How are medical waveforms, including ECG (Electrocardiogram) and arterial blood pressure waveforms, interpreted?

Understanding Medical Waveforms: ECG and Arterial Blood Pressure

Medical waveforms provide critical diagnostic information about cardiac function and hemodynamics, with ECG and arterial blood pressure waveforms being among the most important for clinical decision-making.

Electrocardiogram (ECG) Waveforms

Normal ECG Components

• P wave: Represents atrial depolarization

  • Normal duration <120 ms
  • Normal amplitude <2.5 mm 1
  • Typically upright in leads I, II, and positive precordial leads

• PR interval: Time from P wave onset to QRS complex onset

  • Normal range: 120-200 ms 1
  • Measures AV nodal conduction time

• QRS complex: Represents ventricular depolarization

  • Normal duration <120 ms 1
  • Consists of Q wave (first negative deflection), R wave (positive deflection), and S wave (negative deflection after R)
  • Pathological Q waves: >40 ms or >25% of R wave amplitude 1

• ST segment: Represents early ventricular repolarization

  • Normally isoelectric (at baseline)
  • ST elevation or depression may indicate ischemia or injury 2
  • ST elevation in leads V₁, V₂, and V₃ has different normal limits based on age, sex, and race 2

• T wave: Represents ventricular repolarization

  • Normally upright in leads with upright QRS complexes
  • T wave inversion may indicate ischemia or other pathology

• QT interval: Represents total ventricular depolarization and repolarization

  • Varies with heart rate (use corrected QT or QTc)
  • Normal QTc: <450 ms in men, <460 ms in women 1

ECG Lead Placement and Recording

• Standard 12-lead ECG: Consists of 12 views of cardiac electrical activity 2

  • Limb leads: I, II, III, aVR, aVL, aVF
  • Precordial leads: V₁-V₆

• Simultaneous lead acquisition: Allows precise temporal alignment of waveforms with maximum misalignment of no more than 10 ms 2

• Standard recording parameters:

  • Paper speed: 25 mm/s
  • Amplitude calibration: 10 mm/mV 1

Common ECG Abnormalities

• Myocardial ischemia/infarction patterns:

  • ST elevation: Indicates transmural injury
  • ST depression: May indicate subendocardial ischemia
  • Pathological Q waves: May indicate completed infarction
  • Location determined by lead groups: anterior (V₁-V₄), lateral (I, aVL, V₅-V₆), inferior (II, III, aVF) 1

• Repolarization abnormalities:

  • Primary: Indicate changes in repolarization characteristics of ventricular myocytes
  • Secondary: Result from changes in the sequence of ventricular activation 2

Arterial Blood Pressure Waveforms

• Components:

  • Systolic peak: Highest point, represents maximum pressure during ventricular contraction
  • Dicrotic notch: Small deflection representing aortic valve closure
  • Diastolic runoff: Gradual decline representing elastic recoil of arteries during diastole

• Clinical applications:

  • Provides continuous blood pressure monitoring
  • Waveform morphology can indicate cardiac output, vascular resistance, and volume status
  • Pulse pressure (difference between systolic and diastolic) reflects stroke volume and arterial compliance

Relationship Between ECG and Arterial Pressure

• Pulse Transit Time (PTT): Time interval between ECG R-wave and corresponding point on arterial waveform

  • Strongly correlated with systolic blood pressure (r = -0.712) 3
  • Can be used for non-invasive blood pressure estimation

• Clinical significance:

  • Arterial waveforms can help validate ECG findings
  • Algorithms using both ECG and arterial waveforms can reduce false alarms in critical care settings 4

Interpretation Challenges and Pitfalls

• ECG interpretation accuracy:

  • Poor interreader agreement (kappa = 0.33) for potential STEMI ECGs 5
  • Overall sensitivity for identifying true STEMIs: 65% 5
  • Overall specificity: 79% 5

• Common errors:

  • Misplacement of electrodes, especially V₁/V₂ placed too high 1
  • Computer algorithm limitations with error rates of 40.7% for acute MI 1
  • Failure to recognize normal variants in specific populations (athletes, young individuals) 1

• Best practices:

  • Always interpret ECGs in clinical context 1
  • Focus on understanding pathophysiology rather than memorizing patterns 1
  • Use a systematic approach to interpretation 1
  • Verify all computer interpretations 1

Conclusion

Understanding waveform morphology and interpretation is essential for accurate diagnosis and treatment decisions. Both ECG and arterial pressure waveforms provide complementary information about cardiac function and hemodynamics, with their combined use enhancing diagnostic accuracy and reducing false alarms in critical care settings.