Lung and Chest Wall Compliance | Respiratory Physiology

Lung compliance (CL) measures how easily the lung can be stretched, defined as the change in volume per unit change in transpulmonary pressure (CL = ΔV/ΔP). Normal compliance is about 0.2 L/cm H2O. Compliance varies with lung volume: it is lowest near residual volume and total lung capacity, and highest around functional residual capacity. Hysteresis is the phenomenon where the pressure-volume curve during deflation lies above the curve during inflation, primarily caused by pulmonary surfactant behavior. Surface tension at the air-liquid interface in alveoli creates a powerful force pulling surfaces inward, adding to elastic recoil. Laplace's law states that pressure needed to keep alveoli open is proportional to surface tension and inversely proportional to radius (PS = 2T/R). Pulmonary surfactant, synthesized by type 2 pneumocytes and stored in lamellar bodies, solves this problem. It is composed of 85-90% lipids (mostly dipalmitoylphosphatidylcholine) and 10-15% proteins. Surfactant's unique property is dynamic surface tension reduction: as alveoli shrink during exhalation, surfactant molecules concentrate and reduce surface tension dramatically, counteracting Laplace's law and preventing small alveoli from collapsing while larger ones overdistend.

Lung compliance (200 ml/cm H2O) measures the stretchability of lungs per unit pressure change. It is determined by elastic fibers (promote expansion), collagen fibers (reduce compliance when abundant), and surface tension at the air-liquid interface. According to Laplace's law (P = 2T/r), surface tension creates inward pressure that tends to collapse alveoli. Pulmonary surfactant, a phospholipid secreted by type II alveolar cells, reduces surface tension and prevents alveolar collapse. This is critical in premature infants who may lack sufficient surfactant, leading to neonatal respiratory distress syndrome (NRDS) characterized by alveolar collapse and respiratory failure.

Pulmonary surfactant reduces surface tension in alveoli to prevent collapse. Water molecules form hydrogen bonds creating attractive forces; at the water-air interface, molecules lack upward forces, causing net downward pull and alveolar collapse. Surfactant molecules (90% lipids, 10% proteins) have hydrophilic heads and hydrophobic tails that disrupt water-water interactions. Laplace's Law (P = T/r) states collapsing pressure equals surface tension divided by radius. Without surfactant, smaller alveoli collapse more easily. With surfactant, as alveolar radius increases, surfactant concentration decreases, increasing surface tension and keeping collapsing pressure constant.

Two elastic forces determine lung compliance: (1) Elastic forces in lung tissue from elastin and collagen fibers, and (2) Elastic forces in alveolar walls. Surface tension in alveoli decreases compliance by opposing expansion. Surfactant reduces surface tension, thereby increasing compliance. When saline is introduced, surface tension is eliminated, causing compliance to increase significantly.

Surface tension is the most important determinant of lung compliance. Water in alveolar fluid has high surface tension that tries to collapse alveoli. Pulmonary surfactant produced by type II pneumocytes breaks up surface tension, reduces it, increases lung compliance, and prevents alveolar collapse. Lack of surfactant causes neonatal respiratory distress syndrome in premature babies.

The work of breathing consists of two main components: elastic work (overcoming the elastic recoil of the lungs and chest wall) and resistive work (overcoming airway resistance). The elastic work accounts for approximately 50% of total work, while resistive work accounts for the remainder. Conditions that decrease compliance (like fibrosis) or increase resistance (like asthma) increase the work of breathing. The work of breathing is part of the basal metabolic rate, representing the energy expended even at rest for respiratory muscle activity. Understanding work of breathing is important for assessing respiratory function and the impact of respiratory diseases on energy expenditure.

The work of breathing is the energy required to overcome elastic and non-elastic resistance during inspiration, measured in Joules (not power, which is energy per unit time); normal work of breathing is approximately 3 mL O₂/min with only ~10% efficiency, and it is classified into elastic work (related to lung compliance and chest wall distensibility) and non-elastic work (related to airway resistance, tissue resistance, and external compression), with the pressure-volume curve showing elastic work as a triangular area and non-elastic work as an elliptical area; in restrictive lung disease, increased elastic resistance favors higher respiratory rates, while in obstructive lung disease, increased airway resistance favors lower respiratory rates with longer expiratory times to minimize turbulent flow.

The work of breathing is determined by two primary variables: elastance (lung stiffness) and airway resistance. Increased elastance (as in pulmonary fibrosis) requires more work to stretch the lung, while increased resistance (as in asthma or COPD) requires more work to move air through narrowed airways. The optimal breathing frequency minimizes total work, occurring at the intersection of elastic and resistive work curves. Patients with restrictive lung diseases should breathe faster and shallower, while those with obstructive diseases should breathe slower and deeper to reduce overall work of breathing.

The work of breathing has two components: elastic work (to inflate the lungs against their natural recoil) and resistive work (to overcome airway resistance). In healthy patients, a certain pressure generates a certain tidal volume. In diseased states, the same tidal volume requires higher pressure due to reduced compliance (elastic work) or increased resistance (resistive work). For example, in status asthmaticus with increased resistance, more pressure is spent overcoming resistance rather than inflating the lung.

The work of breathing consists of elastic work (overcoming lung and chest wall elasticity) and resistive work (overcoming airway resistance). Elastic work is proportional to the pressure-volume relationship and is greater during inspiration than expiration. Resistive work is proportional to airflow and airway resistance. The total work of breathing determines the energy cost of ventilation and is minimized at optimal respiratory frequencies.

The pressure-volume loop reveals six critical aspects of respiratory mechanics essential for ventilator management. First, a fishtail pattern at breath initiation indicates triggering difficulties or increased work of breathing, often resolved by adjusting sensitivity settings or increasing flow rate. Second, the lower inflection point (critical opening pressure) marks when alveoli become compliant and ready to accept volume, representing the optimal PEEP setting to minimize pressure-volume trade-offs. Third, a bird beak pattern at breath termination signals overdistension requiring reduced tidal volumes to prevent ventilator-induced lung injury. Fourth, the critical closing pressure determines minimal PEEP to prevent alveolar collapse during exhalation. Fifth, loop orientation indicates lung compliance: upright loops suggest restrictive disease (pneumonia, fibrosis, effusion), while flatter loops indicate emphysema/COPD with improved compliance over recovery. Sixth, front-side protrusion indicates increased airway resistance. Together, these points enable comprehensive assessment of ventilator-patient synchrony and lung mechanics.

Optimal PEEP is the PEEP level that achieves the best oxygenation while maintaining the best static compliance without causing cardiac compromise; it is determined by gradually increasing PEEP and monitoring oxygenation, static compliance, and mean arterial pressure, then identifying the highest PEEP level where all parameters remain stable before cardiac effects occur.

The pressure-volume loop provides comprehensive mechanics assessment, showing dynamic compliance during ventilation and static compliance during pauses. Static compliance is calculated as tidal volume divided by (plateau pressure minus PEEP). Modern ventilators can perform slow inflation maneuvers (6-8 L/min) to measure static compliance accurately. The pressure-volume loop shows the lower inflection point (LIP) where alveoli begin opening and the upper inflection point (UIP) where overdistension begins. Optimal PEEP should be 2 cm H2O above the LIP to prevent cyclic collapse while avoiding barotrauma. Ventilation below the LIP causes cyclic collapse and reopening (atelectrauma), while ventilation above the UIP causes barotrauma. Flow-volume loops plot flow against volume during the respiratory cycle, revealing airway obstruction by showing reduced expiratory flow that doesn't return to baseline. Three main types of patient-ventilator asynchronies exist: trigger, flow, and cycle asynchronies. Trigger asynchronies include ineffective triggering (patient effort not detected), double triggering (patient triggers before previous breath completes), and auto-triggering (ventilator initiates without patient effort). For obstructive lung disease patients, PEEP should be set 70-80% of total PEEP (extrinsic + intrinsic) to prevent airway collapse while minimizing work of breathing.

The PV Loop plots pressure against volume, showing the relationship between pressure and volume during a breathing cycle. Mechanical breaths show positive pressure inspiratory and expiratory limbs. Spontaneous breaths show negative pressure inspiratory limb. PEEP shifts the starting point above zero. The loop has two components: the pink area (inspiratory limb) represents elastic work (compliance), and the violet area (expiratory limb) represents resistive work (airway resistance). The lower inflection point (LIP) marks where alveolar recruitment begins during inspiration. The upper inflection point (UIP) marks where alveolar derecruitment begins during expiration. PEEP titration using PV Loops requires multiple PEEP levels and multiple PV Loops to determine optimal PEEP. The goal is to find the PEEP that maximizes compliance and oxygenation without causing overdistension. The Flow-Volume Loop plots flow against volume. A scooped-out pattern in the expiratory curve indicates expiratory flow limitation (bronchospasm, COPD). Reduced peak expiratory flow rate confirms this. Air leak shows the expiratory curve touching baseline early. Auto-PEEP shows the curve not touching baseline (incomplete exhalation). Circuit secretions cause oscillatory movements in the loop during passive exhalation. In spontaneous breathing, the flow-time scalar shows positive deflection during inspiration with an acceleration curve on the negative side. The pressure-time scalar shows negative deflection during inspiration. The volume-time scalar shows volume increasing from baseline with no negative values. CPAP maintains a baseline pressure above zero (e.g., 5 cm H2O). All breaths are patient-triggered and patient-cycled. Pressure Support Ventilation is patient-triggered and flow-cycled. The pressure is constant during inspiration. The key difference from CPAP is that pressure support provides additional pressure to overcome airway resistance, allowing the patient to achieve a desired tidal volume. The baseline is elevated to a CPAP level plus the pressure support. Pressure support should not be set to zero - a minimum of 6-10 cm H2O is needed. ATC (Automated Tube Compensation) automatically adjusts pressure support based on circuit resistance. Control mode (true control) has no patient triggering - all breaths are machine-triggered at fixed intervals. In assist control mode, tidal volume is constant (preset) regardless of patient effort. SIMV (Synchronized Intermittent Mandatory Ventilation) provides a machine breath with a window for spontaneous breathing, but studies showed higher energy consumption during spontaneous breaths. SIMV is now considered outdated. SIMV with pressure support and PEEP maintains a baseline pressure (PEEP) throughout. The pressure-time scalar shows the baseline elevated to the PEEP level. The pressure support is added on top of the PEEP. This combination allows patient-triggered breaths with pressure support while maintaining alveolar recruitment. PEEP and CPAP are often used interchangeably - PEEP for invasive ventilation, CPAP for NIV.

Pressure-volume loops help assess lung compliance and identify problems. A side-lying appearance indicates poor compliance requiring higher PEEP. A figure-eight or narrow loop suggests air hunger (insufficient tidal volume), indicating the need for extubation or tidal volume increase. The C20/Cdynamic ratio measures compliance consistency during the last 20% of the inspiratory cycle. Values less than 0.8 indicate overdistension and air trapping, requiring adjustment of tidal volume or PEEP.