Development and testing of an artificial arterial and venous pulse oximeter.

The monitoring of patients healthcare is of a prime importance to ensure their efficient and effective treatment. Monitoring blood oxygen saturation is a field which has grown significantly in recent times and more specifically in tissues affected by diseases or conditions that may negatively affect the function of the tissue. This study involved the development and testing of a highly sensitive non-invasive blood oxygen saturation monitoring device. A device that can be used to continuously monitor the condition of tissue affected by diseases which affect the blood flow through the tissue, and the oxygen usage in tissue. The device's system was designed to specifically monitor occluded tissue which has low oxygen saturations and low perfusion. Although with limitted validation the system was unable to accurately measure the venous oxygenation specifically, but it was able to measure the mixed oxygen saturation. With further research it would be possible to validate the system for measuring both the arterial and venous oxygen saturations.

An impedance-guided intra arterial catheter.

Under certain circumstances, the placement of arterial catheters can be difficult. In these instances, a form of guidance is preferred to avoid repeated insertions of the arterial catheter. Ultrasonic guidance is generally used in these instances, but the equipment required is expensive and cumbersome. This study produced an arterial catheter that is guided by the impedance of biological tissue encountered between the patient's skin and the lumen of the artery, with the aim of producing a cheaper and manageable alternative to ultrasonic guidance. Additionally, this study has inspected the impedance of human tissue in order to determine whether or not a sufficient and discernable difference between the impedance of the different tissue types could be identified and thus be used to guide an arterial catheter based on said impedance differences. The results indicate that the difference between subcutaneous tissue, fat tissue and skeletal muscle tissue are not clear enough to make accurate discrimination between tissue types. However, the study shows a clear difference between the impedance of arterial blood and the aforementioned tissue, allowing for the device to determine when accurate placement has been achieved. From the results obtained in the studies, the discrimination between blood and other intermediary tissue can be made with 99,4% confidence.

Assessment of a peak flow whistle in nonasthmatic children.

We tested the agreement of peak expiratory flow (PEF) measurements between an electronic spirometer and a peak flow whistle (Whistle Watch, HarMed, Capetown, South Africa). One hundred and three healthy children between ages 6-13 years and with no previous experience in lung function tests participated in the study. Sequential PEF-readings were obtained from the spirometer and the peak flow whistle; all children had an equal number of attempts using both devices. In the case of the spirometer, the highest PEF reading of three acceptable and reproducible efforts was noted as the best PEF (PEF(SPIRO)). Whistle Watch readings were taken as the highest value when the child could activate the whistle. Despite a strong correlation (r = 0.91; R2 = 83%) between the readings of the spirometer and Whistle Watch, there was a lack of agreement between the two devices. For any individual subject, the 95% probability interval ranged between +30.4 to -47 L.min(-1); 64% of the children obtained higher PEF-values on Whistle Watch, compared to the spirometer. These findings suggest that the whistle sound of the peak flow whistle was a significant incentive, which resulted in greater maximal expiratory efforts.

Effect of compliance on a time-domain estimate of the characteristic impedance of the pulmonary artery during acute pulmonary hypertension.

In 11 pigs under general endotracheal anaesthesia, the time-domain method of determining the pulmonary arterial input impedance was compared with the frequency-domain equivalent under normal conditions as well as acute pulmonary hypertension induced by glass microspheres. The time-domain methods of determining the pulmonary arterial compliance C and pulmonary vascular bed resistance Rp compared favourably with the frequency-domain equivalents (r = 0.774, n = 60, p < 0.001 and r = 0.906, n = 60, p < 0.001, respectively), even at mean pulmonary artery pressures (MPAP) of 35 mmHg and above. A consistent and ever-increasing difference between characteristic impedance Zo(omega), estimated by averaging input impedance modulus values over a selected frequency range, and its time domain equivalent Ro with increasing MPAP was shown to be the cause of the poor fit between the measured and remodelled pulmonary blood flow. By analysing a time-domain estimate of the pulmonary characteristic impedance Ro(C, Rp, t), it was demonstrated that the characteristic impedance was dependent on C. Ro is therefore not an accurate representation of the characteristic impedance, especially under conditions of acute pulmonary hypertension. Ro(C, Rp, t) should therefore be calculated instead.

Relation of measures of asthma severity and response to treatment to outcome in acute severe asthma.

BACKGROUND: It would be helpful if patients with asthma who require admission to hospital for an acute attack could be identified. METHODS: The relation between the severity of an attack of asthma as determined by admission assessment and the eventual outcome was studied in 52 asthmatic patients aged 14 to 44 years and admitted to an asthma emergency room. The patient's history, including medication and previous admissions to hospital, was recorded and a clinical assessment, including a full inspiratory and expiratory flow-volume loop, was performed on four occasions: on admission, at two hours and at 12-18 hours after the start of a standardised treatment, and two weeks later on an outpatient basis. Patients who were discharged and who had an uneventful follow up at the two week assessment were defined as good responders. Patients who had to be admitted to hospital after 12 to 18 hours or were readmitted during the two weeks, or both, were defined as poor responders. RESULTS: Thirty eight patients were good responders and 14 were poor responders (seven admitted at 12 to 18 hours, seven returned to hospital). All four patients with a raised arterial carbon dioxide tension (PaCO2) (> or = 6 kPa) and the three with cyanosis were in the poor responder group, and this group had lower peak expiratory flow (PEF) values (21% v 30% predicted) on admission. There was, however, considerable overlap in PEF between the two groups and no clinical measure was able to distinguish between the good and the poor responders reliably. Poor responders had a history of more chronic systemic steroid administration, regular use of inhaled beta agonists, and asthma related hospital admissions. Nearly all the subjects showing a good response to standardised treatment had a PEF > 75% predicted and an FEV1 > 65% predicted 12 to 18 hours after the start of treatment. CONCLUSION: Prediction of outcome at admission was not possible in individual patients. A history of poor long term control of asthma, a PEF < 30% of predicted, a PaCO2 > or = 6 kPa, the presence of cyanosis, and lack of early response to treatment indicated a group of asthmatic patients who are less likely to respond to conventional emergency treatment over a short period.

Pulmonary artery compliance: its role in right ventricular-arterial coupling.

OBJECTIVE: The aim was to investigate the ventricular/vascular coupling of the intact right heart under conditions of normal operation and acute pulmonary hypertension. METHODS: Right ventricular contractility was obtained by calculating the end systolic pressure-volume relationship (Ees) and the effective pulmonary arterial elastance (Ea), applying the Windkessel parameters of the pulmonary arterial input impedance. Coupling between the ventricle and its load could be determined in terms of Ees and Ea. Acute pulmonary hypertension was induced by injecting glass microspheres into the pulmonary vascular bed until a mean pulmonary arterial pressure of more than 35 mm Hg had been reached. Experimental subjects were Landras/Large white pigs (n = 11), studied under general anaesthesia. Ees was obtained by normalising the right ventricle pressure-diameter equivalent of Ees to stroke volume. The lumped element parameters of the Windkessel analogue were calculated from the pulmonary artery pressure and blood flow. Stroke work was calculated from the pressure-volume loop and oxygen consumption derived from the pressure-volume area. Efficiency was taken to be the ratio between stroke work and oxygen consumption. RESULTS: Ea increased significantly as mean pulmonary artery pressure rose, while Ees remained linear and constant. Stroke work, as well as efficiency, increased, with the maximum of the stroke work curve lying to the right of the efficiency maximum. At the control step (before pulmonary artery hypertension), Ees = 1.71 Ea (n = 11). CONCLUSIONS: Under control conditions, the right ventricle operates at maximum efficiency and submaximal work output. Compliance of the pulmonary artery is a significant factor in decoupling the right ventricle from its vascular load. As the compliance decreases with acute pulmonary hypertension, the maximum stroke work against load point shifted in such a manner that the right ventricle changed its operational status from a flow to a pressure pump, resulting in a decreased stroke volume.

The effect of halothane, enflurane and isoflurane on the circulation.

This study, in open-chested dogs, sought to explore the relationship between whole-body oxygen delivery and oxygen consumption during anaesthesia, using increasing concentrations of halothane, enflurane and isoflurane. Results indicate that the cardiac index and oxygen delivery became critical at less than 1 MAC (minimal alveolar concentration of anaesthetic) for the three commonly used vapours. Halothane caused the least depression of contractility, but the stroke volume was reduced by the well-maintained afterload at 1 MAC. Enflurane and isoflurane were associated with more depression of contractility, but the cardiac output was maintained by an increase in heart rate in the case of isoflurane and reduced mean arterial pressure during the use of enflurane.

The load independence of the end-systolic pressure-length relationship of the heart.

The end-systolic pressure-volume relationship is the state of the art in the measurement of myocardial contractility. This index is load-independent and relatively independent of heart rate. In this study the load-independent character of the end-systolic pressure-length (ESPL) relationship was evaluated in dogs under general anaesthesia. The results indicated that the ESPL is pre- and afterload-independent, since the comparative values of ESPL from afterloaded and reduced preload contractions did not differ significantly (N = 75; P = 0.5993). The application of the ESPL relationship as a means of describing the function of the heart as a muscle as well as a pump is discussed.

[Oxygen balance in the normal dog myocardium with increased concentrations of inhaled anesthetics]. / Die suurstofbalans in die normale hondmiokardium gedurende toenemende konsentrasies van inhalasienarkosemiddels.

Myocardial oxygen supply and demand in the normal dog were evaluated as the concentration of halothane, enflurane or isoflurane was increased. Although coronary blood flow decreased as the anaesthetic gas concentration increased, the O2 supply-to-demand ratio remained stable owing to a decrease in the myocardial O2 consumption.

Determination of airway hyper-reactivity in asthmatic children: a comparison among exercise, nebulized water, and histamine challenge.

An easy and accurate method of assessing bronchial hyper-reactivity could be of great value in identifying and classifying the degree of severity of asthma in children. The sensitivity and specificity of three methods of provocation, ie, histamine, nebulized water, and exercise, were compared in 20 asthmatic and 20 control children between ages 5 and 13 years. Three clinical categories of severity ranging from slight (Group 1) through moderate (Group 2) to severe asthma (Group 3) were identified. The three methods were compared in each subgroup for detecting a tendency to bronchospasm. An inverse correlation (-0.57) was found between the histamine dose and clinical degree of severity, whereas distilled water and exercise proved to be too insensitive for identifying Group 1 asthmatics. Histamine challenge in children is a safe and sensitive technique for identifying asthma and for monitoring the severity of the disease during follow-up.

[Pressure converters in the clinical and research field. A short critical review]. / Drukomsetters in die kliniese en navorsingspraktyk. 'n Kort kritiese beskouing.

The correct measurement of various blood pressures in clinical practice is of obvious importance. A method by which the frequency response and damping of various transducers and catheters can be determined is discussed and reference is made to values obtained in the validation of certain catheters.