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

Abstract

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.

A modification of the Bohr method to determine airways deadspace for non-uniform inspired gas tensions

“…This was not always possible in the past due to the slow response and variable time delay of older-generation sensors and actuators. The proposed algorithm therefore can be used by other gas mixing techniques such as Gültuna et al (1992), Kirmse et al (1998) and Kanhai et al (2003).…”

“…In other techniques, the concentration of indicator in the inspired gas can be made to vary between breaths such as the sinusoidally varying inspired oxygen tensions employed by Williams et al (1997). These inspired gas variations may be achieved by pre-mixing the inspired gas breath by breath, or, in other cases by using devices that allow rapid injection of indicator gases (Gültuna et al 1992, Kirmse et al 1998, Kanhai et al 2003, Clifton et al 2012a into the inspired airflow in real time. In these circumstances, uniform inspired concentrations are difficult or impossible to achieve, which leads to inaccurate estimations of deadspace.…”

Internal Model Control of Inhaled Anaesthesia

“…The research presented in this paper aims to control the concentrations of the gases that are inhaled by the patient, rather than the depth of anaesthesia. Relatively little research has been done in this field [12], [13] and the key challenge of the research is to considerably improve the response time of the low-flow system, and to relieve the burden of the anaesthetist. The focus is on the control of the concentrations of three anaesthetic gases (oxygen O 2 , nitrous oxide N 2 O and isoflurane ISF) within a breath.…”

Design and validation of an analyser to measure sulphur hexafluoride gas during respiration

“…In contrast to other studies, we present a detailed description of the technique and the measurement components themselves. Similarly, a detailed description of a flow-proportional gas injector, to inject tracer gases during respiration for the purpose of lung volume measurement, was published recently (KANHAI et al, 2003). The description and validation of the design may be of use to those designing or using such a sensor.…”