Development andin vitro validation of a device for measuring non-shunt cardiac output by nitrous oxide throughflow

Abstract

A system has been developed for measuring non-shunt cardiac output by the throughflow technique, using nitrous oxide in patients undergoing general anaesthesia. The throughflow measurement technique is a non-invasive method based on inert gas throughflow theory. In vitro validation of the measurement system was performed using a lung gas exchange simulator. The accuracy and precision of the throughflow measurement system was assessed by comparing measured and target values for five simulated values of non-shunt cardiac output, from 2.88 to 9.86 l min(-1). This showed an overall mean bias of -0.031 min(-1) (range -0.00 to -0.101 min(-1)), with a mean coefficient of variation of the difference of 1.39% (1.20-1.93%). These results indicate that the measurement system is suitable for monitoring the non-shunt cardiac output in patients undergoing general anaesthesia using nitrous oxide throughflow.

Continuous measurement of gas uptake and elimination in anesthetized patients using an extractable marker gas

“…The acceptable lower limit for marker N 2 concen-tration depends on the standard deviation of the measurement of N 2 concentration in fresh and exhaust gas streams. We have found that, in our system, which used fully mixed gas streams without tidal variations in gas concentrations, acceptable scatter in measured uptake for all gases for clinical purposes is achieved with fresh gas N 2 concentration of ϳ30% (6,17,21).…”

“…This was chosen as it lay midway within the measurement range of the multigas analyzers for CO 2. For both the extractable marker method and the Haldane method, marker gas flows (CO 2 and N2) were monitored continuously from gas supply pressures (measured with solid-state pressure transducers) at a high-pressure gas source (cylinders of compressed medical air and CO 2 in conjunction with variable pressure regulators, delivering pressures of 400 -460 kPa) and the previously determined pressure-flow characteristics of a series of hollow-wire gas flow resistors (see APPENDIX) (6,21). It was found during bench testing that, at this flow rate, full extraction of CO 2 was ensured for 3-4 h before depletion of the soda lime on the fresh gas side became significant, using two standard 1-liter cannisters in series containing soda lime (Medisorb).…”

“…gas elimination; nitrogen balance; gas dilution MEASUREMENT OF GAS UPTAKE or elimination in a breathing system is performed for a number of purposes. These include metabolic monitoring and exercise testing using O 2 uptake (V O 2 ) and CO 2 elimination (V CO 2 ), measurement of various markers of respiratory function such as diffusing capacity and shunt fraction (15), and measurement of pulmonary blood flow from V CO 2 or inert gas uptake (7,12,17,21). Volumetric techniques employing a fully closed breathing system (2)(3)(4) have been commonly used for physiological studies, but these require careful manipulation for accurate measurement and are not suited for routine clinical use.…”

Non-invasive metabolic monitoring of patients under anaesthesia by continuous indirect calorimetry—an in vivo trial of a new method

“…17 At these points, thorough mixing will ensure stable gas concentrations in the face of tidal fluctuations in concentration at the mouthpiece, which improves accuracy and precision of the measurement. Indirect calorimetric methods such as this can be automated to provide ongoing measurement, 18 but need repeated switching of gas sampling to achieve this, which limits the frequency of measurement and precludes truly continuous gas exchange monitoring. The continuous method tested here is an adaptation of the Haldane transformation and requires the presence of some N 2 in the exhaust gas flow in order to compute Equation (1).…”

Accuracy of the Foldes‐Biro equation for measurement of oxygen uptake during anaesthesia: a laboratory simulation

“…To avoid reliance on visual reading of rotameters which may be imprecise, flows into the system were continuously measured by a previously described technique [4]. This uses continuous monitoring of air supply pressure from a high‐pressure source (medical air cylinders) using a pressure transducer and the previously calibrated pressure‐flow characteristics of one or more gas flow resistors.…”