Basics of a Ventilator
Negative vs. positive pressure ventilation
Negative pressure devices (iron lung, cuirass) expand the chest externally and are mainly useful in neuromuscular disease but cannot precisely control ventilation.
Positive pressure ventilation delivers gas via a cuffed tube or mask, allows precise control of ventilation/oxygenation, and is the critical care standard.
The three components of a ventilator mode
Breath control variable — every mode is either:
Volume control (VC): VT is set and guaranteed; airway pressure varies with resistance/compliance. Pressure waveform is an ascending "shark fin," flow is square. Set pressure alarms to detect changing lung mechanics.
Pressure control (PC): inspiratory pressure limit is set; VT varies with resistance/compliance. Pressure waveform is square, flow is decelerating. Set VT alarms to detect under/overventilation.
Breath delivery sequence:
CMV (assist control): ventilator delivers all breaths at set parameters; patient-triggered breaths are full machine breaths.
IMV (SIMV): set mandatory breaths plus patient's own spontaneous breaths; largely abandoned as a weaning mode in favor of spontaneous breathing trials.
CSV: all breaths spontaneous, always PC; pressure support ventilation (PSV) is the prototype. Weaning to 5–8 cm H₂O offsets circuit/airway work before extubation. Apnea back-up ventilation activates if the patient becomes apneic.
Targeting scheme (set point, adaptive, servo, optimal, intelligent, etc.).
Breath phase variables: trigger (time, flow, or pressure — flow triggering ~2–3 L/min is most sensitive; autotriggering from circuit water, hyperdynamic cardiac activity, or transport can mimic effort, notably during apnea/brain-death testing), target/limit, and cycle (volume in VC, time in PC, flow in PSV).
Initial settings
Tidal volume: dosed by predicted body weight (formulas provided). Low VT (4–8, typically 6–8 mL/kg PBW) reduces mortality vs. traditional 10–15 mL/kg per ARDSnet. Target plateau pressure <30 cm H₂O and driving pressure <15 cm H₂O in ARDS.
Respiratory rate: 10–20/min; adjusted to titrate minute ventilation/PaCO₂. Higher rates shorten expiratory time, risking breath stacking/dynamic hyperinflation (especially asthma/COPD).
Flow rate: low flow in VC can cause flow desynchrony
(air hunger); PC delivers unlimited flow, better tolerated.
I:E ratio: normally 1:2–1:3; inverse ratio (used in PC-IRV and APRV) generates auto-PEEP to recruit alveoli.
FiO₂: 0.21–1.0; start high when oxygenation unknown and wean.
PEEP: usually 5 cm H₂O baseline; titrated in 2 cm H₂O increments to best compliance and lowest driving pressure. Adjuncts include pressure-volume curve inflection points, esophageal pressure, and electrical impedance tomography. Complications: overdistension/barotrauma, pneumothorax, and hemodynamic compromise (reduced RV preload, increased RV afterload, hypotension). Static filling pressures may be falsely elevated with PEEP.
Alarms: patient alarms (high/low pressure, VT, minute ventilation, RR, PEEP) and technical alarms (gas pressure, battery, ventilator failure). Customize limits (e.g., pressure ±10 cm H₂O, RR ±10–15, VT ±10–15%) to avoid alarm fatigue. Respond starting at the patient and working back to the ventilator; use a BVM if the cause is unidentified.
Transport: ventilate patients on a transport-capable ventilator rather than by hand, especially with high minute ventilation, high PEEP, or nontraditional modes (IRV, APRV); add a PEEP valve to the BVM for PEEP >5 cm H₂O.
Nontraditional / advanced modes
Adaptive PC modes (PRVC, VS, VC+, Autoflow, etc. = PC-CMV/CSV adaptive): PC breaths with a minimum VT target achieved by adjusting inspiratory pressure; risk of large uncontrolled VTs.
Adaptive support ventilation (PC-IMV optimal/intelligent): closed-loop mode using patient sex/height to target ideal minute ventilation, optimizing VT/rate combinations; may produce non–lung-protective VTs.
Biphasic/bilevel (PC-IMV set point): spontaneous breathing superimposed on PC breaths with normal I:E.
APRV (PC-IMV set point): prolonged high pressure (T-high 4–6 s) with a very short release (T-low often ≤1 s, PEEP often 0) to recruit and stabilize alveoli; set release to maintain peak expiratory flow ~75% (not below 25%).
Proportional modes — PAV and NAVA (PC-CSV servo): support delivered in proportion to patient effort; NAVA triggers off diaphragm electrical activity, avoiding air-leak/intrinsic-PEEP triggering problems.
Spontaneous breathing benefits: improved venous return, reduced RV afterload, better dependent/peridiaphragmatic ventilation, and less need for deep sedation/paralysis.
Key complications
Tension pneumothorax: rising peak pressure, high-pressure/low-volume alarms, absent breath sounds, JVD, subcutaneous emphysema, hypotension → PEA; treat with immediate needle decompression and chest tube.
Dynamic hyperinflation: air trapping from shortened expiratory time or obstructive disease; manage by reducing VT and rate, shortening inspiratory time, accepting permissive hypercapnia, and sometimes disconnecting to allow full exhalation.
Ventilator-associated events (VAE): CDC surveillance definitions — ventilator-associated complication (≥20% FiO₂ increase or ≥3 cm H₂O PEEP rise after stability), infection-related VAC (adds temperature/WBC criteria plus new antimicrobials), and possible VAP (adds pulmonary infection criteria ≥72 h after intubation). These are quality-reporting definitions, not clinical care guides.
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