Kavassery Mahadevan Krishnamoorthy

Normal-Based Methods for a Gamma Distribution : Prediction and Tolerance Intervals and Stress-Strength Reliability

In this article we propose inferential procedures for a gamma distribution using the Wilson–Hilferty (WH) normal approximation. Specifically, using the result that the cube root of a gamma random variable is approximately normally distributed, we propose normal-based approaches for a gamma distribution for (a) constructing prediction limits, one-sided tolerance limits, and tolerance intervals; (b) for obtaining upper prediction limits for at least l of m observations from a gamma distribution at each of r locations; and (c) assessing the reliability of a stress-strength model involving two independent gamma random variables. For each problem, a normal-based approximate procedure is outlined, and its applicability and validity for a gamma distribution are studied using Monte Carlo simulation. Our investigation shows that the approximate procedures are very satisfactory for all of these problems. For each problem considered, the results are illustrated using practical examples. Our overall conclusion is that the WH normal approximation provides a simple, easy-to-use unified approach for addressing various problems for the gamma distribution.

Assessing occupational exposure via the one-way random effects model with balanced data

A one-way random effects model is considered for the log-transformed shift-long personal exposure measurements, where the random effect in the model represents an effect due to the worker. Following a previous approach, we address a hypothesis-testingproblem involving the proportion of workers for whom the mean exposure exceeds the occupational exposure limit. A confidence interval is constructed for the relevant parameter of interest, following the idea of a previously presented generalized confidence interval. The confidence bound is used for the purpose of testing hypotheses, and the performance of the test is numerically in vestigated. It turns out that the test exhibits satisfactory performance regardless of the sample size, in particular, for small samples. A similar procedure is then employed for testing hypotheses concerning the overall mean exposure. The results are illustrated using examples.

Percutaneous closure of patent ductus arteriosus in children: Immediate and short-term changes in left ventricular systolic and diastolic function

Objective: To evaluate the effect of percutaneous closure of patent ductus arteriosus (PDA) on left ventricular (LV) systolic and diastolic function in children. Background: Limited studies are available on alteration in LV hemodynamics, especially diastolic function, after PDA closure. Methods: Thirty-two consecutive children with isolated PDA treated by trans-catheter closure were studied. The LV systolic and diastolic function were assessed by two-dimensional (2D) echocardiography and tissue Doppler imaging 1 day before the PDA closure, on day 1, and on follow-up. Results: At baseline, none of the patients had LV systolic dysfunction. On day 1 post-PDA closure, 8 (25%) children developed LV systolic dysfunction. The baseline LV ejection fraction (LVEF), LV end-systolic dimension (LVESD), and PDA diastolic gradient predicted the post-closure LVEF. Patients who developed post-closure LV systolic dysfunction had poorer LV diastolic function than those who did not. LV diastolic properties improved after PDA closure; however, the improvement in LV diastolic properties lagged behind the improvement in the LV systolic function. All children were asymptomatic and had normal LVEF on follow up of >3 months. Conclusions: Percutaneous closure of PDA is associated with the reversible LV systolic dysfunction. Improvement in the LV diastolic function lags behind that in the LV systolic function.

Percutaneous treatment of congenital coronary arteriovenous fistulas.

BACKGROUND Congenital coronary arteriovenous fistulas (CAVF), although rare, can present as significant hemodynamic lesions necessitating intervention. METHODS AND RESULTS Six patients (two males) with congenital coronary arteriovenous fistulas (CAVF) underwent percutaneous transcatheter occlusion. The ages ranged from 4 years to 49 years (mean 20.1 years). The fistulas had their origins from the right coronary artery (two), the left anterior descending coronary artery (two), and the left circumflex coronary artery (two). One of the fistulas drained to the right ventricle, four drained to the right atrium, and the remaining one to the left ventricle (LV). The fistulas were closed using the arterial approach with Cook™ coils in two patients and with nitinol ductal occluders (NDOs) using the venous approach in four patients. One patient developed dissection of the wall of the fistula during attempted closure and had spontaneous occlusion of the fistula. Complete occlusion of the fistulas were achieved in all patients. Complications consisted of migration and embolization of the coils in one patient (later closed successfully with NDO) and myocardial infarction occurring two weeks following successful closure in another patient. At mean follow-up of 39.6 +/- 22.9 months, all patients were asymptomatic and echo-Doppler evaluation revealed no residual fistulae. CONCLUSIONS CAVF are very well amenable to percutaneous closure with acceptable morbidity and high success rates.

Unusual Coarctation—the PHACE Syndrome: Report of Three Cases

OBJECTIVES To report the clinical features, nature of vasculopathy observed and the management instituted in three cases of PHACE syndrome. RESULTS All three patients were noted to have diffuse vasculopathy and aortic arch hypoplasia in addition to aortic coarctation adding substantial surgical risk or making surgery palliative. CONCLUSION PHACE syndrome should be considered in the differential diagnosis of aortic coarctation associated with aortic arch hypoplasia.

Congenital bilateral aplasia of external iliac arteries

A child with bilateral aplasia of external iliac arteries with normal internal iliac arteries, demonstrated by vascular Doppler and digital subtraction angiography is presented. Popliteal artery is reformed by collaterals. This anomaly is extremely rare.

Inference for the Lognormal Mean and Quantiles Based on Samples With Left and Right Type I Censoring

Interval estimation of the mean and quantiles of a lognormal distribution is addressed based on a Type I singly censored sample. A special case of interest is that of a sample containing values below a single detection limit. Generalized inferential procedures which use maximum likelihood estimation based approximate pivotal quantities, and some likelihood based methods, are proposed. The latter include methodology based on the signed log-likelihood ratio test (SLRT) statistic and the modified signed log-likelihood ratio test (MSLRT) statistic. The merits of the methods are evaluated for a left-censored sample using Monte Carlo simulation. For inference concerning the lognormal mean, the SLRT is to be preferred for left-tailed testing, generalized inference for right-tailed testing, and all three approaches provide nearly the same performance for two-tailed testing. These conclusions hold even when the proportion of censored values is as large as 0.70. For inference concerning quantiles, both the generalized inference approach and the MSLRT approach are satisfactory. In view of its simplicity and ease of understanding and implementation, the generalized inference procedure is to be preferred. The results are illustrated with two examples. Technical derivations are given on the Technometrics website as supplementary material.

Doppler echocardiographic assessment of TTK Chitra prosthetic heart valve in the mitral position

AIMS TTK Chitra heart valve prosthesis (CHVP), a tilting disc mechanical heart valve of low cost and proven efficacy, has been in use for the last 15 years. Although various studies substantiating its long-term safety and efficacy are available, no study had assessed its echocardiographic characteristics. The purpose of this study was to determine the normal Doppler parameters of CHVP in the mitral position and to assess whether derivation of mitral valve area (MVA) using the continuity equation (CE) and more commonly used pressure half-time (PHT) method is comparable in the functional assessment of this tilting disc mitral prosthesis. METHODS AND RESULTS Doppler echocardiography was performed in 40 consecutive patients with CHVP in the mitral position. All patients were clinically stable, without evidence of prosthetic valve dysfunction such as significant obstruction or regurgitation, endocarditis, left ventricular dysfunction (ejection fraction <40%), or significant aortic regurgitation. Valve sizes studied included 25, 27, and 29 mm. Mitral valve area was derived both by the PHT method and by the CE, using the stroke volume measured in the ventricular outflow tract divided by the time-velocity integral of CHVP jet. The peak Doppler gradient ranged from 5 to 21 (mean 11.0) mmHg, and the mean gradient ranged from 1.7 to 9.2 (mean 4.1) mmHg. Mean gradient negatively correlated with an increase in the actual orifice area (AOA) derived from the valve orifice diameter given by the manufacturer (r = -0.45, P = 0.004). Mitral valve area calculated by both PHT and CE increased significantly with an increase in the AOA (r = 0.42, P = 0.007 and r = 0.32, P = 0.046, respectively). Mitral valve area by the CE averaged 1.55 +/- 0.36 cm(2) (range 0.85 cm(2) for a 25 mm valve to 2.41 cm(2) for a 29 mm valve) and was smaller than by PHT (mean 2.04 +/- 0.41 cm(2), range 1.40-3.14 cm(2); P = 0.0001; t-test), irrespective of whether PHT is less than or >110 ms. CONCLUSION The Doppler parameters obtained with CHVP in the mitral position are comparable with those obtained with the different prosthetic valves in common use. In the selected group of patients with CHVP, assessment of MVA by the PHT method is comparable with that by the CE. Areas by both methods were smaller than the AOA provided by the manufacturer, as seen in other similar design valves.

Multiple use one-sided hypotheses testing in univariate linear calibration

Consider a normally distributed response variable, related to an explanatory variable through the simple linear regression model. Data obtained on the response variable, corresponding to known values of the explanatory variable (i.e., calibration data), are to be used for testing hypotheses concerning unknown values of the explanatory variable. We consider the problem of testing an unlimited sequence of one-sided hypotheses concerning the explanatory variable, using the corresponding sequence of values of the response variable and the same set of calibration data. This is the situation of multiple use of the calibration data. The tests derived in this context are characterized by two types of uncertainties: one uncertainty associated with the sequence of values of the response variable, and a second uncertainty associated with the calibration data. We derive tests based on a condition that incorporates both of these uncertainties. The solution has practical applications in the decision limit problem. We illustrate our results using an example dealing with the estimation of blood alcohol concentration based on breath estimates of the alcohol concentration. In the example, the problem is to test if the unknown blood alcohol concentration of an individual exceeds a threshold that is safe for driving.

Persistent pulmonary artery hypertension in patients undergoing balloon mitral valvotomy.

Pulmonary artery pressure (PAP) is known to regress after successful balloon mitral valvotomy (BMV). Data of persistent pulmonary artery hypertension (PPAH) following BMV is scarce. We analyzed the clinical, echocardiographic, and hemodynamic data of 701 consecutive patients who have undergone successful BMV in our institute from 1997 to 2003. Data of 287 patients who had PPAH (defined by pulmonary artery systolic pressure [PASP] of ≥ 40 mmHg at one year following BMV) were compared to the data of 414 patients who did not have PPAH. Patients who had PPAH were older (39.9 ± 9.9 years vs. 29.4 ± 10.1; P < 0.001). They had higher prevalence of atrial fibrillation (AF; 21.9 vs. 12.1%, P < 0.05), moderate or severe pulmonary artery hypertension (PAH) defined as PASP more than 50 mmHg (43.5 vs. 33.8%, P = 0.00), anatomically advanced mitral valve disease as assessed by Wilkins echocardiographic score > 8 (33.7 vs. 23.2%, P < 0.001), and coexistent aortic valve disease (45.6 vs. 37.9%, P < 0.001) at the baseline. Those patients with PPAH had comparatively lower immediate postprocedural mitral valve area (MVA). On follow-up of more than five years, the occurrence of restenosis (39.3 vs. 10.1%, P = 0.000), new onset heart failure (14% vs. 4%, P < 0.05) and need for reinterventions (9.5% vs. 2.8%, P < 0.05) were higher in the PPAH group. Patients with PPAH were older, sicker, and had advanced rheumatic mitral valve disease. They had higher incidence of restenosis, new onset heart failure, and need for reinterventions on long term follow-up. PPAH represents an advanced stage of rheumatic valve disease and indicates chronicity of the disease, which may be the reason for the poorer prognosis of these patients. Patients with PPAH requires intense and more frequent follow-up.