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Case #51

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An asymptomatic 30-year-old woman.

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What does the ECG reveal?

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The rate is slightly slower than 75 beats/min. P waves are difficult to visualize but can be seen in leads V3, I, and II. The PR interval is slightly prolonged at just greater than 200 milliseconds. Hence, the rhythm is sinus rhythm with first-degree AV block. The intervals are otherwise normal, as is the QRS frontal plane axis. At a glance, there may appear to be low voltage. Before settling on this diagnosis, however, look closely at the voltage standardization of recording, represented by the rectangle at the far left of the tracing and noted in the figure. This rectangle corresponds to 10 mV. The standard 12-lead ECG is recorded such that 1 little box of vertical amplitude is equivalent to 1 mV. Thus, the standardization rectangle would be 10 little boxes tall, as shown in the figure. When an ECG is recorded at "half-standard" voltage, 1 little box is equivalent to 2 mV, and the standardization rectangle would be 5 little boxes tall, as shown in the figure. Thus, this ECG does not represent low voltage, but rather is a normal tracing recorded at half standardization. This case illustrates the importance of a systematic approach to ECG interpretation including an evaluation of recording quality and standardization.

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Case #52

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A 53-year-old woman with long-standing mitral valve prolapse.

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What abnormalities are present on this ECG?

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Sinus rhythm is present with frequent premature ventricular contractions in a bigeminal pattern—a premature ventricular contraction alternating with a sinus beat. The axis is normal. There is an early R-wave transition in the precordial leads with an R wave greater than an S wave in lead V2; normally, the transition from dominant S wave to dominant R wave occurs at lead V4 in the precordium. There are nonspecific ST-segment and T-wave abnormalities in leads V3 to V6.

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How would these abnormalities affect the qualities of the murmur of mitral valve prolapse?

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The classic auscultatory findings of mitral valve prolapse include a midsystolic click and late systolic murmur that continues with constant intensity through S2. These findings are caused by redundant, billowing tissue of the myxomatous mitral valve, much like a parachute in the wind. Maneuvers that increase left ventricular (LV) cavity diameter stretch the mitral valve annulus, leading to a decrease in the amount of redundant tissue (like a parachute being pulled taut), while decreasing LV cavity diameter has the opposite effect, increasing the amount of redundant tissue. A smaller LV cavity will cause the prolapse to occur earlier in systole, moving the click closer to S1 ...

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