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Biomedical subjects

A K Ream

Publications and source records attributed to A K Ream.

30 records · Page 2Linked to original sources
PubMed1979

Epidural measurement of intracranial pressure.

Although the measurement of intracranial pressure (ICP) is gaining widespread acceptance, the most desirable method of measurement is disputed. Subdural fluid-coupled techniques are associated with an increased risk of infection, and epidural techniques are associated with decreased accuracy. We investigated epidural measurement techniques and suggest that the necessary and sufficient criteria for accurate epidural measurement of ICP are adequate transducer size and stiffness, transducer-dura coplanarity, transducer-guard ring coplanarity, complete dural contact, and rigid fixation. An epidural transducer design was developed and prototypes were constructed using these principles. The transducer requires no percutaneous connections, fluid coupling, or batteries. Transducer accuracy was +/- 2.2 torr in bench stability studies lasting up to 198 days, +/- 3.0 torr in acute animal studies of less than 24 hours, and +/- 7.9 torr in chronic animal studies lasting up to 112 days. Error bounds are expressed such that 95% of individual measurements are expected to have error less than the bound; average error is one-third of the bound. Average transducer drift was 0.1 torr per day; our reported accuracy in chronic studies used drift correction from preimplantation data. We conclude that accurate measurement of ICP using an epidural transducer is feasible.

Animals↗
PubMed1978

Cutaneous monitoring of systemic PCO2 on patients in the respiratory intensive care unit being weaned from the ventilator.

A procedure for measuring the partial pressure of CO2 in equilibrium with the epidermis was developed at Hewlett--Packard Laboratories. It consists of determining by infra-red absorption techniques the concentration of CO2 inside a small (50 microliter) chamber applied hermetically over and around an epidermal window (2.25 cm2) stripped of its stratum corneum or horny layer. We have applied the procedure to 25 patients in the Respiratory Intensive Care Unit at Stanford Hospital. Only patients scheduled for weaning from the ventilator were selected for the study. The epidermal windows were on the medial aspect of the forearm and were monitored for 3--6 h. Arterial blood samples were periodically withdrawn from the catheterized radial artery and were analysed by conventional means for subsequent comparison with the cutaneous measurements which were recorded in real time at the rate of one a minute. The range of arterial PCO2 values that were measured varied from 3.33 to 9.30 kPa (25 to 70 mmHg) and correlated well with the corresponding cutaneous PCO2 values that were typically higher than the arterial values by 0.7 kPa (5.2 mmHg) with a standard deviation of 0.2 kPa (1.5 mmHg). Some recordings of cutaneous PCO2 are shown and discussed.

Blood Gas Analysis↗
PubMed1978

Evolution of the solenoid-actuated left ventricular assist system: integration with a pusher-plate pump for intra-abdominal implantation in the calf.

The performance of an implantable left ventricular assist system (LVAS) utilizing a pulsed solenoid energy converter and a pusher-plate blood pump has been characterized in vitro and in vivo. A microprocessor-based electronic control system makes the LVAS completely self-regulating over the range of operating conditions and provides considerable flexibility in various assist modalities. Over forty thousand hours of in vitro and in vivo operating experience has been accumulated with current systems, and significant progress has been acheived in system durability and reliability. A new toggle latch has provided nearly a year of failure-free operation on the bench, without measurable wear. Energy converter efficiencies of 50% have been demonstrated. In vivo evaluation has been highlighted by an animal experiment still in progress after nearly four months of fault-free, continuous synchronous pumping.

Animals↗
PubMed1978

A conductive catheter to improve patient safety during cardiac catheterization.

A 60 Hz current, as small as 20 microamperemeter (rms) is capable of causing ventricular fibrillation when directly applied to the heart. Significant cost and engineering effort has been spent to construct monitoring equipment which satisfies the safety regulations requiring maximum leakage currents below this value. Patients undergoing cardiac catheterization are particularly at risk from electrical hazards, primarily because catheters are made from nonconductive materials. A conductive catheter should allow externally applied currents to leak through its walls before reaching the catheter tip. A new electrically conductive catheter was compared with a standard nonconductive catheter. Five dogs were studied, with 81 attempts to cause fibrillation. Sixty-hertz voltage between the catheter and an external electrode was increased until fibrillation occurred or 130 V was reached. Eight states were studied in randomized sequence: conductive or nonconductive catheter, guidewire or saline-filled and tip touching wall, or free in left ventricle (verified by fluoroscopy and cineangiography). The saline-filled and conductive catheter was safer in that fibrillation never occurred, while fibrillation nearly always occurred with the nonconductive catheter. A conductive guidewire negates the protection of the conductive catheter. The application of conductive catheters could reduce instrumentation costs in laboratories and intensive care units and improve patient safety.

Animals↗
PubMed1978

Telemetry of intracranial pressure.

In the care of patients with abnormal intracranial pressure, it is important to continuously and accurately measure the intracranial pressure so that treatment can be instituted. Strain gage pressure cells have successfully replaced the open-end manometer for chronic use but have lacked the stability for accurate long-term measurements. The system described uses a capacitive pressure transducer with excellent long-term stability. The transducer, electronics, and transmitter package can be implanted to telemeter the pressure transcutaneously, reducing the danger of infection by the elimination of transcutaneous leads. The pressure modulates the frequency of an oscillator in the 8-10 MHz region and provides an easily telemetered signal. To eliminate the problems of periodic battery replacement, the system is energized inductively by a coil placed externally over the implant device, so that the operating life is virtually unlimited.

Animals↗
PubMed1976

An intrathoracic solenoid drive system for chronic left ventricular bypass.

The performance of an implantable Left Ventricular Assist System (LVAS), utilizing an advanced solenoid energy converter, has been extensively studied in vivo. For optimum system efficiency, reliability and responsiveness, the energy converter and blood pump are integrated into a single compact unit, which is implanted intrathoracically in an 80 Kg calf. The MK19 LVAS is battery operated and controlled by a miniature analog computer and is completely self-regulating. The maximum hemodynamic power provided by this circulatory assist system is 4 watts with cardiac outputs up to 10 L/min and a total system efficiency of 30%. LVAS function has been studied in a series of 19 calf experiments progressing through acute, model and chronic evaluations. Development of the implantation protocols and hemodynamic characterization were achieved in a series of 8 acute experiments. Long-term studies were conducted for periods up to 43 days in a series, including 2 control experiments using non-functional models and 9 functional LVAS chronic experiments. Aortic, left ventricular and left atrial pressures, pulmonary artery flow and pump outflow were monitored with chronically implanted transducers. A dedicated minicomputer provided real-time hemodynamic analysis and continuous surveillance of physiologic and LVAS parameters. In acute studies and in the immediate post-operative period of chronic experiments, synchronous LVAS operation reduced peak LV pressures to 30 mm Hg with cardiac outputs of 5 to 7 L/min. In chronic studies, with recovery of normal LV function, long-term synchronous operation was maintained, without capture of the total cardiac output and with significantly reduced ventricular unloading.

Animals↗