Section 3.4: Electrocardiology & Pulmonary Function Tests

Section 3.4: Electrocardiology & Pulmonary Function Tests

Family nurse practitioners must master the interpretation of basic electrocardiography (ECG) and spirometry/pulmonary function tests (PFTs). These diagnostic studies are highly tested on board examinations and are crucial for the management of cardiopulmonary conditions.

Electrocardiogram (ECG) Interpretation

ECG analysis requires a systematic assessment of rate, rhythm, axis, intervals, and morphological changes.

Key Measurements and Intervals

• PR Interval: Reflects conduction time through the AV node. Normal is 0.12 to 0.20 seconds (3 to 5 small boxes). Prolonged PR interval indicates an AV block.
• QRS Duration: Normal is less than 0.12 seconds. Prolongation suggests a ventricular conduction delay, such as a right or left bundle branch block.
• QT Interval: Corrected for heart rate (QTc). Normal is <440 ms in men and <460 ms in women. Prolonged QTc increases the risk of life-threatening ventricular arrhythmias, specifically Torsades de Pointes. Key causes of QTc prolongation include hypokalemia, hypomagnesemia, and drugs (macrolides, fluoroquinolones, zofran, antipsychotics).

Common Arrhythmias and Heart Blocks

• Atrial Fibrillation (AFib): Characterized by an irregularly irregular ventricular rhythm, absence of distinct P waves (replaced by irregular fibrillatory waves), and a variable ventricular rate.
• Atrial Flutter: Characterized by rapid, regular atrial waves that create a classic "sawtooth" pattern (F waves), often with a fixed conduction block (e.g., 2:1 or 4:1 block).
• First-Degree AV Block: Constant prolongation of the PR interval (>0.20 seconds). Every P wave is followed by a QRS complex; no dropped complexes occur.
• Second-Degree AV Block Type I (Mobitz I or Wenckebach): Progressive prolongation of the PR interval until a QRS complex is dropped. The pattern then repeats. This is typically a benign, nodal block.
• Second-Degree AV Block Type II (Mobitz II): The PR interval remains constant, but QRS complexes are dropped randomly or in a fixed ratio (e.g., 2:1 or 3:1). This is a bundle of His block with a high risk of progression to complete heart block. It requires a permanent pacemaker.
• Third-Degree AV Block (Complete Heart Block): Complete AV dissociation. P-P intervals are constant, R-R intervals are constant, but there is no relationship between P waves and QRS complexes. Emergency pacing is required.

Myocardial Ischemia and Infarction

• ST-Segment Elevation: Indicates acute transmural myocardial injury (STEMI). Defined as elevation >1 mm in 2 or more contiguous limb leads, or >2 mm in precordial leads.
• ST-Segment Depression / T-Wave Inversion: Represents myocardial ischemia or non-ST-elevation myocardial infarction (NSTEMI).
• Pathological Q Waves: Indicate completed transmural infarction. Defined as a Q wave >0.04 seconds wide or >25% of the height of the succeeding R wave in contiguous leads.

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Spirometry and Pulmonary Function Tests (PFTs)

Spirometry measures lung volumes and flows, serving as the primary diagnostic tool for obstructive and restrictive lung diseases, guided by GINA (Global Initiative for Asthma) and GOLD (Global Initiative for Chronic Obstructive Lung Disease) criteria.

Obstructive Pattern

Characterized by a reduction in airflow. The diagnostic hallmark is a post-bronchodilator FEV1/FVC ratio of less than 0.70.

• Differential Diagnosis: Asthma, COPD, bronchiectasis.
• Reversibility Testing: Administer 4 puffs of a short-acting beta-agonist (SABA), such as albuterol, and repeat spirometry in 15 minutes. Reversibility is defined as an increase in FEV1 by 12% or more AND an absolute increase of 200 mL or more. Reversibility is highly characteristic of asthma. In contrast, COPD presents with a fixed, irreversible or only partially reversible airflow obstruction.

Restrictive Pattern

Characterized by a reduction in lung volume. The FEV1/FVC ratio is normal or elevated (>0.70), but both FEV1 and FVC are reduced proportionally.

• Diagnostic Confirmation: Requires plethysmography to measure Total Lung Capacity (TLC). A TLC of less than 80% of the predicted value confirms a restrictive pattern.
• Differential Diagnosis: Idiopathic pulmonary fibrosis, sarcoidosis, neuromuscular disorders (amyotrophic lateral sclerosis, myasthenia gravis), obesity, or chest wall deformities.

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Clinical Traps and Worked Scenario

Clinical Traps to Avoid

1. Misidentifying Mobitz II as Mobitz I: Distinguishing between these blocks is critical. Mobitz I (Wenckebach) is often benign and monitored, while Mobitz II is highly unstable and requires a pacemaker. Always look for progressive PR prolongation (Type I) vs. constant PR intervals (Type II).
2. Performing Spirometry During an Exacerbation: Do not order spirometry while a patient is experiencing an acute asthma flare or COPD exacerbation. Results will be inaccurate and the effort is unsafe. Perform testing when the patient is clinically stable.
3. Ruling out Asthma with Normal Spirometry: Do not assume a normal spirometry rules out asthma. In patients with mild intermittent asthma, airway obstruction is transient, and PFTs can be completely normal between flares.

Worked Scenario

A 62-year-old male with a 35 pack-year smoking history presents with progressive exertional dyspnea and a chronic morning cough. On exam, he has decreased breath sounds and a prolonged expiratory phase.

• PFT Results:
  • Pre-bronchodilator FEV1/FVC: 0.61
  • Pre-bronchodilator FEV1: 1.7 L (55% predicted)
  • Post-albuterol FEV1: 1.8 L (58% predicted)
  • Post-bronchodilator FEV1/FVC: 0.62
• Interpretation:
  1. The post-bronchodilator FEV1/FVC ratio is <0.70 (0.62), confirming an obstructive lung disease.
  2. The post-bronchodilator FEV1 increased by only 5.8% (100 mL), which does not meet the reversibility criteria (>=12% and >=200 mL).
  3. This fixed, irreversible airway obstruction in a patient with a heavy smoking history is diagnostic of Chronic Obstructive Pulmonary Disease (COPD).