Chapter 3: How a Breath Is Delivered

Vocabulary and key concepts from Chapter 3 on mechanical ventilation breath delivery mechanics, phase variables, and pressure-volume relationships.

Equation of motion

Describes the relationships among pressure, volume, and flow during a spontaneous or mechanical breath.

Transrespiratory pressure ($P_{TR}$)

The energy (i.e., pressure) required to establish a pressure gradient to move gas into the lungs.

Elastic recoil pressure ($P_E$)

The elastic load offered by the lungs and chest wall that must be overcome to move air into the lungs.

Flow-resistance pressure ($P_R$)

The airway resistance load produced as gas flows through the conducting airways.

Pressure-controlled breathing

A mode where the ventilator maintains the pressure waveform in a specific pattern that is unaffected by lung characteristics.

Volume-controlled breathing

A mode where the ventilator maintains the volume and flow waveform in a specific pattern that is unaffected by lung characteristics.

Trigger variable

The phase variable responsible for beginning inspiration.

Time triggering

A mechanism that delivers a mandatory breath after a set time has elapsed, based on the total cycle time ($TCT$).

Total cycle time ($TCT$)

The sum of inspiratory time ($T_I$) and expiratory time ($T_E$), expressed as $TCT = T_I + T_E$.

Limit variable

A variable (pressure, volume, flow, or time) that can reach a certain value during inspiration but does not end the inspiratory phase.

Maximum safety pressure

A limit typically set at $10\,cm\,H_2O$ above the average peak inspiratory pressure (PIP) to prevent lung damage.

Cycle variable

The phase variable responsible for ending inspiration.

Baseline variable

The parameter (usually pressure) that establishes the baseline during expiration before inspiration is triggered.

Tube compressibility

A factor in volume-cycled ventilation where approximately $1$ to $3\,mL$ of gas is lost to the tubing for every $1\,cm\,H_2O$ of measured airway pressure.

Flow-cycled ventilation

A mechanism where inspiration ends once the flow has decreased to a predetermined percentage of the peak inspiratory flow, commonly between $5\%$ and $80\%$.

Inflation hold (Inspiratory pause)

A maneuver that maintains air in the lungs at the end of inspiration to measure plateau pressure.

Auto-PEEP (Intrinsic PEEP)

Air trapping and hyperinflation that occurs when an adequate amount of time is not provided for exhalation.

Positive End-Expiratory Pressure (PEEP)

The application of baseline pressure above zero (atmospheric) during the expiratory phase.

Expiratory retard

An expiratory adjunct designed to mimic pursed-lip breathing by adding resistance to exhalation.

Spontaneous breaths

Breaths initiated by the patient where volume and flow delivery are determined by patient demand rather than set ventilator values.

Mandatory breaths

Breaths where the ventilator determines the start time (time triggering) or tidal volume (or both).