🩺Biomedical Instrumentation Unit 7 – Respiratory Measurements & Spirometry

Key Concepts in Respiratory Physiology

• Respiration involves gas exchange between the lungs and the environment, enabling oxygen uptake and carbon dioxide removal
• The respiratory system consists of the airways (nose, pharynx, larynx, trachea, bronchi, and bronchioles) and the lungs (alveoli)
  • Alveoli are the primary site of gas exchange, where oxygen diffuses into the bloodstream and carbon dioxide diffuses out
• Ventilation is the process of moving air in and out of the lungs, which is driven by changes in pressure generated by the respiratory muscles (diaphragm and intercostal muscles)
• Lung volumes and capacities describe the amount of air in the lungs at different stages of the respiratory cycle
  • Tidal volume (TV) is the volume of air inhaled or exhaled during normal breathing
  • Residual volume (RV) is the volume of air remaining in the lungs after a maximal exhalation
  • Total lung capacity (TLC) is the maximum volume of air the lungs can hold, equal to the sum of all lung volumes
• Airflow resistance is influenced by factors such as airway diameter, airway smooth muscle tone, and the presence of secretions or obstructions
• Compliance refers to the ease with which the lungs and chest wall expand, affected by factors such as lung elasticity and surface tension

Respiratory Measurement Techniques

• Spirometry is a common pulmonary function test that measures lung volumes, capacities, and flow rates during forced breathing maneuvers
• Peak expiratory flow (PEF) meters measure the maximum speed of expiration, providing a simple assessment of airway obstruction
• Body plethysmography measures lung volumes and airway resistance by having the patient breathe in a sealed chamber
  • Changes in pressure and volume within the chamber are used to calculate lung volumes and resistance
• Gas dilution techniques (helium dilution and nitrogen washout) estimate lung volumes by measuring the concentration of a tracer gas in the lungs
• Impulse oscillometry assesses respiratory mechanics by applying small pressure oscillations to the airways and measuring the resulting flow and pressure changes
• Capnography measures the concentration of carbon dioxide in exhaled air, providing information about ventilation and gas exchange
• Pulse oximetry non-invasively measures the oxygen saturation of arterial blood using light absorption at two wavelengths

Spirometry Basics

• Spirometry measures the volume and flow of air during forced breathing maneuvers, typically a maximal inhalation followed by a rapid and complete exhalation
• Key spirometric parameters include forced vital capacity (FVC), forced expiratory volume in one second (FEV1), and the FEV1/FVC ratio
  • FVC is the total volume of air exhaled during a maximal forced exhalation
  • FEV1 is the volume of air exhaled in the first second of a maximal forced exhalation
  • The FEV1/FVC ratio is the proportion of the FVC exhaled in the first second, indicating airflow limitation
• Flow-volume loops and volume-time curves are graphical representations of spirometry data, providing visual insights into respiratory function
• Obstructive lung disorders (asthma, COPD) are characterized by reduced FEV1 and FEV1/FVC ratio, indicating airflow limitation
• Restrictive lung disorders (interstitial lung disease, chest wall deformities) are characterized by reduced FVC, with a normal or increased FEV1/FVC ratio
• Spirometry results are compared to predicted values based on age, height, sex, and ethnicity to determine the presence and severity of respiratory abnormalities

Spirometry Equipment and Instrumentation

• Spirometers are devices that measure the volume and flow of air during breathing maneuvers
• Volume-type spirometers directly measure the volume of air displaced, using a water-sealed bell or a rolling-seal cylinder
  • These spirometers are accurate but bulky and require frequent calibration
• Flow-type spirometers measure airflow using a pneumotachometer or a turbine, and integrate the flow signal to obtain volume
  • Pneumotachometers measure flow based on the pressure drop across a fixed resistance (screen or capillary tube)
  • Turbine spirometers measure flow based on the rotation speed of a vane or propeller
• Spirometers are calibrated using a known volume (calibration syringe) or a known flow rate (calibrated orifice) to ensure accurate measurements
• Spirometry systems often include software for data acquisition, analysis, and interpretation, as well as for generating reports and comparing results to reference values
• Quality control measures, such as daily calibration checks and regular maintenance, are essential to ensure the accuracy and reliability of spirometry measurements

Performing Spirometry Tests

• Spirometry tests should be performed by trained personnel following standardized guidelines to ensure accurate and reproducible results
• Patient preparation includes explaining the test procedure, obtaining informed consent, and gathering relevant medical history and medication use
• Patients should be seated upright with a nose clip in place to prevent air leakage through the nose
• The spirometer mouthpiece should be placed in the patient's mouth, ensuring a tight seal and avoiding obstruction by the tongue or teeth
• Patients are instructed to take a maximal inhalation, followed by a rapid and forceful exhalation until no more air can be expelled
  • Verbal encouragement and coaching are provided to help patients achieve maximal effort
• The test is typically repeated at least three times to obtain consistent and reproducible measurements
  • The best values for FVC and FEV1 are selected from the acceptable maneuvers
• Acceptability criteria for spirometry maneuvers include a rapid start, a smooth continuous exhalation, a plateau in the volume-time curve, and a duration of at least 6 seconds
• Repeatability criteria require that the two largest FVC and FEV1 values from acceptable maneuvers are within 150 mL of each other

Interpreting Spirometry Results

• Spirometry results are interpreted by comparing measured values to predicted values based on the patient's age, height, sex, and ethnicity
• The percent predicted (%pred) is calculated as (measured value / predicted value) × 100, indicating the extent of deviation from the expected value
• Obstructive patterns are characterized by a reduced FEV1/FVC ratio (typically <0.7) and a reduced FEV1 %pred
  • The severity of obstruction is graded based on the FEV1 %pred (mild: >70%, moderate: 50-69%, severe: 30-49%, very severe: <30%)
• Restrictive patterns are characterized by a reduced FVC %pred (typically <80%) with a normal or increased FEV1/FVC ratio
  • The severity of restriction is graded based on the FVC %pred (mild: 70-79%, moderate: 60-69%, severe: 50-59%, very severe: <50%)
• Mixed patterns show features of both obstruction and restriction, with reduced FVC, FEV1, and FEV1/FVC ratio
• Other parameters, such as peak expiratory flow (PEF), forced expiratory flow at 25-75% of FVC (FEF25-75%), and the shape of the flow-volume loop, provide additional information about respiratory function
• Interpretation of spirometry results should consider the clinical context, including the patient's symptoms, risk factors, and comorbidities

Clinical Applications and Significance

• Spirometry is used for the diagnosis, assessment, and monitoring of various respiratory disorders, such as asthma, COPD, interstitial lung diseases, and neuromuscular disorders
• In asthma, spirometry is used to confirm the diagnosis, assess the severity of airflow obstruction, and monitor the response to treatment
  • Reversibility testing, using bronchodilators, helps distinguish asthma from other obstructive disorders
• In COPD, spirometry is essential for diagnosis, staging, and monitoring disease progression and treatment effectiveness
  • The Global Initiative for Chronic Obstructive Lung Disease (GOLD) classification system uses FEV1 %pred to categorize COPD severity
• Spirometry is used in occupational health screening to detect early signs of respiratory impairment in workers exposed to hazardous substances (dust, fumes, chemicals)
• Preoperative spirometry is used to assess the risk of postoperative pulmonary complications in patients undergoing surgery, particularly thoracic and upper abdominal procedures
• Spirometry is used to monitor the progression of restrictive lung diseases, such as idiopathic pulmonary fibrosis, and to assess the response to treatment
• In neuromuscular disorders, such as amyotrophic lateral sclerosis (ALS) and muscular dystrophy, spirometry helps evaluate the extent of respiratory muscle weakness and the need for ventilatory support

Limitations and Troubleshooting

• Spirometry is an effort-dependent test, requiring patient cooperation and maximal effort to obtain accurate and reproducible results
  • Suboptimal effort, poor technique, or lack of understanding can lead to invalid or misleading results
• Spirometry may not be suitable for certain patients, such as those with severe cognitive impairment, physical limitations, or acute respiratory distress
• Spirometry alone cannot differentiate between obstructive and restrictive patterns, as both can present with reduced FVC
  • Additional tests, such as lung volume measurements and diffusing capacity, may be needed to confirm the underlying pathology
• Spirometry results can be affected by factors such as patient position, mouthpiece obstruction, air leaks, and equipment malfunction
  • Proper patient positioning, instruction, and technique are crucial to minimize these sources of error
• Calibration errors, sensor drift, and software issues can lead to inaccurate measurements
  • Regular calibration, maintenance, and quality control procedures are essential to ensure the reliability of spirometry equipment
• Interpretation of spirometry results should consider the limitations of reference values, which may not accurately represent all populations or individuals
  • Ethnic-specific reference values and lower limits of normal (LLN) should be used when available
• Spirometry results should be interpreted in the context of the patient's clinical presentation, as respiratory symptoms and functional impairment may not always correlate with the severity of spirometric abnormalities