N. D. Harris

Applications of applied potential tomography (APT) in respiratory medicine

Impedance pneumography, electrical impedance measurements of the lung, is a technique which has been widely used to monitor respiration non-invasively and more recently, the onset of pulmonary oedema. Attempts have been made to try to localise the changes in impedance using electrode arrays and electrode guarding. These techniques allow localisation to a particular hemithorax, but the resolution of the majority of the systems remains poor. To assess the performance and possible clinical applications of APT, measurements have been made following increases in lung volume and pulmonary blood volume. During inspiration an increase in both the area and the magnitude of the impedance changes over the area of the lungs was observed. Numerical analysis of the impedance changes in normal subjects reveals a consistently high correlation between the volume of air inspired and the magnitude of the impedance changes. The resolution of the system is sufficient to monitor differences in ventilation in the right and left lung and to measure variations in these levels with posture. Preliminary clinical work suggests that APT may be used to detect ventilatory defects in certain types of lung disease. APT measurements show a decrease in resistivity over the area of the lungs when the pulmonary blood volume is increased by the intravenous infusion of 1.5 litres of isotonic saline. Similar changes in the volume of fluid in the lungs are known to occur in pulmonary oedema. APT measurements of lung impedance may detect the onset of pulmonary oedema in high risk patients.

Spinal-cord involvement in diabetic peripheral neuropathy

The pathogenesis of diabetic distal symmetrical polyneuropathy (DSP) is poorly understood but there is some evidence that the disease process might extend beyond peripheral nerves. We used magnetic-resonance imaging to measure spinal-cord cross-sectional area in diabetic patients with and without DSP and in healthy controls. There were significant differences in cord area between the groups at C4/5 and T3/4 (p=0.004 and p=0.033, respectively), with a smaller cord area in those with DSP compared with controls (p=0.001 and p=0.016 for C4/5 and T3/4, respectively). These results indicate that DSP is not simply a disease of the peripheral nerve and that there is substantial involvement of the spinal cord.

Monitoring patients with left ventricular failure by electrical impedance tomography.

Acute left ventricular failure (LVF) is a common medical emergency but detection and monitoring of pulmonary oedema remains problematic. Fluid is an important determinant of tissue impedance. Electrical impedance tomography (EIT) is a non‐invasive technique allowing localisation of impedance changes within tissue. We have investigated the relationship between LVF and the electrical impedance of lung tissue. Twenty patients with a clinical diagnosis of acute left ventricular failure were compared with 30 normal subjects. Patients were monitored using serial chest radiographs and electrical impedance tomography measurements of lung impedance during hospital admission. Radiographs were graded according to the severity of pulmonary oedema by two independent radiologists. Lung impedance was significantly (P < 0.0001) lower than normal in patients with left ventricular failure. Values returned towards the normal range as LVF resolved. There was a similar improvement in the score of the chest radiographs. The electrical impedance of the lung is low in left ventricular failure and increases following treatment.

Is ACE Inhibition with Lisinopril Helpful in Diabetic Neuropathy

Thirteen diabetic patients with hypertension (mean diastolic blood pressure 96.2 ± 1.1 mmHg) were included in a study to assess the effects of lisinopril (20 mg day−1) on measures of nerve function. Patients had nerve conduction velocity (NCV), temperature discrimination threshold (TDT), and vibration perception threshold (VPT) measurements. At the end of 12 weeks of treatment with lisinopril, there was a significant improvement in median motor NCV (mean change ± SEM 2.7 ± 0.6 m s−1, p < 0.0001), median sensory NCV (2.1 ± 0.9 m s−1, p = 0.03), peroneal motor NCV (1.0 ± 0.4 m s−1, p = 0.03), and sural sensory NCV (1.9 ± 0.7 m s−1 p = 0.01) values. There were also significant improvements in warm TDT and VPT. Diastolic BP decreased significantly, but there was no significant change in HbA1. Double blind controlled studies are now needed to confirm the effect of lisinopril on measures of nerve function.

Applied potential tomography: a new technique for monitoring pulmonary function

An electrical impedance tomographic imaging system has been developed which can monitor changes in the resistivity of the thorax at a rate of 5 frames per second. There is a high correlation (r greater than 0.95) between changes in resistivity of the lungs and the volume of air inspired. Calibration of the system allows continuous monitoring of the level of ventilation on exercise up to a minute volume of 45 l min-1. The volumetric accuracy of the system is generally within +/- 10% of spirometric measurements. Studies of the effect of changes in posture on the calibration of the system show changes of between +9.5% and -3.8% in normal male subjects. The performance of the system compares favourably with existing techniques for the noninvasive monitoring of ventilation.

Clinical applications of electrical impedance tomography

This article is a preliminary review of the possible clinical applications of electrical impedance tomography (EIT). The applications to, for example, the central nervous, respiratory, cardiovascular and digestive systems are covered. It is concluded that the area of greatest potential application of EIT is monitoring cardiopulmonary function, but that studies on much larger groups of patients than have been carried out hitherto are required to fully assess the potential of EIT as a clinical tool.

Model for the dielectric properties of human lung tissue against frequency and air content.

Electrical impedance tomographic spectroscopy measurements of the lungs are taken from nine normal subjects, in the frequency range 9.6 kHz–1.2 MHz. The results show that resistivity ρ′FRC relative to functional residual capacity increases almost linearly with inspiration volume V, with the slope of the curve increasing with frequency f. Resistivity ρ′9.6 kHz relative to 9.6 kHz decreases with f. ρ′9.6 kHz increases with V, at any given frequency. Curves for ρ′9.6 kHz show a roughly linear trend with log10(f). Based on a discussion of the measurement results, a mathematical lung tissue model is designed that involves extra-capillary blood vessels and alveoli, the walls of which consist of blood-filled capillaries, epithelial cells and intercellular liquid. Using this model, the increase in ρ′FRC with V is explained by the thinning of alveolar walls with increasing air content. The almost linear shape of curves for ρ′9.6 kHz is attributed to four partly overlapping main dispersions caused by extra-capillary blood vessels, epithelial cells, blood and the capillary network.

Diuretic induced change in lung water assessed by electrical impedance tomography

Monitoring patients with left ventricular failure can be difficult. Electrical impedance tomography (EIT) produces cross-sectional images of changes in the impedance of the thorax. We measured changes in the electrical impedance of the lung in nine volunteers following a diuretic challenge. The hypothesis was that lung impedance would increase with diuretic induced fluid loss. Heart rate, blood pressure and urine output were also recorded. After diuretic the mean urine output was 1220 ml compared with 187 ml after placebo. Following diuretic administration, mean thoracic impedance increased by 13.6% (p < 0.01) and lung impedance increased by 7.8% (p < 0.05). Taken as a group there was a correlation between overall impedance change and total urine output. However, for each individual, the time course of change in impedance and urine output did not correlate significantly. Our findings show that EIT may offer a better guide to the response of the lung to diuretic treatment than simply measuring urine output. The urine output is neither specific nor sensitive in the assessment of lung water. Mean lung impedance, however, is largely determined by lung water. The study showed that lung impedance can be recorded at supra-normal values. EIT may help in the management of patients with excess lung water.

Feature extraction and classification of electrocardiogram (ECG) signals related to hypoglycaemia

Nocturnal hypoglycaemia has been implicated in the sudden deaths of young people with diabetes. Experimental hypoglycaemia has been found to prolong the ventricular repolarisation and to affect the T wave morphology. It is postulated that abnormally low blood glucose could in certain circumstances, be responsible for the development of a fatal cardiac arrhythmia. We have used automatic extraction of both time-interval and morphological features, from the electrocardiogram (ECG) to classify ECGs into normal and arrhythmic. Classification was implemented by artificial neural networks (ANN) and linear discriminant analysis (LDA). The ANN gave more accurate results. Average training accuracy of the ANN was 85.07% compared with 70.15% on unseen data. This study may lead towards the demonstration of the possible relationship between cardiac function and abnormally low blood glucose.

Measurement of high resolution ECG QT interval during controlled euglycaemia and hypoglycaemia

During hypoglycaemia, typically there is a change in the surface ECG characterized by a flattened and prolonged T wave, often accompanied by a fused U wave. The QT interval is a useful parameter for quantifying the ECG morphology. However, reliable measurement of QT is not straightforward, particularly for hypoglycaemic ECG morphology. The objective of this study was to compare the ability of two methods of QT measurement to distinguish between ECGs recorded during euglycaemia and hypoglycaemia. The first method involves manually setting the intersection of the isoelectric line and the T wave or, where this is not possible, the nadir between the T and U wave. The second method is semi-automatic and fits a tangent to the point of maximum gradient on the downward slope of the T wave. Two independent observers used both methods to measure the QT for high resolution ECG data recorded during a study of 17 non-diabetic subjects undergoing controlled euglycaemia and hypoglycaemia. Using the mean results of the two observers, the mean +/- SD increase in heart rate corrected QT, QTc, for ECGs recorded during euglycaemia and hypoglycaemia was 32 +/- 25 ms for the non-tangent method and 60 +/- 24 ms for the tangent method. Therefore, the tangent method provides greater distinction between ECGs recorded during euglycaemia and hypoglycaemia than the non-tangent method. A potential clinical application could be the non-invasive detection of impending hypoglycaemia at night, which would be of significant benefit to adults and young children with diabetes.