Heart Beats: Rhythm Self-Quiz: Fast and Deadly

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Mr. M, 52, comes to the ED complaining of intermittent palpitations, shortness of breath, and lightheadedness. Triage takes Mr. M back into the treatment area after a quick evaluation. His vital signs are BP, 140/80; pulse, 148 and regular; respirations, 18; and SpO2, 97% on room air.

While a colleague obtains peripheral I.V. access, you attach a cardiac monitor, which shows sinus tachycardia with frequent premature ventricular contractions (PVCs). Next, you use the SAMPLE mnemonic to conduct a secondary survey:

Signs and symptoms: palpitations, shortness of breath with exertion; lightheadedness when changing from a supine to a sitting position; no chest pain, nausea, or other signs and symptoms

Allergies: none known

Medications: none

Past medical history: none

Last meal: light breakfast 2 hours ago

Event history: signs and symptoms started about a half-hour ago while working in his basement.

Mr. M says that now his chest "feels funny," and he feels as if he's "going to pass out." Mr. M becomes unresponsive, apneic, and pulseless and the monitor shows a wide-complex tachycardia.

In a previous Heart Beats, we reviewed the five basic steps of rhythm analysis:

* Determine the rhythm by measuring the distance between R waves and noting any variations in R-wave regularity. Determine if a 0.12-second or greater variance exists between the shortest and longest R-wave variations.

Figure. Rhythm strip

* Calculate the heart rate, using the rapid rate calculation (counting the number of R waves in a 6-second strip and multiplying by 10 to calculate the heart rate per minute), for regular or irregular rhythms. For a regular rhythm, you can also use the precise rate calculation: Count the number of small squares between two consecutive R waves, and divide this number into 1,500 (the number of small squares in a 1-minute rhythm strip) to obtain the heart rate in beats per minute. Report the atrial and ventricular rates separately if they're different.

* Identify and examine P waves to see if one precedes each QRS complex, and if they're all identical in size, shape, and position.

* Measure the PR interval, which should be 0.12 to 0.20 second.

* Measure the QRS complex, which should be 0.10 second or less.

Looking at the mystery rhythm

Let's quickly analyze this new rhythm (see illustration) using the five steps:

* The ventricular rhythm is regular.

* The ventricular rate is 140 to 160 beats/minute.

* P waves aren't seen.

* The PR interval can't be determined.

* The QRS interval is wide at 0.16 to 0.20 second.

The rhythm is monomorphic (consistent QRS morphology) ventricular tachycardia (VT), meaning it comes from a single focus in the ventricles. (Polymorphic VT is an irregular rhythm with varying QRS morphology because of multiple foci.) Sustained VT lasts more than 30 seconds and can result in inadequate cardiac output, leading to hypotension and heart failure.

Causes of VT include acute coronary syndromes, cardiomyopathy, heart failure, myocarditis, valvular heart disease, use of sympathomimetic agents, electrolyte imbalance (especially hyperkalemia, hypokalemia, and hypomagnesemia), and hypoxemia.

Signs and symptoms may start or stop suddenly, and include chest discomfort and palpitations, syncope, dizziness, shortness of breath, hypotension, decreased mentation, and rapid or absent pulse.

Helping Mr. M

Management of monomorphic VT varies depending on whether the patient is hemodynamically stable, unstable, or (as in Mr. M's case), pulseless.

Call a code and begin CPR immediately and continue with minimal interruptions (no more than 10 seconds). Defibrillation is indicated as soon as possible using the manufacturer recommended device-specific energy level. If the device-specific energy level is unknown, use the default setting of 200 joules for a biphasic defibrillator and 360 joules for a monophasic defibrillator....
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