T. Crawford McAslan

Automated respiratory gas monitoring of critically injured patients.

Since 1971, we have used a mass spectrometer system for automatic hourly sampling of airway gases on a 24-hour basis in our 12 bed Critical Care Unit. Used in conjunction with arterial and mixed-venous blood samples, the availability of end-tidal O2 and CO2 values allows early identification of increasing AaDO2 and aADCO2 gradients. The ability to monitor end-tidal CO2 allows the monitoring of the adequacy of alveolar ventilation both in patients on and following removal from mechanical ventilation. Continuous information of the end-tidal PCO2 is of particular value in the management of patients with severe head injury.

The Continuous Measurement of Pulmonary Gas Exchange and Mechanics

Abstract A system for breath-by-breath measurement of respiratory gas exchange and airway mechanics from analog recordings is described. The essential components are a pneumotachometer for measuring air flow, a manometer for airway pressure, and a mass spectrometer for continuous rapid analysis of O 2 , CO 2 , and N 2 . Analog signals from these instruments are recorded, manual measurements done for each breath, and calculations made of variables such as anatomical dead space, alveolar ventilation, oxygen consumption, carbon dioxide production, compliance, and resistance. Use of the Engstrom ventilator permits simplified calculation of nonelastic resistance due to its wave form. The basic calibration routine of the components of this system is described and the mathematical methods are presented. Its usefulness in the management of the critically ill patient is illustrated.

Automatic Respiratory Gas Monitoring

Abstract A unique system is described for automatic sequential sampling of airway gases operating on a 24-hour-a-day basis in a 12-bed Shock Trauma Recovery Unit. A series of 50-foot-long sampling lines run from each bed area to a manifold. A small digital calculator each hour automatically places a mass spectrometer in an operative mode, switches a manifold to a calibrating gas, sequentially samples each occupied bed, prints out calculated data, and then reverts to standby status. The analog waveforms from the spectrometer are conditioned for high and low levels corresponding to inspiratory and peak expiratory values of oxygen and peak expiratory carbon dioxide. The waveforms are counted for respiratory rate, and the respiratory quotient is computed. These data are used, in lieu of frequent blood gas determinations, in making estimates of arterial pCO 2 for adjusting respirators and to monitor airway oxygen levels. The system in conjunction with a pneumotachometer is compatible with on-line computer analysis of respiratory gas exchange and mechanics. Using the system, similar data may be obtained off line by collecting mixed expired gases using more conventional techniques and immediate bedside measurement.

Optimal flow pattern for mechanical ventilation of the lungs. Evaluation with a model lung.

The major components entering into the inspiratory pattern of various respirators were tested on a model lung which had an abnormally high airway resistance on one side. The tests consisted of simultaneous nitrogen washout curves from each lung separately utilizing two mass spectrometers. The components tested included a constant versus accelerating wave form and the presence, duration or absence of an end-inspiratory pause. Respirators tested included the Bennett MA1, Engström 300 and the Elema Schonander Servo Ventilator 900. The results demonstrated the importance of an end inspiratory pause in improving gas distribution and efficiency of washout. No difference was found between a constant or accelerating air flow. Preliminary results in man appear to confirm the importance of an end inspiratory pause.The major components entering into the inspiratory pattern of various respirators were tested on a model lung which had an abnormally high airway resistance on one side. The tests consisted of simultaneous nitrogen washout curves from each lung separately utilizing two mass spectrometers. The components tested included a constant versus accelerating wave form and the presence, duration or absence of an end-inspiratory pause. Respirators tested included the Bennett MA1. Engstrom 300 and the Elema Schonander Servo tventilator 900. The results demonstrated the importance of an end inspiratory pause in improving gas distribution and efficiency of washout. No difference was found between a constant or accelerating air flow. Preliminary results in man appear to confirm the importance of an end inspiratory pause.