Mechanics of Breathing: Pressure, Volume, and the Work of Breathing

Mechanics of Breathing: Pressure, Volume, and the Work of Breathing

Understanding how air moves in and out of the lungs requires a firm grasp of three interrelated concepts: pressure gradients, lung compliance, and airway resistance. Together, these factors determine how much effort—or work—the respiratory muscles must produce with every breath. As a respiratory therapist, you will use these principles daily when you assess patients, set ventilator parameters, and interpret pulmonary function data.

Pressure Gradients and Airflow

Air flows from an area of higher pressure to an area of lower pressure. During a normal spontaneous inhalation, the diaphragm and external intercostal muscles contract, increasing thoracic volume. This expansion lowers alveolar pressure below atmospheric pressure, and air flows in. During exhalation, the muscles relax, the chest wall recoils, alveolar pressure rises above atmospheric, and air flows out.

Three pressure values matter most in clinical practice:

• Atmospheric pressure (Patm): the pressure of the surrounding air, used as the reference point (0 cm H₂O at sea level in clinical terms).
• Alveolar pressure (Palv): pressure inside the alveoli; must be negative relative to atmosphere to inhale.
• Pleural pressure (Ppl): pressure in the pleural space; normally slightly negative, which keeps the lung expanded against the chest wall.

Transpulmonary pressure (Ptp) is the difference between alveolar pressure and pleural pressure (Ptp = Palv − Ppl). It is the pressure that keeps the alveoli open and is central to understanding compliance.

Lung Compliance

Compliance describes the distensibility of the lung—how easily it stretches in response to a given pressure change. It is calculated as:

Static Compliance (Cst) = ΔVolume ÷ ΔPressure

The units are milliliters per centimeter of water (mL/cm H₂O). In a healthy adult, static compliance of the respiratory system (lung plus chest wall together) is approximately 60–100 mL/cm H₂O. The lung alone is typically around 200 mL/cm H₂O.

Decreased compliance (stiff lungs) means more pressure is required to deliver the same tidal volume. Conditions such as pulmonary fibrosis, acute respiratory distress syndrome (ARDS), pulmonary edema, and atelectasis all reduce compliance. On a pressure-volume (P-V) curve, a stiff lung produces a flatter, rightward-shifted curve: the slope is less steep because volume changes little for a given pressure change.

Increased compliance is seen in emphysema, where destruction of alveolar walls reduces elastic recoil. The P-V curve becomes steeper. While this sounds like less work to inflate, the loss of recoil means air trapping and difficulty exhaling.

Airway Resistance

Resistance (Raw) is the opposition to airflow caused by friction between air molecules and the airway walls. It is expressed as cm H₂O per liter per second (cm H₂O/L/s). Normal total airway resistance in an adult is approximately 0.5–2.5 cm H₂O/L/s.

Resistance is governed by Poiseuille's law: it is inversely proportional to the fourth power of airway radius. Halving the radius increases resistance sixteenfold. This is why bronchospasm, secretions, mucosal edema, or a partially kinked endotracheal tube can dramatically increase the work of breathing.

The Work of Breathing

Work of breathing (WOB) is the energy the respiratory muscles expend to overcome both elastic forces (compliance) and resistive forces (airway resistance) during ventilation. Normally, WOB is low and the respiratory muscles consume only a small fraction of total body oxygen. In patients with stiff lungs or obstructed airways, WOB rises significantly. At extreme levels, the respiratory muscles themselves can fatigue and ventilatory failure can result—a key reason for initiating mechanical ventilation.

On a P-V loop (the dynamic curve traced during a full breath), the area enclosed by the loop represents resistive work. The triangle under the static compliance line represents elastic work. Clinicians and ventilator monitoring systems use these loops to guide therapy.

Today's Lab Overview

In this lab you will use a mechanical lung model to simulate normal and decreased compliance. You will record the pressure needed to deliver a set tidal volume, plot P-V curves on graph paper or a spreadsheet, and calculate static compliance from your data. Compare your results to the reference ranges in Egan's Fundamentals of Respiratory Care.

Safety and Professional Practice Points

• Handle equipment carefully. Mechanical lung models and spirometry interfaces can be damaged by excessive pressure. Never exceed the pressure limits stated by your instructor or marked on the device.
• Infection control. If any patient-interface components are used for demonstration, treat them as potentially contaminated. Follow standard precautions and your program's equipment-cleaning protocol.
• Data integrity. Record all readings accurately and honestly. Fabricating or adjusting data—even in a classroom setting—is a violation of professional standards and undermines patient safety habits you are building now.
• Scope of practice awareness. Understanding compliance and resistance is essential before you adjust ventilator settings on a real patient. Always act within your verified competency and under appropriate supervision until fully credentialed.
• Double-check calculations. A tenfold arithmetic error in compliance could lead to a dangerous ventilator setting in clinical practice. Verify your math before recording a final answer.