmH2O baseline, which prevents alveolar collapse). The nurse verifies ETT placement by confirming bilateral breath sounds, CO2 detection, and chest X-ray. ABGs are drawn 20–30 minutes after any setting change to evaluate effectiveness.

Key Distinctions

Don't confuse AC with SIMV: in AC every breath gets the full set tidal volume; in SIMV only mandatory breaths do — spontaneous breaths are unsupported unless PS is added. Students mix up FiO2 (fraction of inspired oxygen, controls oxygenation) with tidal volume/rate (control ventilation and CO2 removal). PEEP keeps alveoli open between breaths — it is NOT the same as peak inspiratory pressure (PIP), which is the pressure delivered during inspiration.

Clinical Pearl

Think 'FiO2 fixes O2, rate fixes CO2.' If the PaO2 is low, adjust FiO2 or PEEP. If the PaCO2 is off, adjust rate or tidal volume.

Alarms & Complications

Ventilator alarms divide into two pressure categories that point you in opposite directions. A high-pressure alarm means something is resisting airflow: the patient is biting the tube, mucus is plugging the airway, the tubing is kinked, bronchospasm has narrowed the airways, or the patient is coughing or fighting the vent. Nursing response moves from simple to complex — check tubing for kinks, suction the airway, reposition the patient, and if unresolved, notify the provider. A low-pressure alarm means air is escaping or not arriving: a circuit disconnection, a loose connection, a cuff leak on the endotracheal or tracheostomy tube, or accidental extubation. This is the more immediately dangerous alarm because the patient may not be receiving breaths at all. First action: check all connections and the patient. If the patient is extubated, manually ventilate with a bag-valve-mask while calling for help. High-volume alarms (high tidal volume or high minute ventilation) often signal anxiety, pain, or tachypnea. Low-volume alarms suggest apnea or shallow breathing. Always assess the patient first, then the equipment — the NCLEX consistently rewards patient-first reasoning.

Key Distinctions

High-pressure alarm = obstruction (something blocking outflow) versus low-pressure alarm = disconnection (air escaping before reaching the patient) — students frequently reverse these. Don't confuse alarm troubleshooting with ventilator setup (mode, FiO2, PEEP settings) — setup lives in the sibling atom. A low-pressure alarm from a cuff leak is not the same as accidental extubation, though both drop pressure; cuff leaks present with an audible air leak around the tube and a voice or gurgling sound.

Clinical Pearl

High pressure = something's blocked, grab the suction. Low pressure = something's loose, grab the bag-valve-mask. Block versus leak — that's your fork in the road.