The medical mass spectrometer.

The attractiveness of mass spectrometer technology comes from its fundamental capability of measuring a large number of gas constituents simultaneously with precision and high speed. During the past two and one-half decades the mass spectrometer has proved to be extremely useful in the laboratory. In this environment the technology has provided the opportunity for innovative research and new discoveries in respiratory physiology. Improved technology (such as new and innovative engineering in vacuum technology, electronics, and microprocessors) has now made this analytic technique practical in the clinical environment. It can be expected that these instruments will continue to play an important role in the future in patient monitoring systems. Because of the specificity and sensitivity in the mass spectrometer, its role in noninvasive monitoring is expected to be most significant. Methods for noninvasive measurements of important physiologic parameters such as cardiac output and lung functions are well established in the research laboratory. These methods depend on the use of small amounts of trace gases, many of which can be effectively measured only by a mass spectrometer. Since the mass spectrometer has been the tool for developing these techniques in the laboratory, it will also be the focus of future noninvasive cardiopulmonary monitoring systems used routinely in the clinical environment.

A computerized mass spectrometer and flowmeter system for respiratory gas measurements.

Instrumentation systems for breath-to-breath analysis of respiratory gas exchange have been faulted by phase lags between various flow and composition signals and by difficulties in gathering and processing large amounts of data. The system described here represents an attempt to overcome these problems. Phase delays have been minimized by using a direct, piezoelectrically operated mass spectrometer inlet rather than an inlet capillary, by locating the mass spectrometer inlet and flow sensors in the same plane, and by incorporating design features which enhance the mass spectrometer response. Convenience in system operation and data analysis has been enhanced by integrating a computer into the system design so that the computer performs on-line data analysis and control functions and provides the primary interface between the experimenter and the instruments. Miniaturization of the instruments permits close coupling to an exercising subject.

Improved phonocardiogram system based on acoustic impedance matching.

We considered that phonocardiographic recording could be improved 1) by minimizing the acoustic impedance mismatch between the precordial tissue and transducer, 2) by optimizing the configuration of the impedance-matching medium and transducer design, and 3) by storing signals in digital form through analog-to-digital conversion of analog recordings made at the bedside. The use of an aqueous coupling medium to improve energy transmission increased signal voltage approximately 100-fold over presently used commercial devices. Further match to the crystal was achieved by a concentrating horn configuration for the aqueous medium. Measured frequency response of the device in the range 1 Hz to 1 kHz was better than two other commercially tested microphones. Inspection of comparative phonocardiograms showed more information from the new device than from the two other commercial devices. Unfiltered digitized signals, using our microphone in normal subjects, demonstrated good beat-to-beat repeatability, but analog filtering to obtain the conventional phonocardiogram showed significant loss of information. The new instrument appears to be superior to those commercial devices tested in recording heart sounds.

Ultrasonic pulsed Doppler transoesophageal measurement of aortic haemodynamics in humans.

Centreline blood velocity and exterior wall motion were measured in the descending aorta of humans using an oesophageal probe, a pulsed ultrasound Doppler velocity meter and an ultrasonic echo tracking system. The development of a method for easily measuring haemodynamics in the thoracic region using an oesophageal probe will provide an essentially non-invasive method for the assessment of cardiac function and the nature of blood vessels in that region. Detailed anatomical studies of the thorax were conducted by cross-sectioning of the thorax of a cadaver. Blood velocity waveforms were recorded from the descending aorta both during rest and exercise. In one volunteer, the peak centreline velocity increased from a resting value of approximately 30 cm s-1 to an exercise value of approximately 50 cm s-1. Vessel diameter waveforms similar to those for pressure were also recorded showing diameter changes of 1.8 mm. The accuracy and resolution of the technique would be improved by multicrystal probes and multigate ultrasonic flowmeters allowing for accurate calculation of the Doppler angle, imaging of vascular flow regions, and measurement of pulse wave velocity.

A closed lung system study of inert gas absorption.

The demonstration that the rate of rise of the alveolar fraction of nitrous oxide is enhanced when the inspired N2O concentration is high is termed the "concentration effect." A similar effect on a second gas has been termed the "second gas effect". These effects have been observed in open systems and attributed to differential changes between inspired and expired ventilation. The purpose of this investigation is to study these effects in a closed system. A breath-holding maneuver was utilized with a high and a low N2O concentration in argon and oxygen. The results indicate that breath holding with a high N2O concentration "concentrates" both the alveolar fraction of N2O and argon. These results are attributable to alveolar volume shrinkage as a consequence of the large absorption of N2O by the pulmonary blood. A mathematical model verifies this interpretation and suggests that volume shrinkage can be important in breath-holding maneuvers designed for noninvasive measurement of cardiac output and lung tissue volume.

Ultrasonic transesophageal measurement of hemodynamic parameters in humans.

A noninvasive method has been developed to monitor centerline blood velocity waveforms and vessel diameter in the descending aorta and pulmonary artery of conscious humans. An esophageal endoscope fitted with miniature ultrasound transducers is swallowed and positioned in the esophagus near vessels of interest. The transducers are connected to ultrasound Doppler velocimeters and echotrack instrumentation to obtain the pertinent hemodynamic parameters. This paper describes the design and fabrication of the esophageal ultrasound transducers and the techniques involved in human applications. In addition, blood velocity and wall motion measurements obtained in conscious men at rest and during exercise are described.

On-line cardiac output by digital computer.

Many devices for fast cardiac output calculations employ special assumptions and departures from the Stewart-Hamilton method. These assumptions and departures can cause substantial errors. This publication describes an on-line computer system for evaluating dye-dilution curves according to the classical Stewart-Hamilton method. Dilution curves are smoothed, using a digital filter to eliminate heartbeat noise. Recirculation is removed using the constant ratio property of exponentials. The quality of the curves is evaluated by determining the length of the base line, the extent and the fit to an exponential downslope, and a figure of merit based on the five tanks-in-series model. Dilution curves, the logarithmic form, and the exponential ratios are displayed on a CRT for visual checks. Comparison of cardiac outputs obtained by this on-line system with results obtained by hand calculations showed a mean difference of 150 ml. Repeated evaluation of one electronically simulated dye curve resulted in SD = 10 ml for computer results and SD = 200 ml for hand calculations.

On-line impedance analysis system for studying the pulmonary vascular response to hypoxia.

The response of the pulmonary circulation to hypoxia is generally assessed in terms of cardiac output and pulmonary vascular resistance. However, due to pulsatility, the opposition to blood flow can be described accurately only by the input impedance to the pulmonary bed. For assessment of impedance, a system was developed consisting of implantable pressure transducers and an improved muitichannel continuous wave Doppler flow meter. A computer system was used to analyze the data, which was based on power output of the right heart, power dissipation in the pulmonary circulation, and impedance spectra. Data have been obtained from dogs exposed to various levels of acute hypoxia.

Diminished ventilatory response to hypoxia and hypercapnia after morphine in normal man.

Although morphine depresses respiration the mechanism of this depression remains unknown. Accordingly, ventilatory responses to hypoxia and to hypercapnia were measured before and after administration of 7.5 mg of morphine sulfate subcutaneously in six normal subjects. This procedure produced resting hypoventilation manifested as a peak rise in alveolar carbon dioxide tension from 42.9 plus or minus 1.7 to 45.4 plus or minus 1.5 mm Hg (plus or minus S.E.M.) at 30 minutes ( greater than 0.01). Hypoxic ventilatory drive, measured by an index of the relation between ventilation and hypoxia (parameter A), decreased from a control of 108 plus or minus 17.6 to 42.8 plus or minus 5.3 at 60 minutes after morphine (p greater than 0.01); Hypercapnic ventilatory drive, measured as the slope of the ventilatory response to hypercapnia, also decreased from 1.69 plus or minus 0.24 to 0;98 plus or minus 0.20 (p greater than 0.01) 75 minutes after morphine. Decreased responsiveness to the chemical stimuli to breathing may contribute to the ventilatory depression frequently seen after administration of morphine.