Respiratory System Mechanics: Compliance and Resistance

A ventilator pressure rises because the respiratory system must expand and gas must move through a resistive pathway. Those requirements change with lung volume, airflow, chest wall behavior, and respiratory effort. Separating them turns a high airway pressure from an isolated number into a physiological explanation.

22.1 Expansion and Flow Require Different Pressures

Inflating the respiratory system requires pressure to overcome elastic recoil. The lung tends to recoil inward, while the chest wall has its own volume-dependent recoil. During a tidal inflation, the combined change in recoil determines how much additional pressure is needed to hold the larger volume.

Moving gas requires a pressure difference along the airway. Pressure falls as gas travels through the endotracheal tube and airways toward the alveoli. A faster flow or a narrower pathway generally requires a larger pressure difference. This resistive requirement exists while flow continues. It differs from the pressure needed to maintain an expanded respiratory system after flow stops.

These two requirements can change independently. Bronchospasm can increase the pressure needed to move gas without initially changing the pressure needed to hold a particular end-inspiratory volume. Loss of aerated lung can increase the pressure needed to expand the remaining ventilated system even when the conducting airway resistance is unchanged. A patient can also develop both abnormalities.

Mead and Whittenberger's human physiological measurements helped distinguish elastic and flow-related components using simultaneous pressure, volume, and flow signals. Their work established the value of examining these variables together rather than interpreting an airway pressure without its accompanying breath. 1

The pressure displayed by a ventilator is usually measured near the airway opening. During flow, that pressure exceeds downstream pressure by the pressure lost along the conducting pathway. When flow stops and communicating regions equilibrate, the resistive pressure difference falls. This is why an inspiratory pause can add information that the peak pressure cannot provide alone.

The starting pressure also matters. Inflation begins from an end-expiratory state, often maintained by positive end-expiratory pressure, or PEEP. The increase above that baseline is the pressure associated with the tidal breath. Comparing an end-inspiratory pressure with atmospheric zero would include both the baseline and tidal components.

22.2 The Equation of Motion Organizes the Breath

A simple model represents the respiratory system as an elastic compartment connected to a resistance. For a passive patient, its pressure during inspiration can be written as:

Airway pressure and total PEEP are expressed in centimeters of water, or cm H₂O. Elastance is the pressure change per liter of volume change, in cm H₂O/L. Multiplying it by volume in liters gives an elastic pressure in cm H₂O. Resistance is expressed in cm H₂O·s/L. Multiplying it by flow in liters per second gives a resistive pressure in the same pressure units.

Consider an illustrative instant during constant-flow inflation. Total PEEP is 8 cm H₂O. The delivered volume above end-expiration is 0.30 L. Elastance is 25 cm H₂O/L, and resistance is 10 cm H₂O·s/L. Inspiratory flow is 0.60 L/s.

The elastic contribution is 25 × 0.30 = 7.5 cm H₂O. The resistive contribution is 10 × 0.60 = 6 cm H₂O. Adding the baseline gives 8 + 7.5 + 6 = 21.5 cm H₂O at that instant. The example describes a model, not a ventilator prescription.

With constant flow and fixed resistance, the resistive contribution remains approximately constant during inflation. The elastic contribution grows as volume increases. With a decelerating flow, both contributions change over time. A pressure shape therefore cannot be interpreted without knowing how the ventilator delivered the breath.

Real lungs contain regions with different mechanics. Elastance can change with inflation, and resistance can depend on flow and volume. Tissue stress relaxation adds time dependence. Gas acceleration contributes inertial pressure, which the simple equation neglects. These omissions often permit useful bedside estimates, but they prevent the model from being a complete anatomical description.

Respiratory muscle activity adds another pressure source. The passive equation applies only when that source is absent or negligible. A calculation performed during strong effort can produce a plausible number while attributing the patient's contribution to the ventilator or to passive mechanics.

22.3 Compliance and Elastance Describe the Same Relationship in Opposite Directions

Compliance describes how much volume changes for a given pressure change. A more compliant system accepts more volume for the same increase in distending pressure. Elastance describes how much pressure changes for a given volume change. A system with high elastance requires a larger pressure increase to deliver the same volume.

Suppose an illustrative passive respiratory system gains 400 mL when its elastic pressure increases by 10 cm H₂O. Its compliance is 400 ÷ 10 = 40 mL/cm H₂O. Expressing 400 mL as 0.40 L gives an elastance of 10 ÷ 0.40 = 25 cm H₂O/L. The reciprocal agrees because 40 mL/cm H₂O is 0.040 L/cm H₂O, and 1 ÷ 0.040 = 25 cm H₂O/L.

These are reciprocal descriptions only when they refer to the same pressure–volume relationship and interval. Taking the reciprocal of a compliance in milliliters produces a different numerical value from taking the reciprocal after converting to liters. Units prevent that apparent contradiction.

The pressure–volume relationship is usually nonlinear over the full inflation range. A tidal compliance is an average slope across the tidal interval. It does not establish the slope at every volume within that interval. Changing PEEP changes the starting volume and may move the next breath onto a different part of the relationship.

Low respiratory system compliance can reflect reduced ventilated lung volume, changes in the behavior of open lung tissue, increased surface forces, or a stiffer chest wall. The number does not identify which contribution dominates. Conversely, a high compliance does not establish normal lung structure. Emphysematous lungs may inflate readily because elastic recoil has been lost, while expiration becomes difficult.

Compliance also depends on what is being expanded. A small functioning lung exposed to an ordinary tidal volume can show a low overall compliance even if the remaining open units have relatively preserved material properties. The measured slope describes the accessible system; it is not a direct assay of tissue stiffness per unit of lung.

22.4 Lung and Chest Wall Pressures Add

The ventilator expands the lung and moves the chest wall through the same tidal volume. The pressure required for that expansion is divided between them. Lung distension depends on transpulmonary pressure, the difference between alveolar and pleural pressure. Chest wall expansion depends on the pressure across the chest wall, with pleural pressure forming its inner boundary.

When the patient is passive and measurements describe the same volume interval, the pressure changes add. Their elastances consequently add:

Consider lung compliance of 100 mL/cm H₂O and chest wall compliance of 200 mL/cm H₂O in an illustrative linear model. A 400-mL inflation requires 400 ÷ 100 = 4 cm H₂O across the lung and 400 ÷ 200 = 2 cm H₂O across the chest wall. The combined pressure increase is 6 cm H₂O. Respiratory system compliance is therefore 400 ÷ 6 ≈ 67 mL/cm H₂O. Adding the two compliances would give the wrong result because the pressure requirements add.

A rise in abdominal pressure can push the diaphragm upward and alter chest wall mechanics. The same tidal volume may then require more airway pressure even without an equivalent increase in transpulmonary pressure. Obesity, posture, edema, and pleural disease can also affect the mechanical conditions, but body habitus alone cannot quantify the chest wall contribution.

Gattinoni and colleagues measured lung and chest wall mechanics in patients with pulmonary and extrapulmonary causes of ARDS. Their findings demonstrated that similar overall respiratory impairment can contain different lung and chest wall contributions. The small physiological study does not make the cause of ARDS an accurate substitute for measuring an individual patient's mechanics. 2

Esophageal pressure can help estimate pleural pressure under appropriate measurement conditions. It also requires calibration, attention to position, and recognition of regional differences. The next chapter develops those pressure relationships. At this stage, the essential distinction is that airway pressure includes the chest wall and cannot be equated automatically with lung-distending pressure.

22.5 An Inspiratory Hold Separates Flow from Expansion

During passive constant-flow inspiration, the peak airway pressure contains both elastic and flow-related components. An end-inspiratory occlusion stops flow while briefly retaining the delivered volume. Airway pressure then falls toward a plateau. The plateau provides an estimate of the pressure supporting the inflated respiratory system after the major conducting-airway pressure loss has disappeared.

The pressure decline is not always a single instantaneous step. The rapid initial fall relates mainly to cessation of flow through the conducting pathway. A slower decline can follow because tissue stress relaxes and gas redistributes between regions. The selected measurement time therefore influences the reported plateau.

Bates and colleagues analyzed interruption in mechanical models and showed that the initial and later pressure changes need not represent the same resistance. Their analysis explains why the entire peak-to-plateau difference cannot always be assigned exclusively to airway narrowing. 3

D'Angelo and colleagues studied rapid occlusion in anesthetized, paralyzed humans. Their pressure measurements separated airway-related resistance from time-dependent contributions in lung and chest wall tissues. This human physiological evidence supports interpreting the pause as a dynamic process rather than treating every point after flow stops as interchangeable. 4

A useful hold requires a stable volume, negligible leak, and absence of disruptive inspiratory or expiratory muscle activity. Pressure that rises, falls erratically, or changes with an obvious effort is not a reliable passive plateau. The shape of the tracing helps judge validity before the number enters a calculation.

Zero flow at the airway opening does not guarantee identical pressure in every alveolus. Regions with different time constants may continue to exchange gas internally. Completely closed or noncommunicating regions may not contribute to the measured pressure at all. The plateau is a global measurement of the communicating respiratory system under the conditions of that pause.

Longer occlusions can allow more equilibration but also alter the breath and provoke effort. Consistent measurement technique is therefore important when comparing trends. A change produced by a different hold duration should not immediately be labeled a change in disease severity.

22.6 Static Compliance Requires the Correct End-Expiratory Baseline

Static respiratory system compliance uses tidal volume and the difference between plateau pressure and total end-expiratory pressure. In bedside usage, “static” usually refers to this no-flow estimate over a finite hold. It does not mean that the respiratory system has reached an indefinitely unchanging equilibrium.

Consider an illustrative passive breath with tidal volume of 420 mL, plateau pressure of 24 cm H₂O, and total PEEP of 10 cm H₂O. The pressure increase supporting the tidal inflation is 24 − 10 = 14 cm H₂O. Compliance is 420 ÷ 14 = 30 mL/cm H₂O.

Suppose the ventilator is set to PEEP of 6 cm H₂O, while the measured total PEEP remains 10 cm H₂O because expiration is incomplete. Using the set value would give 24 − 6 = 18 cm H₂O, followed by 420 ÷ 18 ≈ 23 mL/cm H₂O. That calculation would make the system appear less compliant because it incorrectly attributes the intrinsic end-expiratory pressure to the tidal inflation.

Total PEEP can be estimated with an end-expiratory occlusion in a passive patient. As flow stops, pressure in communicating regions can equilibrate with the airway opening. Active expiration, inspiratory attempts, leaks, severe heterogeneity, or airway closure complicate that interpretation. A hold does not reveal pressure trapped behind every closed airway.

Tidal volume should represent the volume actually entering and leaving the patient. Circuit compression, leaks, and differences between inspired and expired volume can introduce errors. Modern compensation can reduce some errors, but the clinician still needs to recognize when the volume signal is unreliable.

A compliance trend is most interpretable when volume, PEEP, posture, effort, and measurement method are comparable. If any of these changes, the new slope may describe a different operating condition. That information can be clinically useful, but it requires an explanation beyond “the lung improved” or “the lung worsened.”

22.7 Resistance Depends on Flow and on the Pathway Being Measured

Resistance expresses the pressure difference needed to produce a given flow. During constant-flow passive inflation, an effective inspiratory resistance can be estimated from the peak-to-plateau difference divided by the immediately preceding flow:

Suppose peak pressure is 34 cm H₂O, plateau pressure is 24 cm H₂O, and constant inspiratory flow is 30 L/min. The pressure difference is 34 − 24 = 10 cm H₂O. Converting flow gives 30 ÷ 60 = 0.50 L/s. The estimated resistance is 10 ÷ 0.50 = 20 cm H₂O·s/L.

The result includes the pathway between the pressure sensor and the communicating lung. An endotracheal tube, secretions, a kink, an obstructed filter, and narrowed airways can each increase that pressure difference. The full peak-to-plateau estimate also includes time-dependent pressure losses discussed earlier. It is an effective respiratory measurement, rather than a pure measurement of bronchial smooth-muscle tone.

Flow conversion is essential. Dividing by 30 without converting minutes to seconds would give a different unit and a misleading numerical comparison with values expressed per liter per second. The formula also assumes that the peak pressure represents the relevant end-inspiratory pressure at a known flow. A varying-flow pressure-control breath does not satisfy that assumption simply because a peak is displayed.

Resistance is not always constant as flow changes. Narrow tubes and irregular pathways produce nonlinear pressure losses, especially at higher flows. Increasing flow can therefore increase peak pressure even when the airway anatomy and elastic properties have not changed. Compare breaths at a similar flow before interpreting a peak-to-plateau increase as new obstruction.

In a straight rigid tube with laminar flow, resistance depends strongly on radius. Real airways branch, deform, and contain flows that may be transitional or turbulent. The ideal tube relationship explains why narrowing can have a large effect, but it cannot calculate an intubated patient's total resistance from airway diameter alone.

22.8 Dynamic Compliance Does Not Isolate Elastic Properties

The term dynamic compliance has several uses. On many ventilators, it means tidal volume divided by peak pressure minus PEEP. This displayed value includes the pressure required to sustain flow. It consequently changes when resistance or inspiratory flow changes, even if passive elastic compliance remains unchanged.

Using the illustrative breath with tidal volume of 420 mL, peak pressure of 34 cm H₂O, and total PEEP of 10 cm H₂O gives 420 ÷ (34 − 10) = 17.5 mL/cm H₂O. The static estimate from plateau pressure of 24 cm H₂O remains 420 ÷ 14 = 30 mL/cm H₂O. The lower displayed dynamic value reflects the inclusion of flow-related pressure.

If a narrower tube raised peak pressure while plateau pressure and delivered volume stayed the same, the displayed dynamic compliance would fall. The static estimate would remain unchanged. That pattern points toward an increased flow-related pressure requirement rather than an isolated increase in the elastic pressure needed to hold the volume.

In physiological studies, dynamic lung compliance may instead be defined using transpulmonary pressure between points of zero flow during ongoing breathing. That quantity differs from a ventilator's peak-pressure calculation. Reports should specify the pressure signals, timing, and formula rather than assuming one universal definition.

Frequency dependence adds another distinction. When different regions have different filling rates, a faster respiratory pattern can leave slow regions less fully ventilated. The measured dynamic volume–pressure relationship then changes because the accessible contribution of those regions changes. A single global number cannot distinguish this redistribution from a uniform change in tissue properties.

Automated ventilator calculations may fit the equation of motion to many points in a breath. Such fitting can be useful, but it still depends on its model, sensor quality, leaks, and assumptions about patient effort. A precise display is not proof of a valid physiological estimate.

22.9 Time Constants Connect Resistance with the Time Available for Breathing

A compartment fills and empties at a rate determined by both its resistance and compliance. Their product is the time constant, represented by the Greek letter tau. In a simple linear compartment, the time constant describes the speed of an exponential response to a pressure change.

Resistance in cm H₂O·s/L multiplied by compliance in L/cm H₂O gives seconds. If resistance is 15 cm H₂O·s/L and compliance is 0.040 L/cm H₂O, the time constant is 15 × 0.040 = 0.60 seconds.

After one time constant, about 63% of a step-driven volume change has occurred, leaving about 37% remaining. After three time constants, about 95% has occurred. In this example, three time constants occupy 3 × 0.60 = 1.8 seconds. These percentages describe an ideal exponential response, not a requirement that every patient receive a particular inspiratory or expiratory duration.

A region with a long time constant responds slowly. High resistance slows flow, while high compliance means more volume must move for a given pressure change. A region can therefore empty slowly despite being easy to inflate. A stiff region with low compliance may have a short time constant, although airway closure or changing resistance can invalidate that simple prediction.

When expiration ends before the system has emptied toward its relaxation state, the next breath begins from a higher volume and pressure. Repetition can produce dynamic hyperinflation and intrinsic PEEP. Pepe and Marini described the clinical importance of this concealed end-expiratory pressure in mechanically ventilated patients with airflow obstruction, including its circulatory consequences. 5

Expiratory flow that has not returned to zero before the next inspiration suggests incomplete emptying. It does not quantify every region's trapped pressure. Regional time constants, expiratory flow limitation, active expiration, and airway closure can make the system more complicated than one exponential compartment. Later chapters develop dynamic hyperinflation and its treatment in detail.

22.10 Global Mechanics Can Conceal Regional Stress and Patient Effort

Compliance measured at the airway opening describes the behavior of the whole communicating respiratory system. It averages regions that may differ markedly in aeration, stiffness, opening pressure, and time constant. The same tidal volume is not necessarily distributed evenly across those regions.

If much of the lung is unavailable for tidal ventilation, the delivered volume enters a smaller functioning region. Its local expansion can be substantial even when the whole-body tidal volume appears modest. Recruitment can make more lung participate, while overdistension can stiffen regions that were already open. Both processes can occur during the same PEEP change, so the resulting compliance reflects their combined effects.

Chiumello and colleagues measured lung mechanics, stress, and strain in patients with ARDS and comparison subjects. Their findings showed substantial variation in the relationship between airway pressures and lung-distending conditions. This supports distinguishing global respiratory measurements from the actual pressure and volume changes experienced by the lung. 6

Patient effort creates a further source of hidden variation. Inspiratory muscles lower pleural pressure and can increase transpulmonary pressure without producing a corresponding rise in airway pressure. A low airway pressure during assistance therefore does not establish a small lung-distending pressure or little respiratory work.

Effort can also redistribute gas internally. Yoshida and colleagues combined a clinical observation with experiments in injured pigs showing movement of gas from nondependent toward dependent regions during spontaneous effort. This pendelluft occurred without requiring an equivalent increase in tidal volume at the airway opening. The experiment identifies a potential mechanism of regional overinflation; it does not establish that every spontaneous breath is injurious. 7

A passive mechanics measurement and an assisted breath consequently answer different questions. Passive measurements estimate the system's pressure requirement without muscle contribution. Assisted-breath assessment must also consider respiratory drive, effort, timing, and regional effects. Neither should be substituted for the other without stating the conditions.

22.11 A Change in Pressure Becomes Useful When Its Components Are Identified

Consider an illustrative passive patient receiving a 420-mL constant-flow breath at 0.50 L/s. Total PEEP is 10 cm H₂O, plateau pressure is 24 cm H₂O, and peak pressure is 34 cm H₂O. The measurements imply a tidal elastic pressure of 14 cm H₂O, static respiratory system compliance of 30 mL/cm H₂O, and effective inspiratory resistance of 20 cm H₂O·s/L.

Now suppose peak pressure rises to 42 cm H₂O while plateau pressure, tidal volume, total PEEP, and flow remain unchanged. The extra 8 cm H₂O appears in the flow-related component. The next assessment concerns the artificial airway, circuit, secretions, bronchospasm, and other causes of increased effective resistance. The pattern supports that direction of investigation; it does not by itself locate the obstruction.

In a different trajectory, peak and plateau pressures both rise while their difference remains similar. With unchanged volume and total PEEP, the larger plateau-to-PEEP difference indicates a greater tidal elastic requirement. Evaluate changes in aeration, lung water, position, chest wall conditions, and other causes. An acute pressure change accompanied by instability also demands prompt assessment for a new mechanical complication, rather than reliance on a calculation alone.

Plateau minus total PEEP is commonly called respiratory system driving pressure. In passive conditions it equals tidal volume divided by static compliance. Amato and colleagues analyzed individual data from earlier ARDS trials and found a strong relationship between driving pressure and survival. This secondary analysis supports the clinical importance of volume relative to the functioning system, but it did not randomize patients to an isolated driving-pressure target. 8

A lower driving pressure can result from a smaller tidal volume or a more compliant system. Those changes have different explanations. A PEEP adjustment can change recruitment, overdistension, total lung volume, and circulation simultaneously. Choosing settings therefore requires gas exchange, hemodynamics, regional risks, and the wider treatment evidence as well as the mechanical response.

The practical sequence is to confirm the measurement, identify which component changed, and relate that change to the patient's course. A valid physiological explanation should account for pressure, volume, flow, and timing together. The following chapters build on this framework to examine transpulmonary pressure, recruitment, airflow obstruction, PEEP, and respiratory work.

References

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