Chest Radiography
See Medicine Recall,
3rd ed.
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Table 3-1. Calculating the Heart Rate on the
ECG
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Electrocardiogram
| What is the normal size of the P wave measured in II? | ≤120 ms (3 boxes)—if longer, consider LA
enlargement ≤0.25 mV (2.5 boxes)—if taller, consider RA enlargement |
| What is the 300, 150, 100 rule? | The 300, 150, 100 rule is used to quickly calculate the rate on the ECG. The HR = 150/(number of small boxes between R waves) or 300/(number of large boxes between R waves) |
| What is the normal range for the PR interval? | 120 to 200 ms (3 to 5 boxes) |
| List three causes of a short PR interval. |
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| List five causes of a long PR interval. |
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| What is the normal range for the QRS interval? | 60 to 119 ms (1.5 to 3 boxes). A conduction delay is present if the QRS is ≥110 ms, and a BBB is present if the QRS is ≥120 ms. |
| What is the QTc? | The corrected QT interval (corrects for the HR) |
| Why is it important to measure intervals? | The measurement allows determination of blocks (e.g., first-degree or RBBB). |
| What prolongs the PR interval? | AV blocks |
| What prolongs the QRS interval? | BBBs Premature ventricular beats LVH Preexcitation syndromes (e.g., WPW syndrome) Electrolyte abnormalities (e.g., hyperkalemia) Paced beats Medications (e.g., amiodarone, procainamide, tricyclic antidepressants) |
| What prolongs the QT interval? | Medications [tricyclic antidepressants, antiarrhythmics (e.g.,
quinidine, procainamide, amiodarone, sotalol, dofetilide), methadone,
antipsychotics, terfenadine plus macrolide antibiotics (e.g.,
erythromycin)] Electrolyte deficiencies (e.g., potassium, magnesium, calcium) Congenital long QT IHD Hypothermia MVP Intracranial events (e.g., subarachnoid hemorrhage) |
| What is the I, aVF rule? | From the previous example, if leads I and aVF are both more positive than negative, the axis is normal. |
| What is a significant Q wave? | 40 ms or one third the height of the QRS |
| What does the Q wave represent? | A previous MI |
| Does the absence of a Q wave mean that no infarction has occurred? | Some infarctions tend to lose Q waves over time: inferior infarctions lose Q waves 50% of the time; anterior infarctions lose Q waves <10% of the time. |
| Describe the QRS complex in the following: | |
| RBBB | QRS ≥120 ms rSR′ pattern in V1 Deep slurred S wave in V6, I |
| LBBB | QRS ≥120 ms All negative in V1 All positive in V6, I |
| LAFB | QRS <120 ms (unless associated with an RBBB) LAD (>–30 degrees) Small Q waves in I, aVL R waves in II, III, aVF (i.e., no IMI) |
| LPFB | QRS <120 ms (unless associated with an RBBB) RAD (>120 degrees) Small Q waves in II, III, aVF R waves in I, aVL (i.e., no lateral MI) |
| If an RBBB pattern is observed with paced beats (transvenous), what should be considered? | Septal perforation by the wire, a high RV septal location (the impulse crosses to the LV side) and biventricular pacing |
| Name the causes of ST elevation. | AMI, ventricular aneurysm, pericarditis, Prinzmetal's angina, myocardial contusion, hypothermia (Osborne or J wave), hyperkalemia, early repolarization, and artifact. ST elevation is also occasionally seen with LBBB, LVH, myocardial neoplasms, hypertrophic cardiomyopathies, and after VT. |
| Name the causes of ST depression. | Acute posterior MI, ischemia, digitalis, LVH, and BBB |
| What does the S1S2S3 pattern (i.e., S waves in leads I, II, and III) or the S1Q3T3 pattern (i.e., S wave in lead I and Q wave and inverted T wave in lead III) suggest? | Acute pulmonary embolus causing acute RV strain |
| How frequently is this ECG abnormality seen? | In 10% of cases |
| What is electrical alternans? | Alternating ECG voltage beat by beat |
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