Evaluation of the effects of gas volume and composition on accuracy of volume measurement by two flow sensors and delivery by a piston-driven large-animal ventilator

Use of Electrical Impedance Tomography (EIT) to Estimate Tidal Volume in Anaesthetized Horses Undergoing Elective Surgery

Simple Summary

The aim of this study was to explore the usefulness of electrical impedance tomography (EIT), a novel monitoring tool measuring impedance change, to estimate tidal volume (volume of gas in litres moved in and out the airways and lungs with each breath) in anaesthetised horses. The results of this study, performed in clinical cases, demonstrated that there was a positive linear relationship between tidal volume measurements obtained with spirometry and impedance changes measured by EIT within each subject and this individual relationship could be used to estimate tidal volume that showed acceptable agreement with a measured tidal volume in each horse. Thus, EIT can be used to observe changes in tidal volume by the means of impedance changes. However, absolute measurement of tidal volume is only possible after establishment of the individual relationship.

Abstract

This study explores the application of electric impedance tomography (EIT) to estimate tidal volume (VT) by measuring impedance change per breath (∆Z_breath). Seventeen healthy horses were anaesthetised and mechanically ventilated for elective procedures requiring dorsal recumbency. Spirometric VT (VT_SPIRO) and ∆Z_breath were recorded periodically; up to six times throughout anaesthesia. Part 1 assessed these variables at incremental delivered VT of 10, 12 and 15 mL/kg. Part 2 estimated VT (VT_EIT) in litres from ∆Z_breath at three additional measurement points using a line of best fit obtained from Part 1. During part 2, VT was adjusted to maintain end-tidal carbon dioxide between 45–55 mmHg. Linear regression determined the correlation between VT_SPIRO and ∆Z_breath (part 1). Estimated VT_EIT was assessed for agreement with measured VT_SPIRO using Bland Altman analysis (part 2). Marked variability in slope and intercepts was observed across horses. Strong positive correlation between ∆Z_breath and VT_SPIRO was found in each horse (R² 0.9–0.99). The agreement between VT_EIT and VT_SPIRO was good with bias (LOA) of 0.26 (−0.36–0.88) L. These results suggest that, in anaesthetised horses, EIT can be used to monitor and estimate VT after establishing the individual relationship between these variables.

Accuracy of tidal volume delivery by five different models of large-animal ventilators

OBJECTIVE

To determine the accuracy of tidal volume (V_T) delivery among 5 different models of large-animal ventilators when tested at various settings for V_T delivery, peak inspiratory flow (PIF) rate, and fresh gas flow (FGF) rate.

SAMPLE

4 different models of pneumatically powered ventilators and 1 electrically powered piston-driven ventilator.

PROCEDURES

After a leak flow check, each ventilator was tested 10 times for each experimental setting combination of 5 levels of preset V_T, 3 PIF rates, and 4 FGF rates. A thermal mass flow and volume meter was used as the gold-standard method to measure delivered V_T. In addition, circuit systems of rubber versus polyvinyl chloride breathing hoses were evaluated with the piston-driven ventilator. Differences between preset and delivered V_T (volume error [ΔV_T]) were calculated as a percentage of preset V_T, and ANOVA was used to compare results across devices. Pearson correlation coefficient analyses and the coefficient of determination (r²) were used to assess potential associations between the ΔV_T and the preset V_T, PIF rate, and FGF rate.

RESULTS

For each combination of experimental settings, ventilators had ΔV_T values that ranged from 1.2% to 22.2%. Mean ± SD ΔV_T was 4.8 ± 2.5% for the piston-driven ventilator, compared with 6.6 ± 3.2%, 10.6 ± 2.9%, 13.8 ± 2.97%, and 15.2 ± 2.6% for the 4 pneumatic ventilators. The ΔV_T increased with higher PIF rates (r² = 0.69), decreased with higher FGF rates (r² = 0.62), and decreased with higher preset V_T (r² = 0.58).

CONCLUSIONS AND CLINICAL RELEVANCE

Results indicated that the tested ventilators all had ΔV_T but that the extent of each of ΔV_T varied among ventilators. Close monitoring of delivered V_T with external flow and volume meters is warranted, particularly when pneumatic ventilators are used or when very precise V_T delivery is required.

Comparison of various types of inert gas components on efficacy of an alveolar recruitment maneuver in dorsally recumbent anesthetized horses

OBJECTIVE

To assess effects of nitrogen and helium on efficacy of an alveolar recruitment maneuver (ARM) for improving pulmonary mechanics and oxygen exchange in anesthetized horses.

ANIMALS

6 healthy adult horses.

PROCEDURES

Horses were anesthetized twice in a randomized crossover study. Isoflurane-anesthetized horses in dorsal recumbency were ventilated with 30% oxygen and 70% nitrogen (treatment N) or heliox (30% oxygen and 70% helium; treatment H) as carrier gas. After 60 minutes, an ARM was performed. Optimal positive end-expiratory pressure was identified and maintained for 120 minutes. Throughout the experiment, arterial blood pressures, heart rate, peak inspiratory pressure, dynamic compliance (C_dyn), and Pao₂ were measured. Variables were compared with baseline values and between treatments by use of an ANOVA.

RESULTS

The ARM resulted in significant increases in Pao₂ and C_dyn and decreases in the alveolar-arterial gradient in the partial pressure of oxygen in all horses. After the ARM and during the subsequent 120-minute phase, mean values were significantly lower for treatment N than treatment H for Pao₂ and C_dyn. Optimal positive end-expiratory pressure was consistently 15 cm H₂O for treatment N, but it was 10 cm H₂O (4 horses) and 15 cm H₂O (2 horses) for treatment H.

CONCLUSIONS AND CLINICAL RELEVANCE

An ARM in anesthetized horses might be more efficacious in improving Pao₂ and C_dyn when animals breathe helium instead of nitrogen as the inert gas.