Respiratory Mechanics and Lung Function Notes

Respiratory Mechanics and Lung Function

Objectives

• Discuss the principles of static and dynamic lung compliance.
• Define airway resistance.
• Explain the relationship among ventilation, lung compliance, and airway resistance.
• Discuss Hooke’s law and its application to elastic recoil.
• Describe how pressure–volume curves illustrate airway dynamics.
• Explain the three patterns of gas flow through the airways and their impact on airway resistance.
• Explain the difference between positive pressure ventilation, negative pressure ventilation, and intermittent abdominal pressure ventilation.
• List examples of conditions that may cause shifts in pulmonary pressure–volume curves.

Mechanics of the Lung and Chest Wall

• Respiratory mechanics describes the interaction of pressures and forces for breathing.
• Changes in lung physiology due to pulmonary disease are identified via changes in respiratory mechanics from baseline.
• Lung function is an expression of respiratory mechanics measured by pressure, volume, and flow.
• Lungs and chest wall act as two springs with counteracting pressures and forces.
• Inspiration: muscles pull chest wall up and out, expanding lungs.
• Exhalation: lungs pull inward, contracting the chest wall.
• Factors influencing lung and chest wall interaction:
  • Compliance (flexibility to expand)
  • Airflow velocity and volume
  • Pressure exerted by airflow
  • Respiratory muscle activity
  • Resistance to airflow
  • Elastic recoil

Compliance

• Compliance is the measurement of elastic capability, or ease of stretching, of an organ/system.
• Defined as change in volume per unit change in pressure.
• Both lungs and chest wall have compliance.
• Total respiratory compliance is the combined compliance of lungs and chest wall.

Lung Compliance

• Lung compliance is also called lung distensibility; high compliance means easy inflation, low compliance means difficult inflation.

• High compliance: normal aging/emphysema.

• Low compliance: stiffer lungs accepting small air volume.

• Reduced compliance causes:

  • Pulmonary fibrosis (stiffer tissue)
  • Increased pulmonary venous pressure (engorged vasculature)
  • Restricted alveolar expansion (atelectasis/pulmonary edema)

• Lung compliance calculation: change in lung volume (L) / change in transmural pressure (cm H2O).

• Compliance = \frac{\Delta Volume}{\Delta Pressure}

• Example:

  • Adult inhales 500 mL of air.
  • Intrapleural pressure: -5 cm H2O (pre-inspiration) to -10 cm H2O (end-inspiration).
  • Convert mL to L: 500 mL = 0.5 L
  • \Delta Volume = 0.5 L
  • \Delta Pressure = -10 cm H2O - (-5 cm H2O) = -5 cm H2O
  • Compliance = \frac{0.5 L}{-5 cm H2O} = -0.1 L/cm H2O

• Static compliance: measured without airflow (end of inspiration/exhalation); normal adult = 200 mL/cm H2O.

• Dynamic compliance: measured during airflow (inhalation); always less than or equal to static compliance.

• Normal static to dynamic compliance ratio: 1:1.

• Adult lung compliance values are generally higher than those in infants and children.

• Specific compliance: corrects for lung size differences.

• Specific \space compliance = \frac{Compliance}{FRC}

  • FRC = functional residual capacity.

Chest Wall Compliance

• Thorax bone/muscle elastic properties directly affect ventilation.
• Chest wall compliance: transmural pressure across chest wall vs. chest cavity volume.
• Like lung compliance, it can be static or dynamic.

Total Respiratory Compliance

• Individual chest/lung compliance ≈ 0.2 L/cm H2O.
• Total respiratory compliance (CT): normal compliance of chest wall and lungs working together ≈ 0.1 L/cm H2O.
  • CT = CL + Ccw
    • CT = Total compliance
    • CL = Lung compliance
    • Ccw = Chest wall compliance

Pressure-Volume Curves

• Changes in ventilation mechanics (volume, pressure, airflow) plotted on pressure-volume curve.
• Volume on y-axis, pressure on x-axis.
• Lung compliance expressed as slope of pressure-volume curve.
• Increased compliance: curve shifts upward.
• Decreased compliance: curve shifts downward.
• Chest wall compliance: volume changes on y-axis, pressure changes on x-axis.
• Chest wall movements counterbalance lungs.
• Transmural pressure closer to zero: chest cavity and volume decrease.
• Increased transmural pressure: chest wall expands, volume increases.

Elastance

• Lung compliance: measure of distensibility.
• Elastance/elastic recoil: ability of lungs to spring back after expansion.
  • EL = \frac{\Delta P}{\Delta V}
    • EL = Elastance
    • \Delta P = change in pressure in liters
    • \Delta V = change in volume in cm H2O
• Chest wall elastance can also be measured.
• Total elastance of respiratory system:
  • ET = EL + Ecw
    • ET = Total elastance
    • EL = Lung elastance
    • Ecw = Chest wall elastance

Hooke’s Law

• Spring stretch distance is proportional to applied force/load.
• Lung pressure is proportional to incoming air volume.
• More pressure = more lung expansion = greater air volume.
• Volume increases with pressure until elastic limits are reached.
• Elastance is the inverse of compliance; high compliance = low elastance, and vice versa.

Clinical Focus: Lung Hysteresis

• Inspiratory/expiratory arches in pressure-volume curve show lung volume differences during inspiration vs. expiration.
• Lung volumes at given pressure are less during inspiration than expiration.
• The difference between the two curves is called hysteresis.

Pressure Gradients

• Pressure gradient is the change in pressure per unit distance; air flows from high to low pressure.
• Pressure gradient: difference between high and low pressure.
• Measuring pressure gradients is essential to understanding breathing and managing ventilation.
• Baseline airway pressure (reference point) = zero.
• Zero airway pressure = 1 atm or 760 mm Hg at sea level.
• Pressure below 1 atm/760 mm Hg: negative/subatmospheric.
• Pressure above 1 atm/760 mm Hg: positive/supra-atmospheric.
• Normal breathing pressures at mouth (Pam) and nose (Pno) are usually zero.
• Pressure outside the chest/body surface (Pbs) is also zero.
• Alveolar pressures (PA) change with chest wall and lung movement.
• Inspiration: thoracic muscles lift chest, creating negative alveolar pressure, moving air from mouth to alveolus.
• When PA equals mouth pressure, gradient is zero, and airflow stops.
• Exhalation: muscles relax, lung recoil pulls chest inward, creating positive pressure from alveoli to mouth/nose.
• Air moves out until gradient reaches zero, and the cycle restarts.
• Three pressure gradients during ventilation:
  • Transrespiratory pressure gradient
  • Transpulmonary pressure gradient
  • Transthoracic pressure gradient

Pressure Gradients: Transrespiratory Pressure

• Transrespiratory pressure (Prs): difference between alveolar pressure and body surface area.
  • Prs = PA − Pbs
    • Prs = Transrespiratory pressure
    • PA = Alveolar pressure
    • Pbs = Body surface area
• Describes pressure required to inflate lungs/airways; occurs early in inspiration.
• Also called transairway pressure (PTA); Prs and PTA are used interchangeably in clinical practice.
• Used when discussing positive pressure mechanical ventilation.

Pressure Gradients: Transpulmonary Pressure

• Transpulmonary pressure (PL): pressure needed to maintain alveolar inflation; difference between alveolar and pleural space.
• Also called alveolar distending pressure.
• Increased PL = increased alveolar air volume.
• Excessive PL = overextended alveolus.
• Insufficient PL = decreased alveolar air, potentially causing atelectasis.
• PL = PA − Ppl
  • PL = Transpulmonary pressure
  • PA = Alveolar pressure
  • Ppl = Intrapleural pressure

Pressure Gradients: Transthoracic Pressure

• Transthoracic pressure (PW): total pressure to expand/contract lungs & chest wall; difference between transpulmonary pressure and body surface.
  • Pw = PL − Pbs
    • Pw = Transthoracic pressure
    • PL = Transpulmonary pressure
    • Pbs = Body surface area

Airway Resistance

• Airway resistance: friction of airways/lung tissue against airflow during inhalation/exhalation.
• Factors affecting airway resistance:
  • Airway radius
  • Airflow velocity
  • Airflow pattern
  • Gas physical properties
• Airway resistance is inversely proportional to airway radius; larger airway = lower resistance, smaller airway = higher resistance.
• Gas velocity affects airway resistance.
• Narrowed airway (e.g., asthma) increases gas velocity, causing turbulent flow.
• Turbulent flow increases friction, thus increasing airway resistance.
• Inhaled gas properties alter airway resistance.
• Heliox (helium/oxygen mix) reduces resistance due to lower density, easing flow past obstructions in conditions like asthma or COPD.

Airway Resistance: Patterns of Gas Flow

• Airflow pattern affects RAW; classified as laminar, turbulent, or tracheobronchial/transitional.
• Laminar flow: uninterrupted, parallel movement of particles/molecules; also called streamline flow.
• Associated with even, unobstructed gas movement; molecules move parallel to airway walls.
• Creates parabolic/cone-shaped movement with faster molecules in the center; common in small airways (< 2 mm).
• Laminar flow occurs with calm, relaxed, low-flow, low-pressure breathing and is associated with low RAW.
  • Q = \frac{\Delta P \pi r^4}{8 \mu L}
    • Q = flow rate
    • \Delta P = change in pressure
    • r = radius
    • \mu = viscosity
    • L = length
• The Hagen–Poiseuille equation explains pressure increases as air moves into smaller airways for effective gas exchange.
• Turbulent flow: erratic/choppy movement with molecules churning, bumping into each other and airway walls; molecules not in synchrony.
• Higher airway resistance due to erratic molecular movement; occurs in high-flow, high-pressure breathing and larger airways (> 2 mm diameter).
• Tracheobronchial/transitional flow: mix of laminar/turbulent flow at airway branches.
• Air molecules moving in laminar pattern hit branching portion, creating resistance.

Airway Resistance: Reynolds Number

• Airflow type can be calculated using the Reynolds number.
  • Re = \frac{Density \times Velocity \times Diameter}{Viscosity}
• Low Re is associated with laminar flow, whereas a high Re is associated with turbulent flow.
• If Re < 2000, flow is laminar; if Re > 4000, flow is turbulent; if 2000 < Re < 4000, flow is transitional.

Ventilation Time Constants

• Ventilation time constant: time in seconds to inflate a lung portion.
• Measure of time for alveolar pressure to reach 63% of airway pressure change.
  • Time \space Constant = Resistance \times Compliance
• Time constant equation reflects: time for lungs to fill during inspiration at a predictable percentage due to the exponential nature of the filling process (inspiration)
• Normal inspiration divided into five intervals:
  • 1st interval: ~63% filled
  • 2nd interval: ~86% filled
  • 3rd interval: ~95% filled
  • 4th interval: ~98% filled
  • 5th interval: ~99% filled
• Increased RAW/compliance means longer inflation time and longer time constants.
• Decreased RAW/compliance means rapid inflation and shorter time constants.
• Time constants measure respiratory disorder effects on compliance.
• Restrictive disorders (ARDS, atelectasis, ILD, pneumonia, pulmonary edema, effusions) have decreased lung compliance and decreased time constants.
• Patients experience increased respiratory rate to offset decreased compliance and maintain constant air volume.
• Obstructive disorders (asthma, bronchitis, emphysema) have increased RAW and increased time constants.
• Patients take slower, deeper breaths to ease air past obstructions and maintain volume.
• Time constants are used to calculate dynamic compliance.

Representing Lung Dynamics with Graphics

• Airway dynamics illustrated via pressure-volume and pressure-time curves.
• Static/dynamic compliance can be plotted as pressure-volume curves, also called flow volume loops.
• Pressure-volume curve during mechanical ventilation assesses lung compliance/resistance.
• In dynamic pressure-volume curve: bottom portion is inhalation, upper is exhalation, line bisecting is static compliance, lower line is dynamic compliance.
• Increased baseline pressure above zero is called positive end-expiratory pressure (PEEP).
• PEEP is the pressure in the lungs above atmospheric pressure that exists at the end of expiration.
• Extrinsic PEEP: intentionally added by ventilator operator, elevating baseline pressures to alleviate alveolar collapse.
• Intrinsic PEEP/auto-PEEP: incomplete exhalation leaving air in lungs, elevating functional residual capacity and increasing work of breathing.
• Pressure-time curve depicts pressure on y-axis, time on x-axis.
• Peak airway pressure (Ppeak): maximum pressure applied during inspiration.
• Plateau pressure (Pplat): lower airway pressure after inspiratory pause, reflecting elastic recoil.
• Plateau pressure estimates transalveolar pressure and alveolar distention.
• High transalveolar pressures increase risk of barotrauma and lung injury.

Clinical Focus: Positive Pressure Ventilation, Negative Pressure Ventilation, and Intermittent Abdominal Pressure Ventilation

• Mechanical ventilation: external device supporting air movement into/out of the lungs.
• Goal: adequate gas exchange with minimal complications.
• Positive pressure ventilation: gas into airways inflates lungs; achieved via manual resuscitator or mechanical ventilator with mask/tube.
• Negative pressure ventilation: external subatmospheric pressure lifts chest wall upward/outward (e.g., iron lung, cuirass).
• Intermittent abdominal pressure ventilation: external intermittent pressure pushes up on diaphragm to support ventilation.

Summary

• Ventilation is an interaction of pressure, flow, volume, compliance, and resistance.
• Respiratory mechanics describes these interactions.
• Compliance is the ability to stretch/expand; can be measured in both lungs and chest wall.
• Static compliance: measured without gas flow, illustrated as the slope on a pressure-volume curve.
• Dynamic compliance: measured during gas flow, illustrated by the curved inspiration line on a pressure–volume curve.
• Lungs that are easily inflated have high compliance, whereas lungs that are difficult to inflate have low compliance.
• Lung compliance in adults, infants, and children is proportionally equal.
• Compliance values are higher in adults because they can accept higher pressures and volumes than a child’s.
• Specific compliance corrects for size differences.
• Elastance is the opposite of compliance.
  Elastic recoil is the ability of the lungs and chest wall to spring back after expansion during inspiration.
• Pressure gradients play a significant role in ventilation.
• Transrespiratory pressure gradient is the pressure difference between the airway opening and the alveolus, also called the transairway gradient.
• Transrespiratory pressure is the pressure that must be generated to overcome airway resistance.
• Transpulmonary pressure gradient is the pressure difference between the alveolar space and the pleural space, also called the alveolar distending pressure.
• Transthoracic pressure gradient is the pressure difference between the alveolar space and the body’s surface and is the pressure needed to expand or contract the lungs and the chest wall.
• Airway resistance is the friction of the airways and lung tissue to airflow during inhalation and exhalation.
• Factors that affect airway resistance are the diameter of the airways, the velocity of the gas, the pattern of airflow, and the physical properties of the gas.
• The flow pattern in which the air moves through the airways has a significant impact on airway resistance.
• The three types of flow patterns are laminar, turbulent, and tracheobronchial or transitional.
• Changes in the respiratory mechanics, including the changes in volume, pressure, and airflow that occur during ventilation, can be plotted on either a pressure–volume curve or a pressure–time curve.
• In particular, the compliance, peak inspiratory pressure, plateau pressure, and PEEP can be graphically represented in these graphics.