Pulmonary Function Tests

Introduction

• PFTs comprise a set of standardized, noninvasive diagnostic tools routinely used to assess the respiratory system by measuring airflow, lung volumes, and gas transfer efficiency. These tests provide insight into both mechanical and gas exchange abnormalities of the lungs and are fundamental in clinical and research settings.

Indications

• PFTs are used to diagnose and distinguish obstructive from restrictive pulmonary disease. They can be used to further evaluate pulmonary symptoms such as unexplained dyspnea, chronic cough, or exercise intolerance. They help clinicians assess disease severity and progression, monitor response to therapeutic interventions, and determine perioperative and intraoperative pulmonary risk.

Core Components

• Spirometry assesses airflow limitation and ventilatory capacity through measurements such as forced expiratory volume in one second (FEV₁), forced vital capacity (FVC), and the FEV₁/FVC ratio.
• Static lung volumes, which include total lung capacity (TLC), residual volume, and functional residual capacity help in providing information about lung expansion, air trapping, and restrictive physiology
• DLCO assesses the integrity of the alveolar-capillary membrane and the pulmonary vascular bed, providing insight into gas-exchange abnormalities.

Clinical Significance

• Distinct PFT patterns aid in disease classification, risk stratification, and longitudinal monitoring in conditions such as asthma, COPD, interstitial lung disease, pulmonary vascular disease, and neuromuscular disorders.

Lung Volumes

• Lung volumes describe the volume of air in the lungs at set points in the respiratory cycle. Please see the OA summary on lung mechanics for more details (Link)

Spirometry

• Spirometry is a foundational test used to measure pulmonary function and assess airflow into and out of the lungs.
• The airflow-related volumes it measures are residual lung volume, which is the volume of air left in the lung after a full exhale, TLC, FVC, which is the total exhaled volume, FEV₁, and the ratio FEV₁/FVC.¹
• The FEV₁/FVC ratio value can be used to establish whether a lung condition is restrictive, obstructive, or normal.
  • If the FEV₁/FVC ratio is below the lower limit of normal, the condition is most likely obstructive.²
  • If the FEV₁/FVC ratio is within the normal range, the condition may be either normal or restrictive. If this is the case, the FVC is assessed; if it is below the lower limit of normal, it indicates a restrictive pattern.²
• Characteristic spirometry findings differentiate obstructive and restrictive ventilatory patterns, as shown in Table 1.

• In addition to quantitative spirometry parameters, flow-volume loops help visually depict the relationship between airflow and lung volume during forced inspiration and expiration. This allows for characteristic patterns of obstructive vs restrictive lung diseases to be readily identified (Figure 2).

Diffusion Testing

• Diffusion testing, commonly known as the DLCOs for carbon monoxide, quantifies the rate at which carbon monoxide diffuses from the alveoli into red blood cells per minute.³ Carbon monoxide is used because of its high affinity for hemoglobin (Hb) compared to oxygen³ (Table 2).
• Several variables can affect the DLCO, such as exercise, position, pulmonary hemorrhage, polycythemia, obesity, asthma, anemia, emphysema, and pregnancy.⁴
• The main determinants of DLCO include alveolar surface area, capillary surface area, alveolar membrane thickness, Hb concentration, and capillary blood volume.⁵
• The Fick equation for gas diffusion describes variables that affect lung diffusion capacity.
  • V = (k*(A)(∆P) / T
  • V represents the volume of gas exchange per unit time, K is the gas diffusion coefficient, A is the surface area, ∆P represents the partial pressure difference between gases, and T is the alveolar membrane thickness.³
• DLCO is a valuable clinical tool for measuring gas transfer across the alveolar-capillary membrane. It provides insight into the integrity of the pulmonary parenchyma and vasculature. A reduction in DLCO suggests impaired alveolar-capillary diffusion due to decreased pulmonary capillary blood volume or increased membrane thickness. This finding may be present earlier in the course of lung disease, sometimes even before abnormalities are seen on spirometry.

Exercise Testing

• Cardiopulmonary exercise testing (CPET) is widely used to assess the maximal level of physical exertion a patient can tolerate. CPET measures the responses of the lungs, heart, and muscles to increase exercise demand. It is a crucial and comprehensive test because it evaluates the body’s physiological response under non-resting conditions.
• Testing is performed on a treadmill with a gradually increasing workload and measures respiratory oxygen uptake (VO₂), carbon dioxide output (VCO₂), blood pressure, and ventilatory parameters during the exercise test.⁶
• VO₂ max measures maximal oxygen uptake and is an accurate indicator of aerobic exercise capacity. The CPET can distinguish between pulmonary and cardiac limitations and detect diffusion impairments during physical exertion.
• CPET can be used to identify dyspnea, monitor pulmonary hypertension, heart failure, and various other cardiopulmonary conditions that might get overlooked due to a lack of physical conditioning in patients.
• CPET is valuable in contemporary practice for its ability to distinguish among symptoms related to pulmonary disease, heart disease, and exercise.
• Indications for CPET are based on the American College of Cardiology and American Heart Association’s classification system. Some indications include evaluating exercise capacity in patients with heart failure who are being considered for heart transplant, differentiating between cardiac vs pulmonary causes of exercise-induced dyspnea, and evaluating exercise capacity for medical reasons when other tests are unreliable. There are also several absolute and relative contraindications to CPET, which are listed at this link.

• The respiratory exchange ratio (RER) is one of the variables frequently analyzed from a CPET. The RER is the ratio of VCO₂ to oxygen uptake. In a normal state, the RER is typically equivalent to the respiratory quotient. RER is measured via gas exchange at the mouth and can provide insight into metabolic events during exercise. Typically, during exercise, RER increases due to either buffered lactic acid or hyperventilation.⁶
• CPET assesses pulmonary, cardiovascular, and metabolic responses to exertion. This allows clinicians to objectively evaluate patients’ exercise capacity and distinguish among the various limitations that may contribute to their exercise intolerance.

Pulse Oximetry

• Pulse oximetry is a noninvasive monitoring technique that measures arterial oxygen saturation in blood by shining light at specific wavelengths through tissue.⁷
• The most common site is the fingernail bed due to its accessibility. The finger-mounted sensor enables clinicians to measure a patient’s oxygen saturation in real time and to guide clinical decision-making.
• This method allows for rapid and continuous data. It is frequently used in both inpatient and outpatient settings and can help identify hypoxemia.
• Although pulse oximetry provides valuable information on oxygen saturation, it does not allow measurement of ventilation or carbon dioxide levels.
• Regarding pulmonary function testing, pulse oximetry serves as an adjunctive tool to assess oxygenation both at rest and during exertion. It can be commonly used alongside spirometry and exercise testing to detect exertional desaturation and assess functional gas exchange abnormalities.
• Although pulse oximetry does not directly measure ventilation or carbon dioxide retention, it provides physiologic information when interpreted alongside PFTs and arterial blood gases.
• There are limitations in the utilization of pulse oximetry, and its accuracy can be impacted by factors such as poor perfusion, skin pigmentation, and nail polish.⁷
• Despite limitations, pulse oximetry remains a first-line tool for monitoring a patient’s oxygenation.

Arterial Blood Gases

• An arterial blood gas (ABG) test measures pH, PaO₂, PaCO₂, and bicarbonate. This allows clinicians to assess a patient’s oxygenation, ventilation, and acid-base status.
• A simple “blood gas analysis” can be drawn from any vessel, but an ABG refers to blood directly drawn from an artery. It is obtained via arterial puncture or arterial catheterization and must be placed on ice and analyzed promptly for accurate results.⁸
• In an acute care setting, ABGs allow for the analysis of hypoxemia, hypercapnia, and metabolic disturbances.
• PaO₂ on the ABG reflects oxygenation and helps assess conditions such as shunting, V/Q mismatch, or diffusion problems, whereas PaCO₂ provides insight into ventilation and allows analysis of respiratory failure.
• An ABG’s ability to provide precise measurements of both oxygenation and ventilation makes it essential for monitoring and managing patients with respiratory failure and for guiding ventilator settings.
• ABG results are also used to calculate clinical values such as the A-a gradient and the PaO₂/FiO₂ ratio. The A-a gradient can help narrow down the causes of hypoxemia, while the PaO₂/FiO₂ ratio can help assess a patient’s overall oxygenation.⁸
• When used alongside spirometry and DLCO, ABG can be useful in further evaluating abnormalities in gas exchange, particularly when noninvasive tests yield poor results. ABG remains a mainstay for diagnosing, assessing disease severity, and guiding clinical management.
• Arterial blood gas provides detailed physiologic insight into a patient, enabling accurate monitoring and management across a range of pulmonary conditions.