Respiratory Investigations and Pulmonary Function Tests Flashcards

Respiratory Function Overview

Respiratory system investigations evaluate either structural integrity or functional performance. Functional studies specifically assess ventilation, gas exchange, lung volumes, and respiratory compliance.

Pulmonary Function Tests (PFTs)

Indications

  • Assessment of respiratory symptoms (cough, shortness of breath) or physical signs (wheezing, crackles).

  • Screening at-risk populations (smokers, occupational hazards).

  • Monitoring pulmonary drug toxicity (e.g., Amiodarone).

  • Follow-up for previously abnormal studies.

  • Preoperative risk evaluation.

  • Severity grading, therapeutic monitoring, and disability assessment.

Contraindications

  • Confusion or inability to follow technical instructions.

  • Acute severe illness or severe pain.

  • Current pneumothorax.

  • Recent cardiothoracic, abdominal, or ophthalmic surgery.

  • Recent acute myocardial infarction (<1 month< 1\,\text{month}).

Diagnostic Applications

  • Obstructive Diseases: Characterized by airflow limitation (e.g., Asthma, COPD, Bronchiectasis).

  • Restrictive Diseases: Characterized by reduced lung expansion due to parenchymal causes (e.g., Pulmonary Fibrosis) or extrapulmonary causes (e.g., kyphoscoliosis, neuromuscular disease).

Lung Volumes and Capacities

  • Residual Volume ($RV$): Cannot be measured directly by simple spirometry.

  • Functional Residual Capacity ($FRC$): Measured via closed-circuit helium dilution, open-circuit nitrogen washout, or body plethysmography (the fastest method).

  • Calculation:

RV=FRC−ERVRV = FRC - ERV


Lung Volumes and Capacities Diagram

Dynamic Airflow Tests & Spirometry

Severity Grading Scheme ($FEV_1$ or $FVC$ % of predicted)

  • Mild: 70−79%70-79\%

  • Moderate: 60−69%60-69\%

  • Moderately Severe: 50−59%50-59\%

  • Severe: 35−49%35-49\%

  • Very Severe: <35%< 35\%

Key Parameters

  • FEV1/FVCFEV_1/FVC Ratio: Normal values are ≥70−80%\ge 70-80\%. A ratio <70%< 70\% defines an obstructive deficit. In restrictive lung disease, the ratio remains normal or increased.

  • FEF25−75%FEF_{25-75\%} (Forced Expiratory Flow 25–75%): Measures mid-expiratory airflow in small airways; it is effort-independent. Values <50%< 50\% of predicted indicate early small airway disease even when $FVC$ and $FEV_1$ are normal.


Comparison of Normal, Obstructive, and Restrictive Spirometry Curves

Flow-Volume Loops

  • Fixed Obstruction (Intra- or Extrathoracic, e.g., large goiter, tracheal stenosis): Flattening of both inspiratory and expiratory loops.

  • Variable Extrathoracic Obstruction (e.g., vocal cord paralysis): Flattening confined to the inspiratory loop.

  • Variable Intrathoracic Obstruction (e.g., tracheomalacia): Flattening confined to the expiratory loop.

  • Restriction: Preserved normal shape, but narrowed along the volume axis ($TLC$ is reduced).


Fixed vs Variable Obstruction Flow Volume Loops

Reversibility and Provocation Testing

Bronchial Reversibility Test

  • Spirometry is recorded before and 15 minutes after administering a short-acting bronchodilator.

  • Positive Result: Increase in FEV1≥12%FEV_1 \ge 12\% AND ≥200 mL\ge 200\,\text{mL}, confirming Asthma.

  • Post-Bronchodilator FEV1/FVC<70%FEV_1/FVC < 70\%: Diagnostic criteria for COPD.

Bronchoprovocation Testing

  • Indicated for suspected Asthma with normal baseline spirometry to evaluate Airway HyperReactivity (AHR).

  • Agents used include methacholine, histamine, antigen, mannitol, or exercise.

  • Positive Result: A fall in FEV1>20%FEV_1 > 20\%.

  • Contraindications: Baseline severe reduction in airflow ($FEV_1 < 60\%relativeorrelative or< 50\% absolute).\n\n\n# Peak Expiratory Flow Rate (PEFR) & Diffusion Capacity\n\n## Peak Expiratory Flow Rate (PEFR)\n- Measures maximal expiratory airflow velocity.\n- **Uses**: Diagnosis of occupational asthma, therapy response tracking, and asthma exacerbation prediction.\n- **Limitation**: Highly effort-dependent.\n\n## Diffusion Capacity ($DL_{CO}$ / $TL_{CO})\n- Measures gas transfer across the alveolar-capillary membrane.\n- **Causes of Decreased TLCOTL_{CO}**: Emphysema (loss of surface area), Interstitial Lung Disease/Pulmonary Fibrosis (membrane thickening), Pulmonary Hypertension (reduced capillary perfusion), Anemia (reduced hemoglobin), Pneumonectomy.\n- **Causes of Increased TLCOTL_{CO}**: Pulmonary Hemorrhage (intra-alveolar blood), High cardiac output states.\n- **Transfer Coefficient (KCO=TLCO/VAK_{CO} = TL_{CO} / V_A)**: Normal/elevated in extrapulmonary restriction (chest wall deformities, obesity, neuromuscular weakness); reduced in intrapulmonary parenchymal restriction.\n\n\n# Arterial Blood Gas (ABG) Analysis & Pulse Oximetry\n\n## Pulse Oximetry\n- Non-invasive peripheral oxygen saturation (SpO_2$) measurement.\n- **Inaccuracy Factors**: Hypothermia, shock, carbon monoxide poisoning, methemoglobinemia, bright ambient lighting.\n\n## ABG Reference Values\n- **\text{pH}∗∗:**:7.35 - 7.45\n- **PO_2∗∗:**:10 - 13\,\text{kPa}\n- **PCO_2∗∗:**:4.5 - 6.0\,\text{kPa}\n- **\text{Bicarbonate } (HCO_3^-)$Buffer**: 21−26 mmol/L21 - 26\,\text{mmol/L}

  • Base Excess: −2 to +2 mmol/L-2 \text{ to } +2\,\text{mmol/L}


ABG Diagnostic Flowchart

Causes of High Anion-Gap Metabolic Acidosis (MUDPILES)

  • M: Methanol

  • U: Uremia

  • D: Diabetic Ketoacidosis

  • P: Paraldehyde & Phenformin

  • I: Isoniazid

  • L: Lactic Acidosis

  • E: Ethylene Glycol

  • S: Salicylates

Alveolar-Arterial (A-a) Gradient

Formula

PAO2=PIO2−(PaCO20.8)PAO_2 = PIO_2 - \left(\frac{PaCO_2}{0.8}\right)

DA−aO2=PAO2−PaO2D_{A-a}O_2 = PAO_2 - PaO_2

Interpretation

  • Normal Range: 2−4 kPa2 - 4\,\text{kPa} (5−20 mmHg5 - 20\,\text{mmHg}).

  • Elevated A-a Gradient: Indicates underlying Ventilation/Perfusion ($V/Q$) mismatch, anatomic or physiologic shunt, or diffusion barrier impairment.

  • Normal A-a Gradient with Hypoxia: Indicates pure hypoventilation (e.g., drug-induced CNS depression) or low ambient oxygen tension ($FiO_2$).