When an alternating current of high frequency is applied to the thorax, the first derivative of the impedance to this current is affected by the cardiac cycle resulting in a characteristic wave form.
Phonocardiograms, electrocardiograms, and first derivative thoracic impedance cardiograms were recorded simultaneously in 91 subjects. The first derivative thoracic impedance cardiograms were found to have sharply demarcated points which occur synchronously with the first heart sound, aortic second sound, pulmonic second sound, mitral opening snap, third heart sound, and fourth heart sound. The first derivative thoracic impedance cardiogram may thus be used not only as a reference tracing to help identify heart sounds on the phonocardiogram, but also for directly timing the intervals within the cardiac cycle.
Clinical differentiation of Duehenne MD (DMD), Becker MD, limbgirdle (LGMD) and benign congenital MD (BCMD), in the early stages is not always possible and may require observation for several years before the correct diagnosis can be reached. DMD, with its rapid downhill course, has a t the age of 10 years reached the stage which is f a r beyond t h a t seen usually in patients affected with one of the other three MDs. Becker MD, LGMD, and BCMD, at that age, may still present similar clinical features. Helpful differential diagnostic criteria discussed in this article are summarized in Table 8. If the patient is a girl, i t would be unlikely for her to have X-linked MD. She most likely represents a case of LGMD or BCXID, which may still not be distinguishable from each other in the early stages. Highly elevated CPK and Ald levels arc typical f a r early X-linked MDs and BCMD and uncommon in LGMD. LDH and SGOT are very high in the early stages of DMD. high in Becker MD and BCMD and distinctly less elevated in I.GMD. l h u s serum enzyme levels are helpful in distinguishing LGMD from the other three hIDs. Electrocardiographic abnormalities are common in the X-linked MDs. Tall right precordial R-waves with or without deep Q waves in the left precordial lcads are found in the majority of patients with X-linked MD. It is, however, not a constant finding of X-linked MD. Prcvxnce of the ECG abnormalities appears to exclude XID with autosomal inheritance, notably LGMD and BCMD.
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