Study of Heart Rate Variability in Control and Hypertensive Subjects

Autonomic nervous system comprising sympathetic and parasympathetic divisions has important role inregulating the cardiovascular system. Heart rate variability has been shown to give objective assessment ofautonomic function. Peripheral Pulse Analyzer has been used to study heart rate variability in controls andhypertensive subjects for understanding manifestations of hypertension on autonomic activity. Subjects havebeen divided in two age groups; 18-30 years and 31-44 years and variability parameters have been comparedwith respect to gender stratification, age stratification and disease stratification. Statistical analysis hasshown marked reduction in the coefficient of variation for variability parameter expressed as logarithm (to thebase 10) in comparison to raw or average value of the parameter and has yielded higher discriminatorystrength in various stratified groups. Excluding age and gender sensitive parameters, significant differenceis observed in amplitude of low frequency component for lower age group male/female hypertensives andamplitude as well as area of low frequency component for higher age group female hypertensives. Theseobservations are in agreement with similarly targeted previous studies. Higher age group male controls andhypertensives, however, could not be discriminated by variability study probably due to similar changesmanifested by senility and hypertension. Thus amplitude and area of low frequency component in heart ratevariability spectrum is identified as hypertension specific parameter.

Seventy-five Years of Use of Impedance Plethysmography in Physiological Data Acquisition and Medical Diagnostics.

Impedance plethysmography (IPG) came into existence in 1940 as a result of Jan Nyboer’s pioneering work in the noninvasive assessment of central and peripheral blood flow. The technique got an impetus after introduction first-time derivative of the impedance for accurate determination of stroke volume (SV) and various cardiac intervals. Later, this signal was employed by Parulkar et al for estimation of blood flow index (BFI) and differential pulse arrival time (DPAT) in various segments of the extremity, which were adequate to detect the aortic and peripheral arterial blocks and estimate collateral circulation and distal arterial runoff. The technique was widely used for measurement of respiration and body water. All these applications have resulted into use of bioimpedance for body composition analysis and continuous monitoring of cardiac output as US Food and Drug Adminstration (FDA) approved technologies, which are being used worldwide. Physiological variability has added more value to this technique as single data acquisition gives variability in heart rate and SV (or peripheral blood flow). Morphology index thus derived is very useful in screening patients suspected with coronary artery disease (CAD). All these milestones are briefly described in this paper.

Estimation of Body Fat Mass from Bioelectrical Impedance Analysis in Indian Adults Aged 23 to 81 Years: A Comparison with Dual Energy Xray Absorptiometry.

The purpose of this study was to validate a bioelectrical impedance analysis (BIA) equation for prediction of body fat mass (FM) against dual energy Xray absorptiometry (DXA) in healthy Indian adults with large variations in body mass index and age. Healthy subjects (28 males and 85 females) were investigated by two methods: FM was measured by a dual energy Xray absorptiometry and segmental bioelectrical parameters at various frequencies were measured by a commercial segmental multifrequency BIA instrument. Total body parameters were derived from segmental bioelectrical parameters. As correlation was high and prediction error was low, a single equation was developed for FM as follows: FM = 15.45 + [0.0074 × (Rbody250)] - (3.89 × sex); men = 1, women = 0) + (0.844 × w) - [6938 × (h2/Zbody50)] - (22.22 × h) + [3 × (Xbody250 - Xbody5)/age)] + [1.53 × (Φbody5)] - [0.126 × (Xbody50/h)]. Fat mass predicted with dual energy Xray absorptiometry was 28.11 ± 9.30 kg. BIApredicted FM was 28.12 ± 9.11 kg (R = 0.9794, adjusted R2 = 0.9561, standard error of estimate = 1.95 kg, total error = 1.87 kg). In conclusion, the new developed BIA equation was valid for prediction of FM in healthy subjects aged 23 to 81 years with body mass indices between 15.62 and 39.98 kg.m-2. Inclusion of reactance in the kg.m-2 single prediction equation appeared to be essential for use of BIA equation in adults with large variations in body mass and age.

Design and Development of Ultrasound-based Wheelchair.

A wheelchair is a most commonly used machine in hospitals for patients, mainly those patients who cannot move or walk or who work with great difficulty. A patient on wheelchair has to depend on another person required to control the motion and direction of the wheelchair to avoid obstacles on their path since the patient is not in a condition to control the wheelchair himself. This makes the patient on the wheelchair dependant.1 The prototype of ultrasound-based wheelchair allows patient on the wheelchair to be self-dependant and the wheelchair itself controls the motion by avoiding obstacles in its path. This wheelchair is mainly designed for handicapped people (blind, limp patients mainly) for their domestic purposes. Ultrasoundbased wheelchair avoids obstacles on its path using ultrasound sensors.2 Once an obstacle is detected, the microcontroller gives signal to the motors of the wheel to perform appropriate actions as per the developed algorithm. The designed prototype causes no inconvenience to the patient and allows patient to move within the house or a specified hospital environment. The design cost of ultrasound-based wheelchair prototype is Rs 7000.

Peripheral Pulse Morphology for Early Detection of Coronary Artery Disease.

Based on the past observation of recording abnormal impedance cardiogram (ICG) in 13% normal subjects and subsequent detection of coronary artery disease (CAD) in majority of these cases, led the authors to record peripheral impedance plethysmograms (IPG) in control subjects and patients using impedance cardiovasograph, developed by electronics division, Bhabha Atomic Research Centre (BARC). Analysis of peripheral plethysmograms, thus recorded, has shown 8 dominant morphological patterns of the peripheral pulses depending upon their status of health. In cognizance of these observations, different methods of pattern analysis were used for pattern identification. Fourier Transform based method has been observed to yield higher diagnostic yield. Morphology index (MI) of the peripheral pulse derived from this method was observed to vary from 0.28 to 1, the former indicating the poorest and later the normal health. Among 100 subjects suffering from various disorders, 8 patients with coronary artery disease have recorded average index to be between 0.30 and 0.45.

Impedance plethysmography of thoracic region: impedance cardiography.

Impedance plethysmograms were recorded from thoracic region in 254 normal subjects, 183 patients with coronary artery disease, 391 patients with valvular heart disease and 107 patients with congenital septal disorder. The data in 18 normal subjects and 55 patients showed that basal impedance decreases markedly during exercise in patients with ischaemic heart disease. Estimation of cardiac index by this technique in a group of 99 normal subjects has been observed to be more consistent than that of the stroke volume. Estimation of systolic time index from impedance plethysmograms in 34 normal subjects has been shown to be as reliable as that from electrocardiogram, phonocardiogram and carotid pulse tracing. Changes in the shape of plethysmographic waveform produced by valvular and congenital heart diseases are briefly described and the role of this technique in screening cardiac patients has been highlighted.

Evaluation of state of circulation in radiation injury using impedance plethysmography.

Seven patients with history of radiation burn were subjected to impedance plethysmographic investigation at Non-invasive Vascular Laboratory, K. E. M. Hospital. Impedance plethysmograms were recorded from various locations in both the upper extremities in supine and digits of both the hands in supine as well as on hyper-abduction in sitting position. Control values of blood flow index (BFI) and differential pulse arrival time (DPAT) were derived from similar data in 15 normal subjects. It was observed that digits having thinning of epidermis of skin recorded marked decrease in values of BFI and significant change in value of DPAT and these observations correlated well with thermography and vascular scintigraphy. Digits recording significant decrease in BFI, which were clinically normal, were found to have changes in the skin during follow up examination. Thus IPG provided a sensitive modality for detecting ischaemia in early stages in patients with history of radiation injury.

Effect of pulmonary circulation on vector impedance cardiogram.

Vector impedance cardiograms in horizontal lead configuration [VICG(H)] were recorded in 34 normal subjects, 18 patients with mitral stenosis, 9 patients with mitral regurgitation, 14 patients with aortic regurgitation and 13 patients with non- cyanotic septal disorders. Data in normal subjects revealed that most of the phase reversal points in VICG(H) waveform did not coincide with those of conventional impedance cardiogram. The shape of VICG(H) waveform and values of VICG indices were observed to be markedly affected in patients having significant alteration in the pulmonary circulation and the changes observed were specific of the type of the disorder. The results of the study suggested that right side of the heart had dominant contribution in generation of VICG(H) waveform.

Diagnosis of peripheral arterial occlusive diseases using impedance plethysmography.

Impedance plethysmographic observations have been compared with arteriographic findings in 216 patients with peripheral arterial occlusive diseases. Impedance plethysmographic diagnosis in these patients was obtained by Parulkar's method without apriori knowledge of arteriographic diagnosis. But for minor discrepancy in the anatomical location of the block in few patients, impedance plethysmographic observations correlated very well with arteriographic findings. Impedance plethysmographic diagnosis was found to be correct in 312, wrong in 53, false negative in 8 limbs respectively, yielding a sensitivity of 97.5% and specificity of 98.1% of this technique.

Diagnosis of venous disorders using impedance plethysmography.

Impedance plethysmography (IPG) was carried out in one hundred and forty-one patients suspected of venous disorders using Parulkar's method. In these patients occlusive impedance phlebography (OIP) and venography were also carried out using standard procedures. Comparison of IPG and OIP observations with venographic findings revealed sensitivity of these methods to be 65% and 77.7% in the diagnosis of primary varicosity of veins and chronic deep vein thrombosis respectively with a specificity of 85%. Occlusive impedance phlebograms showing unilateral decrease in OIP parameters were observed to be sufficiently diagnostic. IPG observations in 5 patients with arterio-venous malformation were observed to be different from those in patients with deep vein thrombosis.

Impedance plethysmographic observations in thoracic outlet syndrome.

Forty patients with symptoms of neuro-vascular compression in the upper extremities were subjected to impedance plethysmographic study using Parulkar's method. Two patients recorded decreased blood flow (BFI) in supine position and were diagnosed as having partial occlusion at subclavian level. Sixteen of the patients recorded decreased BFI on 90 degrees abduction and hyper-abduction. Twelve of these patients had radiological evidence of anomalous cervicle ribs. In remaining four patients extrinsic impression on the subclavian artery due to fibrous deposits was confirmed by arteriography. Remaining 22 patients recorded normal impedance plethysmograms. Impedance plethysmography thus provided a non-invasive modality for confirmation of vascular compression in thoracic outlet syndrome.

Diagnosis of aortic occlusive diseases using impedance plethysmography.

Impedance plethysmographic observations have been correlated with aortographic observations in 57 patients suspected of aortic occlusive diseases. Aortic occlusions have been characterised by marked decrease in blood flow index and significant increase in differential pulse arrival time at thigh level bilaterally. Atherosclerotic affection of the aorta has been featured by a bilateral decrease in the value of blood flow index as well as differential pulse arrival time at thigh level. Leriche's syndrome, however, has been found to decrease the blood flow index moderately at thigh in both the legs without any significant change in differential pulse arrival time. Aortography in all the patients has confirmed the diagnosis made by impedance plethysmography.

Physiological correlates of impedance plethysmographic waveform.

A review of investigations into the origin of impedance plethysmographic waveform is presented in this article. Attempts made by several investigators in the comparison of impedance plethysmographic estimations of peripheral blood flow with that obtained by standard methods are briefly described. Investigations indicating the negligible contribution from contact impedance at body electrode interface are highlighted. Temporal correlation of impedance plethysmographic waveform recorded from thorax with various important events of the cardiac cycle is summarised and various hypotheses on the genesis of this waveform are presented.

Impedance plethysmography: basic principles.

Impedance Plethysmography technique has been discussed with explanation of two compartment model and parallel conductor theory for the estimation of peripheral blood flow and stroke volume. Various methods for signal enhancement to facilitate computation of blood flow are briefly described. Source of error in the estimation of peripheral blood flow is identified and the correction has been suggested.

Technical aspects of impedance plethysmography.

This paper describes the basic methods for measurement of body impedance, electrodes and their configuration, and the measuring instrument with its limitations. A microcomputer assisted impedance plethysmograph system, developed at BARC and different lead configurations for impedance plethysmographic investigation are also described. Typical impedance plethysmographic waveforms recorded from a normal subject and measurement of their amplitude and various time intervals are illustrated.