Noninvasive monitoring of nonshunted pulmonary capillary blood flow in the acute respiratory distress syndrome

A fast, digitally controlled flow proportional gas injection system for studies in lung function

The aim of this paper is to describe a device for flow proportional injection of tracer gas in the lungs of mechanically ventilated patients. This device may then be used for the study of the multiple breath indicator gas washout technique to determine the end-expiratory lung volume. Such a tracer gas injection device may also be used in the study of other techniques that rely on uptake and elimination of tracer gas by the lungs. In this paper, an injector is described which enables injection of indicator gas at a predetermined concentration in a breathing circuit independent of the type of breathing. The presented setup uses a control computer to produce steering signals to a multivalve array in proportion to the input breathing signals. The multivalve array consists of ten circular valves, each with a different diameter, which can be opened or closed individually according to the input signal of the array. By opening of a certain combination of valves an amount of sulphur hexafluoride gas proportional to the inspiratory breathing signal is released. The rate of transmission between the components of the injection system was 80 Hz. The injector has a full flow range between 0-10 L/min. The delay time between the breathing signal and the flow response was 70 ms. The aimed washin gas concentration of 1% SF6 was achieved after 0.5 s. The study describes the results of tests to determine valve-flow ratios, step response and dynamic response of the injector. The flow output response of the injector system was shown to increase in input frequencies above 3 Hz. The valve flow ratios showed the largest relative deviation in the two smallest valves of the 10 valve array, respectively 0.005 L/min (25%) and 0.002 L/min (20%). We conclude that the injector can achieve a stable concentration of indicator gas in a breathing system with an accuracy of 0.005 L/min to execute the multiple breath indicator washout test in human subjects. The results of the study indicate that the injector may be of use in other application fields in respiratory physiology in which breathing circuit injection of indicator gas is required.

The effect of diffusion in the respiratory tree on the alveolar amplitude response technique (AART)

Theoretical data for the alveolar amplitude response technique (AART) (J. Appl. Physiol. 41 (1976) 419-424) for assessing lung function was simulated using a single path lung model. This model takes account of stratified inhomogeneities in gas concentrations within the respiratory tree. The data was inserted into previously published parameter recovery techniques that may be used to estimate dead-space volume, alveolar volume and cardiac output. These parameter recovery techniques are based on much simpler mathematical models that do not allow stratified inhomogeneities in gas concentrations. It was found that: (i) recovered dead-space volume depended significantly on the ventilation pattern and on the distribution of volume within of the conducting airways; (ii) alveolar volume was recovered to a good degree of accuracy; and (iii) the recovered value of cardiac output was highly dependent on both the choice of inert gas and parameter recovery technique.