Airway Resistance, Gas Flow, and Time Constants Flashcards

Vocabulary flashcards covering topics on airway resistance, laminar vs. turbulent gas flow, Poiseuille's law, time constants, auto-PEEP, and inspiratory/expiratory muscle strength testing.

Normal Airway Resistance (Adult)

The normal resistance to airflow in a spontaneously breathing adult, ranging from $0.6$ to $2.4\text{ cmH}_2\text{O/L/sec}$.

Laminar Flow

A streamlined, organized gas flow pattern occurring primarily in the lower conducting airways, where the required driving pressure depends on gas viscosity.

Turbulent Flow

A chaotic, disorganized gas flow pattern occurring in the upper airways, large airways (trachea and main stem bronchi), and at bifurcations, requiring higher driving pressure influenced by gas density.

Poiseuille's Law

A law describing laminar flow stating that driving pressure is directly proportional to tube length, flow rate, and gas viscosity, and inversely proportional to the radius raised to the fourth power ($r^4$).

Effect of Halving Airway Radius

According to Poiseuille's Law, reducing the airway radius by half causes the required driving pressure to increase by $16$ times ($2^4$).

Bronchospasm

The constriction of bronchial smooth muscle in the airways, resulting in a decreased airway radius and increased airway resistance.

Heliox

A low-density gas mixture used to reduce the driving pressure needed to overcome upper and large airway obstructions where turbulent flow is present.

Time Constant

The calculated time necessary for the lungs to fill and empty, calculated as compliance multiplied by airway resistance ($\text{Time Constant} = \text{Compliance} \times \text{Airway Resistance}$).

One Time Constant

The time required for the lung to fill or empty by $63\%$.

Five Time Constants

The total time required for the lungs to achieve almost complete ($99.3\%$) filling or emptying.

Normal Adult Inspiratory Time (I-time)

The standard duration set for inhalation on a mechanical ventilator for adults, which is typically $1\text{ second}$.

Auto-PEEP (Intrinsic PEEP)

Trapping of gas in the lungs due to inadequate expiratory time or high expiratory resistance, leading to increased functional residual capacity (FRC) and residual volume (RV).

Diaphragm Fatigue

A reversible decrease in the force-generating capacity of an actively working diaphragm that occurs due to sustained high work of breathing.

Abdominal Paradox

An abnormal sign of respiratory muscle fatigue where the abdomen sucks inward during inspiration while the rib cage expands.

Maximum Inspiratory Pressure (MIP / NIF / PI max)

A clinical test measuring inspiratory muscle (diaphragm) strength performed by inhaling against an occluded valve to generate negative pressure.

Maximum Expiratory Pressure (MEP)

A test measuring expiratory muscle strength (primarily abdominal muscles) performed by exhaling against a closed valve to generate positive pressure.