Table 2: Mechanical ventilation classifications.


Positive pressure ventilation  
Airway pressure applied via face mask or nasal prongs; positive pressure beginning in the upper airway causes gas flow to the lungs until the ventilator breath is terminated; as the airway pressure drops to zero, elastic recoil of the chest accomplishes passive exhalation by pushing the tidal volume out.
Two types: Bi-level Positive Airway Pressure (BiPAP)—provides both inspiratory and expiratory positive airway pressure; Continuous Positive Airway pressure (CPAP)—provides only single level of airway pressure.    
Negative pressure ventilation  
Delivered via cuirass (shell); vacuum pump creates negative pressure in the chamber, resulting in reduced intrapulmonary pressure and allowing air flow into the lungs and expansion of the chest; when vacuum terminated, negative pressure applied to chest becomes zero, recoil of chest and lungs permits passive exhalation.


Positive pressure ventilation   
Airway pressure applied through an endotracheal tube or tracheostomy tube; gas flows along pressure gradient between upper airway and alveoli; achieved by delivering either a pre-determined volume or a set pressure to the lungs on a regular or a demand basis; PPV modes defined by inspiratory parameters.
Volume modes: Controlled Mechanical Ventilation (CMV)—100% control by ventilator, patient has minimal or no respiratory effort); Assist Control Ventilation (ACV)—if patient fails to initiate breathing within prescribed time, ventilator triggers breath); Intermittent Mandatory Ventilation (IMV)—combination of spontaneous breathing and CMV as the ventilator provides a positive pressure breath at a pre-determined frequency); Synchronized Intermittent Mandatory Ventilation (SIMV)—patient triggers mandatory breath in assist mode thus synchronizing with own respiratory effort.
Pressure modes: Pressure Support  Ventilation (PSV)—patient-triggered breath with a preset positive pressure thus using patient’s own inspiratory timing and tidal volume; used for weaning; Pressure Control Ventilation (PCV)—time-initiated, pressure-limited and time-cycled mode; intended for patients fully dependent on MV; Pressure Assist Control Ventilation (PACV)—all breaths machine delivered at preset tidal volume; allows for patient triggering of breaths as described with PCV.
Positive End Expiratory Pressure (PEEP)—primary expiratory parameter; can be added to all above modes; used to increase functional residual capacity and distend alveoli.