| Jeffrey P Schaefer MSc MD FRCPC FACP |
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Electrocardiogram Interpretation The method used to interpret an ECG will vary with experience and knowledge. For someone starting out, I recommend a systematic approach. It is my judgment that the interpretation of all para-clinical findings ('lab tests') should include four phases. I. Identification * Frequently overlooked, this is critically important. Failure to properly identify the patient or study may have adverse consequence. * identify the patient * identify the study: "this is a 12 - lead ECG done at standard voltage and paper speed" (standard voltage: 10.0 mm = 1 mV) (standard paper speed is 25.0 mm / second) II. Observations and Measurements * This is purely about making observations. Just collect the observations and save making a diagnosis until all the relevant observations and measurement have been made. * ventricular rate (can use the 300, 150, 100, 75, 60, 50 method) * atrial rate (can use the same method, not necessary if in normal sinus rhythm) * inspect the P waves * measure the P - R interval (milliseconds preferred) normal 120 - 210 msec * inspect the QRS complexes (are there q waves, is there an rSr' pattern, are there delta waves, are there Osborne waves, any other abnormalities?) * measure the QRS duration (normal is less than 120 msec) * inspect the ST segments for elevation or depression * measure the QT interval (think about how to correct the QT according to ventricular rate * inspect the T waves (elevated, inverted, isoelectric, other) * ventricular axis (easiest to use leads I and aVf, more skilled interpreters will consider all limb leads) * sometimes U waves can be observed III. Electrocardiographic Diagnoses * Based upon the observations collected, what are the 'Electrocardiographic Diagnoses? Constrain your diagnoses to ECG diagnoses, save comments about clinical diagnoses for the next step. For example... * what is the rhythm? * is there brady or tachycardia or normal rate? * is there AV block? * is there conduction delays or block? * is there evidence for pericarditis * is there evidence for ischemic heart disease? * is there evidence for chamber hypertrophy * are there repolarization abnormalities * there are many other possibilities, these are the more common ECG related diagnoses... IV. Cardiac Diagnoses * Now that we have made ECG diagnoses, let's go back to the bedside. What is the presentation? What differential diagnosis is associated with the patient presentation? Does diagnoses are suggested by the history, physical examination, and para-clinical data (lab tests)? Do the ECG findings make potential diagnoses more likely or less likely? For example: * A patient presents with chest pain of 3 days duration. Chest pain of 3 days duration is associated with a differential diagnosis. The history and physical examination reveal the presence of risk factors for coronary artery disease. There are historical features of coronary artery disease (quality, severity, radiation, associated symptoms, onset, progression, relieving factors, and aggravating factors). The physical examination is negative for findings that are associated with competing diagnoses. How do the ECG findings fit with our working diagnosis? Do the finding increase or decrease the likelihood of our working diagnoses? What else might be done to assist with the diagnosis or is there sufficient information to make a diagnosis and move on toward a consideration of therapy? ECG Sites of Interest The Allan E. Lindsay ECG Learning Center USC ECG Learning Center |