Pulmonary function testing in veterinary practice

Introduction

Pulmonary function testing (PFT) constitutes a vital set of diagnostic procedures extensively utilized in human respiratory medicine, sports medicine, and occupational health. These tests objectively evaluate lung function by examining not only the large and small airways but also the pulmonary parenchyma, inclusive of the alveolar capillary bed. In veterinary medicine, particularly concerning small animals, PFTs serve multiple critical roles: assessing lung function in animals with suspected or confirmed respiratory disorders, aiding pre-anesthetic evaluations, guiding diagnosis, and monitoring therapeutic responses to optimize and tailor ongoing treatments.

Indications and Safety Considerations

While PFTs are generally safe and non-invasive, there are specific contraindications that must be observed to avoid potential risks. These contraindications include conditions such as pneumothorax, unexplained hemoptysis, hemodynamic instability especially related to pulmonary thromboembolism, recent surgical procedures involving the thoracic or abdominal cavities, recent ophthalmic surgeries, and the presence of infectious respiratory diseases. Importantly, the effectiveness of these tests relies heavily on patient cooperation and effort, with pain and discomfort in the thoracic or abdominal regions potentially compromising test quality.

Purpose and Classification of Pulmonary Function Tests

Fundamentally, pulmonary function tests are designed not to establish specific diagnoses but rather to quantify the degree of respiratory dysfunction. Additionally, in various cases, they assist in localizing the anatomical site of respiratory diseasedifferentiating whether the upper or lower airways are involved. Advances in veterinary technology and refinement in testing protocols have significantly lowered the barriers to performing PFTs accurately and safely in conscious small animals, making these tests increasingly accessible in clinical settings. Diseases such as acute respiratory distress syndrome (ARDS), pneumonia, pulmonary fibrosis, and pulmonary oedema result in reduced lung compliance, necessitating higher airway pressures to achieve adequate ventilation.

Pulmonary function tests are broadly categorized into two

Lung Resistance

Lung resistance quantifies the pressure required to maintain airflow against airway narrowing or obstruction. It primarily reflects small airway patency and is elevated in obstructive respiratory conditions such as chronic bronchitis or asthma. Similar to compliance measurements, lung resistance assessments benefit from specialized equipment capable of precise airflow and pressure recordings.

Whole-Body Plethysmography

Considered the gold standard for measuring lung volumes, plethysmography quantifies total lung capacity, functional residual capacity, and residual volume. Traditional methods involving sealed chambers present challenges in animals due to intolerance of increased airway resistance; therefore, modifications such as head-out plethysmography and barometric whole-body plethysmography (BWBP) have been developed. BWBP is a non-invasive method where the animal is enclosed within a sealed chamber that measures pressure oscillations from breathing-induced thoracic volume changes. It records parameters like respiratory rate, pseudo-flow rates, pseudovolume, inspiratory and expiratory times, and provides surrogates for airway resistance using calculated indices such as enhanced pause (Penh). This technique has found clinical applications in evaluating brachycephalic obstructive airway syndrome (BOAS) and in assessing airway responsiveness in both dogs and cats. Despite some sensitivity to movement artifacts, BWBP offers an effective, non-sedated diagnostic option for airway function evaluation.

Forced Oscillation Technique (FOT)

Widely used in human medicine but not yet extensively applied in veterinary practice, FOT assesses respiratory system impedance by applying low-frequency sound waves during tidal breathing. Different frequencies characterize large versus small airway function and lung parenchymal properties. Although it complements spirometry, its inability to differentiate between restrictive and obstructive lung disease limits its diagnostic specificity.

Tests of Pulmonary Gas Exchange Diffusing Capacity of Carbon Monoxide (DLCO)

DLCO measures the efficiency of gas diffusion across the alveolar-capillary membrane by exploiting haemoglobin's high affinity for carbon monoxide in comparison to oxygen.

When lung volumes and spirometry appear normal but DLCO is reduced, impaired diffusion is indicated. This test is vital in diagnosing conditions like pulmonary arterial hypertension and interstitial lung disease in humans, though its use in veterinary medicine has been limited mostly to large animals.

Arterial Blood Gas Analysis

Arterial blood gas (ABG) analysis remains the gold standard for quantifying oxygenation and ventilation status in small animals. Blood is typically sampled from arteries such as femoral, dorsal metatarsal, aural, or sublingual arteries using pre-heparinized syringes with meticulous care to avoid contamination and sample degradation. ABG testing provides key parameters including arterial oxygen tension (PaO 2 ), carbon dioxide tension (PaCO 2 ), blood pH, and derived acid-base indicators like bicarbonate concentration and base excess. Understanding the physiology behind hypoxemia (low PaO 2 ) requires considering factors such as reduced inspired oxygen fraction, ventilationperfusion mismatch, intrapulmonary shunting, diffusion impairment, and hypoventilation. Clinicians distinguish hypoxemia (low oxygen in blood) from hypoxia (insufficient oxygen at tissues) and further classify hypoxia into types including hypoxic, circulatory, anaemic, and histotoxic, each with distinct pathophysiology.

ABG data interpretation involves

• Assessing acid-base status (acidosis vs alkalosis and their metabolic or respiratory origins). • Evaluating ventilation adequacy via PaCO 2 .

• Examining oxygenation via PaO 2 and derived measures like the alveolar-arterial oxygen gradient (A-a gradient) and PaO 2 /FiO 2 ratio (PF ratio), which inform on the severity of pulmonary dysfunction.

The PF ratio is especially useful in grading acute lung injury (ALI) and acute respiratory distress syndrome (ARDS), aiding clinical decision-making based on oxygenation impairment severity.

Pulse Oximetry

Pulse oximetry is a non-invasive, widely available modality providing continuous assessment of haemoglobin oxygen saturation (SpO 2

References

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