Mechanical Ventilation and Respiratory Mechanics Vocabulary

Vocabulary definitions derived directly from Pilbeam's Mechanical Ventilation Chapter 1 notes covering respiratory mechanics, compliance, resistance, pressure gradients, time constants, and ventilation modes.

Compliance ($C$)

The ease with which a structure such as the lungs distends, calculated as $C = \frac{\Delta V}{\Delta P}$.

Normal compliance for spontaneously breathing patients

50 to 170 mL/cm H2O

Elastance ($e$)

The ease with which a structure returns to its original shape after being stretched; the opposite of compliance.

Static Compliance ($C_s$)

Compliance measured during static or no-flow conditions in mechanically ventilated patients, calculated as:

Cs = (Exhaled Vt) / (Pplat - EEP)

Normal compliance for intubated male patients:

40 to 50 mL/cmH2O, up to 100 mL/cmH2O

Normal compliance for intubated female patients:

35 to 45 mL/cmH2O, up to 100 mL/cmH2O

Airway Resistance ($R_{AW}$)

The pressure required to overcome resistance in the airways to gas flow, calculated as:

Raw= Pta/ Flow   OR    Raw= (PIP - Pplat) / Flow

Note: Pta= Paow - Palv

Note: Flow needs to be converted into L/s by diving the L/m by 60 seconds

Raw in the conductive airways depends on:

• gas viscosity, gas density, the length and diameter of the tube, and the flow rate of the gas through this tube. 

• In mechanically ventilated patients the viscosity, density, and tube of airway length remain fairly constant. However, the airway lumen can still change from things like secretions or bronchospasms. 

Normal Resistance for spontaneously breathing patients

0.6 to 2.4 cmH2O/(L/s)

Normal Resistance for Intubated patients:

Around 6cmH2O/(L/s) or higher (airway resistance increases as endotracheal tube size decreases)

Viscous Resistance

Resistance to flow occurring when new gas molecules flow through airways over older gas molecules, causing energy (pressure) against airway walls.

Transairway Pressure ($P_{TA}$)

The pressure difference between the airway opening and the alveolus ($P_{TA} = P_{AWO} - P_{ALV}$)

• Represents the gradient required to produce airflow in conductive airways by overcoming resistance.

Transthoracic Pressure ($P_{TT}$ or $P_W$)

The pressure difference between the alveolar space and the body surface ($P_{TT} = P_{ALV} - P_{BS}$).

• Represents the pressure required to overcome the elastic recoil of the lungs and chest wall to expand both at the same time.

Transpulmonary Pressure ($P_{TP}$ or $P_L$)

The pressure difference between the alveolar space and the pleural space ($P_{TP} = P_{ALV} - P_{PL}$), also called alveolar distending pressure.

• Represents the pressure required to maintain alveolar inflation, also called the alveolar distending pressure. 

Transrespiratory Pressure ($P_{TR}$)

The pressure difference between the airway opening and the body surface.

(Ptr) = Pawo - Pbs or (Pawo - Pbs) = (Palv - Pbs) + (Pawo - Palv)

• Represents the pressure required to inflate the lungs during positive pressure ventilation. 

Time Constant

The product of compliance and airway resistance ($\text{Time Constant} = C \times R_{AW}$), representing the time required for a lung unit to fill or empty.

Note: For the calculation, C needs to be converted into L, by diving the mL by 1000.

1 Time Constant

63%

2 Time Constants

86%

3 Time Constants

95%

4 Time Constants

98%

5 Time Constants

99%

Note: After 5 Time Constants the lung is considered to contain 100% of Vt or 100% Vt has been exhaled.

Fast Lung Units

Lung units with short time constants that fill and empty rapidly, typically associated with normal or low $R_{AW}$ and decreased compliance, such as in Interstitial Fibrosis.

Slow Lung Units

Lung units with long time constants that take longer to fill and empty, typically associated with increased $R_{AW}$, increased compliance, or both, such as in Emphysema or Bronchospasms.

Negative Pressure Ventilation

Ventilation mode that lowers pressure at the body surface ($P_{BS}$) to generate negative pressure around the thoracic cavity, drawing air into the alveoli similar to spontaneous breathing.

Positive Pressure Ventilation

Ventilation mode where a mechanical ventilator pushes air into the lungs by increasing pressure at the airway opening ($P_{AWO}$).

High Frequency Jet Ventilation (HFJV)

A type of high frequency ventilation that delivers pressurized jets of gas into the lungs at high frequency.

High Frequency Oscillatory Ventilation (HFOV)

A type of high frequency ventilation that pushes gas in during inspiration and draws gas out during exhalation.

High Frequency Percussive Ventilation (HFPV)

A type of high frequency ventilation providing high frequency ambient pressure pulses superimposed on conventional positive pressure breaths.

Peak Inspiratory Pressure (PIP)

The highest pressure recorded at the end of inspiration on a mechanical ventilator, equal to $P_{TA} + P_{ALV}$.

Plateau Pressure ($P_{\text{plat}}$)

Pressure measured during an inspiratory hold at the end of inspiration when flow is zero, reflecting elastic recoil of the lungs and chest wall on alveolar gas volume.

Baseline Pressure (EEP)

The end-expiratory pressure level on top of which delivered ventilatory pressures build.

Extrinsic PEEP

Positive end-expiratory pressure deliberately applied by the operator setting a baseline pressure above $0\,\text{cmH}_2\text{O}$.

Intrinsic PEEP (Auto-PEEP)

Unintended baseline pressure above $0\,\text{cmH}_2\text{O}$ caused by incomplete exhalation and air trapping when no extrinsic PEEP has been set.

Proximal Airway Pressure

Airway pressure measured close to the mouth, providing the most accurate reading of patient airway pressure.

Inspiratory Accessory Muscles

Muscles used to assist inspiration when work of breathing is increased, including the Scalene, Sternocleidomastoid, Pectoralis minor and major, and Trapezius.

Expiratory Accessory Muscles

Muscles used to assist exhalation, including the Rectus abdominis, External Oblique, Internal Oblique, Transverse abdominal, Serratus anterior and posterior, Latissimus dorsi, and Internal intercostals.

End inspiration pressures

• The intrapleural pressure drops from -5cmH2O to -10cmH2O

• Note: The negative intrapleural pressure is transmitted to the Intrapulmonary/intraalveolar pressure, so it becomes around -3 to -5cmH2O.

• Pta becomes 5cmH2O. (Pta = Pawo - Palv)

(0) - (-5)= 5cmH2O

• Pressure at the mouth or nose (airway opening) is 0 and it is greater than the pressure inside the alveoli which is -5cmH2O. 

  • This creates a pressure gradient that leads to air moving from the airway opening into the alveoli causing them to expand. 

  • Volume of gas (air) builds up inside the alveoli until the intraalveolar pressure increases to 0 cmH2O. 

    • This causes airflow to stop as now the atm/ airway opening pressure and the alveoli pressure are both equal to zero. Marking the end of inspiration. 

End Expiration

• Intrapleural pressure increases to -5cmH2O

• Intraalveolar pressure now increases to +5cmH2O

  • Since intraalveolar pressure is now greater than the airway opening pressure (0), the pressure gradient causes air to now move to the outside. 

  • Air continues exiting the lungs until the intraalveolar and airway opening pressures are both equal to zero. Marking the end of expiration.