Noninvasive Mechanical Ventilation

Noninvasive mechanical ventilation (NIV) is the provision of mechanical ventilatory support without invasive endotracheal intubation, delivered through external patient interfaces using positive pressure ventilation. NIV represents one of the major advances in pulmonary and critical care medicine in the past three decades, allowing effective ventilatory support while avoiding the complications of invasive ventilation including ventilator-associated pneumonia, sedation requirements, hemodynamic instability, and prolonged ICU stays. NIV has demonstrated mortality benefits in specific patient populations including COPD exacerbation, cardiogenic pulmonary edema, and select cases of acute respiratory failure.

Modes and equipment: 1) CPAP (continuous positive airway pressure) — single pressure (typically 5-15 cmH2O) maintained throughout respiratory cycle; primary indications include cardiogenic pulmonary edema (decreases venous return, decreases work of breathing, improves V/Q matching, reduces afterload), obstructive sleep apnea, post-extubation prophylaxis in selected patients, hypoxemic respiratory failure with intact ventilation; 2) BiPAP (bilevel positive airway pressure) — different inspiratory (IPAP, typically 10-25 cmH2O) and expiratory (EPAP, typically 4-10 cmH2O) pressures; IPAP supports inspiration providing pressure support reducing work of breathing, increasing tidal volumes, improving alveolar ventilation and CO2 elimination; EPAP maintains airway patency, prevents alveolar collapse, decreases auto-PEEP, increases functional residual capacity; standard mode for hypercapnic respiratory failure; 3) Pressure support ventilation (PSV) — patient-triggered breath with set inspiratory pressure delivered until flow decreases to threshold; 4) Pressure assist-control with backup rate — provides minimum number of breaths per minute even if patient does not trigger; useful for neuromuscular weakness, central apneas; 5) Average volume-assured pressure support (AVAPS) — automatically adjusts pressure delivery to maintain target tidal volume despite changing patient mechanics; useful for variable lung compliance; 6) AutoPAP for OSA — algorithm-driven variable pressure based on detected apneas/hypopneas; 7) Volume cycling rare in NIV; 8) Equipment includes ventilator (dedicated NIV ventilator like Trilogy, Astral or critical care ventilator with NIV mode), interface (mask, helmet), tubing, humidifier, oxygen source.

Patient interfaces: 1) Nasal mask — covers nose only, comfortable for chronic use, lower mouth leak with normal lip seal, less claustrophobia, but mouth opening reduces effectiveness; 2) Oronasal (full face) mask — covers nose and mouth, prevents mouth leak, often preferred for acute respiratory failure where mouth breathing is common; 3) Total face mask — covers entire face including chin and forehead; useful for facial deformities, claustrophobia in standard masks; 4) Nasal pillows — for chronic stable disease and home use; 5) Helmet — transparent helmet with sealed cushion at neck; advantages include reduced facial pressure, no leak, better tolerance for long-term use, reduced claustrophobia for some; disadvantages include higher dead space requiring higher pressure support, need for high-volume CO2 washout, cost; some literature suggests helmet superior outcomes in some respiratory failure populations; 6) Mouthpiece NIV — for daytime support in neuromuscular disease, sip ventilation; 7) Custom-molded interfaces for specific patient needs.

Mechanism of action and physiologic effects: 1) Reduction of work of breathing — pressure support augments inspiratory effort decreasing diaphragmatic and accessory muscle work; 2) Increased alveolar ventilation — improved tidal volumes increase CO2 elimination especially in hypercapnic patients; 3) Recruitment of atelectatic alveoli — EPAP recruits alveoli increasing functional residual capacity; 4) Improved oxygenation — increased FRC, decreased shunt, improved V/Q matching, supplemental oxygen via mask; 5) Decreased afterload (CPAP) — beneficial in cardiogenic pulmonary edema; 6) Decreased venous return — useful in pulmonary edema; 7) Auto-PEEP compensation — EPAP counteracts dynamic hyperinflation in COPD; 8) Diaphragmatic rest — particularly important in fatigued respiratory muscles; 9) Compared with invasive ventilation, advantages include preserved upper airway defense mechanisms, ability to communicate, swallow, and cough, reduced sedation requirements, reduced ventilator-associated pneumonia, reduced ICU mortality in appropriate patients.