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 pressure 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 6exhalation.
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.