Characterize Left Ventricular Outflow Obstruction in Hypertrophic Cardiomyopathy: DISCUSSION part 2

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Relationship of Doppler Velocity to Gradient

The Doppler peak left ventricular outflow tract velocity relates to the pressure gradient at the site of obstruction. If the Doppler signal is obtained nearly parallel to the flow of blood, this relationship is described by the modified Bernoulli equation: PG = 4 (V)², where PG is the maximum instantaneous pressure gradient at the site of obstruction and V is the peak Doppler velocity. There is experimental and clinical evidence to support a good relationship of the Dopp- ler-derived peak gradient with that measured at ma­nometry both at rest and following interventions in patients with HCM.

The mean ± 1 SD for peak left ventricular outflow velocity in patients with resting LVOTO was 4.2 ± 1.3 m/s, with a peak velocity of 8.0 m/s measured in one patient. By the modified Bernoulli equation, these would correspond to a mean value for peak instanta­neous gradient of 71 mm Hg and a maximally recorded peak instantaneous gradient of 256 mm Hg. Previous investigators have also noted that patients with HCM may demonstrate unusually high left ventricular out­flow tract velocities and gradients. Ad­ditionally, Doppler left ventricular outflow tract signals demonstrated characteristic late systolic peaking, in­dicative of the development of the maximal gradient in mid to late systole. Analogous to patients with resting LVOTO, patients with latent LVOTO provoked by Amyl also developed unusually high peak left ventricular outflow velocities, late systolic peaking of Doppler waveforms, and elevations in peak instanta­neous gradients to levels previously re­ported.
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Precautions and Limitations

Differentiating the left ventricular outflow tract velocity from the Doppler signal of mitral insufficiency may be difficult. They may both show late peaking and unusually high velocities. In such cases, image- directed continuous wave imaging may be useful. Identification of mitral insufficiency by pulsed wave or color flow methods may also be of value. However, there is no substitute for experience in properly locating and identifying Doppler velocities assessed by continuous wave Doppler methods.

Another consideration is the well-known lability of gradients in patients with HCM. Gradients have been reported to vary not only day to day, but also during the course of a cardiac catheterization. We made no attempt to reproduce the Amyl-induced gradients, but we would expect that the same lability noted by previous investigators would exist in this group of patients. Furthermore, no attempts were made to alter medications in use at the time of echo-Doppler studies. Nevertheless, the lability of the gradient does not appear to detract from the therapeutic and prognostic significance of identifying patients as having latent LVOTO.

Patients with HCM included in this study had either asymmetric septal or concentric left ventricular hypertrophy. While this accounts for 95 percent of patients with HCM, the findings herein are not applicable to the small subset of patients with midven- tricular, apical, or posteroseptal and/or lateral wall hypertrophy.

Of note is that patients whose cases were reported in this study were older than those whose cases were previously reported. This may be a result of including patients who were more symptomatic at the time of referral for Doppler echocardiography evaluation, in which case an older age group would be expected. The increased female predominance would also be accounted for by this, as older patients with HCM tend to have an unusually high female proportion. Another explanation, however, for the older age group and female proportion would be inclusion of patients with the recently described syndrome of hypertensive cardiomyopathy of the elderly. These patients have been characterized as being elderly, mostly female, and having long-standing hypertension. Recently, such patients have also been characterized as having elevated left ventricular outflow tract velocities and similar Doppler waveforms as patients with classic HCM. Based on their described pathophysiologic features, it has been recommended that such patients also be treated in a manner similar to patients with classic HCM.

Although we encountered no serious complications in patients receiving Amyl inhalation, all inhalations were performed under the direct supervision of a physician and with continuous electrocardiographic monitoring. Importantly, of 82 patients eligible to receive Amyl, 31 (37 percent) were excluded because of contraindications or recent use of p-blockers. Be­cause of the potential for serious side effects, the decision to administer Amyl should be made in concert with the patient s attending physician, who may be the only person with complete knowledge of the patients clinical condition.
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Conclusions

The results of this study confirm previous studies in identifying echocardiography characteristics of he­modynamic subsets of patients with HCM. Impor­tantly, the additional use of Doppler evaluation, com­bined with inhalation of amyl nitrite in selected patients, permits an entirely noninvasive characteri­zation of the nature and severity of left ventricular outflow obstruction. Information obtained in this man­ner may then be used to assist in the treatment and prognostic assessment of patients with HCM.