Respiratory Failure & Mechanical Ventilation

Phase variables with trigger, limit, and cycle criteria can be used to characterize breath types during mechanical ventilation, four different breath types can be generated based on different phase variables; spontaneous, supported, assisted and controlled breaths.

Spontaneous Breath

Spontaneous breath is completely regulated by the patient with no contribution of the ventilator. The breath is triggered and cycled by the patient with no set target on the ventilator. The baseline variable can be set with positive pressure (Continuous Positive Airway Pressure: CPAP) (A)

Supported Breath

Supported breath is triggered by the patient (pressure or flow trigger), the target (limit) is set as pressure and the cycle variable being a percentage of the peak inspiratory flow (patient-triggered, pressure-limited and flow-cycled breath) (B)

Cycle variables determine how a breath is ended. A positive pressure breath will always end because a variable has reached a set value; this can be pressure, volume, flow or time. When a ventilator is set to pressure cycle, it delivers the flow until a preset pressure is reached, at which time inspiratory flow ends and expiratory flow begins. When a ventilator is set to volume cycle, it delivers the flow until a preset volume has passed through the control valve, at which time inspiratory flow stops and expiratory flow begins. The flow may be used to terminate the inspiratory phase when it reaches a preset level usually 25% of the maximum inspiratory flow, at which time the inspiration ends. A pressure-cycled ventilator is one that stops pushing gas into a patient’s lungs when a preset airway pressure is reached. An example of a pressure-cycled ventilator is a Bird ventilator,…

Limit variable means restricting the magnitude of a variable during inspiration. A limit variable is one that can reach and maintain a preset level before inspiration ends (i.e. it does not end inspiration). Pressure, flow or volume can be limit variable but time cannot be a limit variable because limiting inspiratory time would cause inspiration to end. A limit variable does not terminate inspiration; it only sets an upper bound for pressure, flow or volume.

During mechanical ventilatory support, there are four phases during each ventilatory cycle: a) the trigger phase (breath initiation), b) the flow delivery phase, c) the cycle phase (breath termination), and e) the expiratory phase. Mechanically delivered breaths can be described by what determines the trigger, flow delivery, and cycle parameters for that breath.

Triggers are of three types: machine timer, pressure change, and flow change. With a machine timed trigger, the clinician sets a rate and mechanical breaths are initiated by a machine timer. With a pressure trigger, a patient effort pulls airway/circuit pressure negative and mechanical breaths are initiated when pressure exceeds the set negative pressure threshold (pressure sensitivity). With a flow trigger, a patient effort draws flow from the circuit (often from a continuous bias flow) and mechanical breaths are initiated when flow into the patient exceeds the set flow threshold (flow sensitivity). In the above figure the…

Analysis of pressure tracing curve of volume control mechanical ventilation.