Monitoring of tidal ventilation by electrical impedance tomography in anaesthetised horses

Validation of three-dimensional thoracic electrical impedance tomography of horses during normal and increased tidal volumes

Objective. Data from two-plane electrical impedance tomography (EIT) can be reconstructed into various slices of functional lung images, allowing for more complete visualisation and assessment of lung physiology in health and disease. The aim of this study was to confirm the ability of 3D EIT to visualise normal lung anatomy and physiology at rest and during increased ventilation (represented by rebreathing).Approach. Two-plane EIT data, using two electrode planes 20 cm apart, were collected in 20 standing sedate horses at baseline (resting) conditions, and during rebreathing. EIT data were reconstructed into 3D EIT whereby tidal impedance variation (TIV), ventilated area, and right-left and ventral-dorsal centres of ventilation (CoVRLand CoVVD, respectively) were calculated in cranial, middle and caudal slices of lung, from data collected using the two planes of electrodes.Main results. There was a significant interaction of time and slice for TIV (p< 0.0001) with TIV increasing during rebreathing in both caudal and middle slices. The ratio of right to left ventilated area was higher in the cranial slice, in comparison to the caudal slice (p= 0.0002). There were significant effects of time and slice on CoVVDwhereby the cranial slice was more ventrally distributed than the caudal slice (p< 0.0009 for the interaction).Significance. The distribution of ventilation in the three slices corresponds with topographical anatomy of the equine lung. This study confirms that 3D EIT can accurately represent lung anatomy and changes in ventilation distribution during rebreathing in standing sedate horses.

Deciphering Mechanisms of Respiratory Fetal-to-Neonatal Transition in Very Preterm Infants

Rationale: The respiratory mechanisms of a successful transition of preterm infants after birth are largely unknown. Objectives: To describe intrapulmonary gas flows during different breathing patterns directly after birth. Methods: Analysis of electrical impedance tomography data from a previous randomized trial in preterm infants at 26-32 weeks gestational age. Electrical impedance tomography data for individual breaths were extracted, and lung volumes as well as ventilation distribution were calculated for end of inspiration, end of expiratory braking and/or holding maneuver, and end of expiration. Measurements and Main Results: Overall, 10,348 breaths from 33 infants were analyzed. We identified three distinct breath types within the first 10 minutes after birth: tidal breathing (44% of all breaths; sinusoidal breathing without expiratory disruption), braking (50%; expiratory brake with a short duration), and holding (6%; expiratory brake with a long duration). Only after holding breaths did end-expiratory lung volume increase: Median (interquartile range [IQR]) = 2.0 AU/kg (0.6 to 4.3), 0.0 (-1.0 to 1.1), and 0.0 (-1.1 to 0.4), respectively; P < 0.001]. This was mediated by intrathoracic air redistribution to the left and non-gravity-dependent parts of the lung through pendelluft gas flows during braking and/or holding maneuvers. Conclusions: Respiratory transition in preterm infants is characterized by unique breathing patterns. Holding breaths contribute to early lung aeration after birth in preterm infants. This is facilitated by air redistribution during braking/holding maneuvers through pendelluft flow, which may prevent lung liquid reflux in this highly adaptive situation. This study deciphers mechanisms for a successful fetal-to-neonatal transition and increases our pathophysiological understanding of this unique moment in life. Clinical trial registered with www.clinicaltrials.gov (NCT04315636).

Thoracic electrical impedance tomography identifies heterogeneity in lungs associated with respiratory disease in cattle. A pilot study

Respiratory disease in cattle is a significant global concern, yet current diagnostic methods are limited, and there is a lack of crush-side tests for detecting active disease. To address this gap, we propose utilizing electrical impedance tomography (EIT), a non-invasive imaging technique that provides real-time visualization of lung ventilation dynamics. The study included adult cattle from farms in Western Australia. The cattle were restrained in a crush. A standardized respiratory scoring system, which combined visual, auscultation, and clinical scores, was conducted by two non-conferring clinicians for each animal. The scores were blinded and averaged. During assessment, an EIT electrode belt was placed around the thorax. EIT recordings of ten suitable breaths were taken for analysis before the cattle were released back to the herd. Based on the combined examination scoring, the cattle were categorized as having healthy or diseased lungs. To allow visual interpretation of each breath and enable the creation of the quartile ventilation ratio (VQR), Flow/Tidal Impedance Variation curves (F/TIV) were generated for each breath. The analysis focused on two EIT variables: The novel VQR over time during inhalation and exhalation and global expiratory impedance (TIVEXP) adjusted by breath length. A mixed effects model was used to compare these variables between healthy and diseased cattle. Ten adult cattle of mixed ages were used in the current analysis. Five cattle were scored as healthy and five as diseased. There was a significant difference in the examination scores between the healthy and diseased group (P = 0.03). A significant difference in VQR during inhalation (P = 0.03) was observed between the healthy and diseased groups. No difference was seen in VQR over time during exhalation (P = 0.3). The TIVEXP was not different between groups (P = 0.36). In this study, EIT was able to detect differences in inhalation mechanics when comparing healthy and diseased cattle as defined via clinical examination, highlighting the clinical utility of EIT.

Lung volume changes during apnoeas in preterm infants

OBJECTIVE

Mechanisms of non-invasive high-frequency oscillatory ventilation (nHFOV) in preterm infants are unclear. We aimed to compare lung volume changes during apnoeas in preterm infants on nHFOV and nasal continuous positive airway pressure (nCPAP).

METHODS

Analysis of electrical impedance tomography (EIT) data from a randomised crossover trial comparing nHFOV with nCPAP in preterm infants at 26-34 weeks postmenstrual age. EIT data were screened by two reviewers to identify apnoeas ≥10 s. End-expiratory lung impedance (EELI) and tidal volumes (V_T) were calculated before and after apnoeas. Oxygen saturation (SpO₂) and heart rate (HR) were extracted for 60 s after apnoeas.

RESULTS

In 30 preterm infants, 213 apnoeas were identified. During apnoeas, oscillatory volumes were detectable during nHFOV. EELI decreased significantly during apnoeas (∆EELI nCPAP: -8.0 (-11.9 to -4.1) AU/kg, p<0.001; ∆EELI nHFOV: -3.4 (-6.5 to -0.3), p=0.03) but recovered over the first five breaths after apnoeas. Compared with before apnoeas, V_T was increased for the first breath after apnoeas during nCPAP (∆V_T: 7.5 (3.1 to 11.2) AU/kg, p=0.001). Falls in SpO₂ and HR after apnoeas were greater during nCPAP than nHFOV (mean difference (95% CI): SpO₂: 3.6% (2.7 to 4.6), p<0.001; HR: 15.9 bpm (13.4 to 18.5), p<0.001).

CONCLUSION

Apnoeas were characterised by a significant decrease in EELI which was regained over the first breaths after apnoeas, partly mediated by a larger V_T. Apnoeas were followed by a considerable drop in SpO₂ and HR, particularly during nCPAP, leading to longer episodes of hypoxemia during nCPAP. Transmitted oscillations during nHFOV may explain these benefits.

TRIAL REGISTRATION NUMBER

ACTRN12616001516471.

The optimal PEEP after alveolar recruitment maneuver assessed by electrical impedance tomography in healthy horses

Background

Electrical impedance tomography (EIT) has been an essential tool for assessing pulmonary ventilation in several situations, such as the alveolar recruitment maneuver (ARM) in PEEP titration to maintain the lungs open after atelectasis reversion. In the same way as in humans and dogs, in horses, this tool has been widely used to assess pulmonary aeration undergoing anesthesia, mechanical ventilation, recruitment maneuver, standing horses, or specific procedures.

Objectives

The present study aimed to evaluate the distribution of regional ventilation during ARM based on lung monitoring assessment by EIT, with a focus on better recruitment associated with less or no overdistention.

Methods

Fourteen horses of 306 ± 21 kg undergoing isoflurane anesthesia in dorsal recumbency were used. The animals were mechanically ventilated with a tidal volume of 14 ml kg^-1 and a respiratory rate of 7-9. An alveolar recruitment maneuver was instituted, increasing the PEEP by five cmH₂O every 5 min until 32 cmH₂O and decreasing it by five cmH₂O every 5 min to 7 cmH₂O. At each step of PEEP, arterial blood samples were collected for blood gas analysis, EIT images, hemodynamic, and respiratory mechanics.

Results

Associated with the CoV-DV increase, there was a significant decrease in the DSS during the ARM and a significant increase in the NSS when PEEP was applied above 12 cmH₂O compared to baseline. The Compl_ROI showed a significant increase in the dependent area and a significant decrease in the non-dependent area during ARM, and both were compared to their baseline values. The driving pressure decreased significantly during the ARM, and Cst, PaO₂, and PaO₂/FiO₂ ratio increased significantly. The V_D/V_T decreased significantly at DEPEEP17 and DEPEEP12. There was an HR increase at INPEEP27, INPEEP 32, and DEPEEP17 (p < 0.0001; p < 0.0001; and p < 0.05, respectively), those values being above the normal reference range for the species. The SAP, MAP, DAP, CI, and DO₂I significantly decreased INPEEP32 (p < 0.05).

Conclusion

The ARM by PEEP titration applied in the present study showed better ventilation distribution associated with better aeration in the dependent lung areas, with minimal overdistention between PEEP 17 and 12 cmH₂O decreasing step. Those changes were also followed by improvements in static and regional compliance associated with increased oxygenation and pulmonary ventilation. ARM promoted a transitory decrease in arterial blood pressure and depression in CI with a concomitant drop in oxygen delivery, which should be best investigated before its routine use in clinical cases.

Performance evaluation of electrode design and material for a large animal electrical impedance tomography belt

BACKGROUND

Electrical impedance tomography (EIT) produces lung ventilation images via a thoracic electrode belt. Robust electrode design and material, providing low electrode skin contact impedance (SCI), is needed in veterinary medicine. The aim of this study was to compare three EIT electrode designs and materials.

METHODS

Simulations of cylindrical, rectangular and spiked electrode designs were used to evaluate electrode SCI as a function of electrode size, where skin contact was uneven. Gold-plated washers (E_GW ), zinc-plated rivets (E_ZR ) and zinc-galvanised spikes (E_ZS ) were assigned randomly on two interconnected EIT belts. Gel was applied to the cranial or caudal belt and placed on 17 standing cattle. SCI was recorded at baseline and 3, 5, 7, 9 and 11 minutes later.

RESULTS

Simulations that involved electrodes with a greater skin contact area had lower and more uniform SCI. In cattle, SCI decreased with all electrodes over time (p < 0.01). Without gel, no difference was found between E_GW and E_ZS , while SCI was higher for E_ZR (p < 0.03). With gel, SCI was lower in E_GW and E_ZR (p < 0.026), with the SCI in E_GW being the lowest (p < 0.01).

LIMITATIONS

Low numbers of animals and static electrode position may affect SCI.

CONCLUSIONS

Electrode design is important for EIT measurement, with larger electrode designs able to compensate for the use of less conductive materials. Gel is not necessary to achieve acceptable SCI in large animals.

Thoracic Electrical Impedance Tomography-The 2022 Veterinary Consensus Statement

Electrical impedance tomography (EIT) is a non-invasive real-time non-ionising imaging modality that has many applications. Since the first recorded use in 1978, the technology has become more widely used especially in human adult and neonatal critical care monitoring. Recently, there has been an increase in research on thoracic EIT in veterinary medicine. Real-time imaging of the thorax allows evaluation of ventilation distribution in anesthetised and conscious animals. As the technology becomes recognised in the veterinary community there is a need to standardize approaches to data collection, analysis, interpretation and nomenclature, ensuring comparison and repeatability between researchers and studies. A group of nineteen veterinarians and two biomedical engineers experienced in veterinary EIT were consulted and contributed to the preparation of this statement. The aim of this consensus is to provide an introduction to this imaging modality, to highlight clinical relevance and to include recommendations on how to effectively use thoracic EIT in veterinary species. Based on this, the consensus statement aims to address the need for a streamlined approach to veterinary thoracic EIT and includes: an introduction to the use of EIT in veterinary species, the technical background to creation of the functional images, a consensus from all contributing authors on the practical application and use of the technology, descriptions and interpretation of current available variables including appropriate statistical analysis, nomenclature recommended for consistency and future developments in thoracic EIT. The information provided in this consensus statement may benefit researchers and clinicians working within the field of veterinary thoracic EIT. We endeavor to inform future users of the benefits of this imaging modality and provide opportunities to further explore applications of this technology with regards to perfusion imaging and pathology diagnosis.

Determination of tidal volume by electrical impedance tomography (EIT) after indirect two-point calibration

OBJECTIVE

A linear relationship between impedance change (△Z) measured by thoracic electrical impedance tomography (EIT) and tidal volume (VT) has been demonstrated. This study evaluated the agreement between the displayed VT calculated by the EIT software (VT_EIT) and spirometry (VT_SPIRO) after an indirect two-point calibration.

APPROACH

The EIT software was programmed to execute a bedside two-point calibration from the subject-specific, linear equation defining the relationship between △Z and VT_SPIROand displaying VT_EITbreath-by-breath in 20 neutered male, juvenile pigs. After EIT calibration VTs of 8, 12, 16 and 20 mL kg^-1were applied to the lungs. VT_EITand VT_SPIROwere recorded and analysed using Bland-Altman plot for multiple subject measurements. Volumetric capnography (VCap) and spirometry data were explored as components of variance using multiple regression.

MAIN RESULTS

A mean relative difference (bias) of 0.7% with 95% confidence interval (CI) of -10.4 - 10.7% were found between VT_EITand VT_SPIROfor the analysed data set. The variance in VT_EITcould not be explained by any of the measured VCap or spirometry variables.

SIGNIFICANCE

The narrow CI estimated in this study allows the conclusion that EIT and its software can be used to measure and accurately convert △Z into mililitre VT at the bedside after applying an indirect two-point calibration.

Effects of PEEP on the relationship between tidal volume and total impedance change measured via electrical impedance tomography (EIT)

Electrical impedance tomography (EIT) is used in lung physiology monitoring. There is evidence that EIT is linearly associated with global tidal volume (VT) in clinically healthy patients where no positive end-expiratory pressure (PEEP) is applied. This linearity has not been challenged by altering lung conditions. The aim of this study was to determine the effect of PEEP on VT estimation, using EIT technology and spirometry, and observe the stability of the relationship under changing lung conditions. Twelve male castrated cattle (Steer), mean age 7.8 months (SD ± 1.7) were premedicated with xylazine followed by anaesthesia induction with ketamine and maintenance with halothane in oxygen via an endotracheal tube. An EIT belt was applied around the thorax at the level of the fifth intercostal space. Volume controlled ventilation was used. PEEP was increased in a stepwise manner from 0 to 5, 10 and 15 cmH₂O. At each PEEP, the VT was increased stepwise from 5 to 10 and 15 mL kg^-1. After a minute of stabilisation, total impedance change (VT_EIT), using EIT and VT measured by a spirometer connected to a flow-partitioning device (VT_Spiro) was recorded for the following minute before changing ventilator settings. Data was analysed using linear regression and multi variable analysis. There was a linear relationship between VT_EIT and VT_Spiro at all levels of PEEP with an R² of 0.71, 0.68, 0.63 and 0.63 at 0, 5, 10 and 15 cmH₂O, respectively. The variance in VT_EIT was best described by peak inspiratory pressure (PIP) and PEEP (adjusted R² 0.82) while variance in VT_Spiro was best described by PIP and airway deadspace (adjusted R² 0.76). The relationship between VT_EIT and VT_Spiro remains linear with changes in tidal volume, and stable across altered lung conditions. This may have application for monitoring and assessment in vivo.

Assessment of Postnatal Pulmonary Adaption in Bovine Neonates Using Electric Impedance Tomography (EIT)

Several aspects of postnatal pulmonary adaption in the bovine neonate remain unclear, particularly the dynamics and regional ventilation of the lungs. We used electric impedance tomography (EIT) to measure changes in ventilation in the first 3 weeks of life in 20 non-sedated neonatal calves born without difficulty in sternal recumbency. Arterial blood gas variables were determined in the first 24 h after birth. Immediately after birth, dorsal parts of the lungs had 4.53% ± 2.82% nondependent silent spaces (NSS), and ventral parts had 5.23% ± 2.66% dependent silent spaces (DSS). The latter increased in the first hour, presumably because of gravity-driven ventral movement of residual amniotic fluid. The remaining lung regions had good ventilation immediately after birth, and the percentage of lung regions with high ventilation increased significantly during the study period. The centre of ventilation was always dorsal to and on the right of the theoretical centre of ventilation. The right lung was responsible for a significantly larger proportion of ventilation (63.84% ± 12.74%, p < 0.00001) compared with the left lung. In the right lung, the centrodorsal lung area was the most ventilated, whereas, in the left lung, it was the centroventral area. Tidal impedance changes, serving as a surrogate for tidal volume, increased in the first 3 weeks of life (p < 0.00001). This study shows the dynamic changes in lung ventilation in the bovine neonate according to EIT measurements. The findings form a basis for the recognition of structural and functional lung disorders in neonatal calves.

Exercise-induced airflow changes in horses with asthma measured by electrical impedance tomography

BACKGROUND

Equine asthma (EA) causes airflow impairment, which increases in severity with exercise. Electrical impedance tomography (EIT) is an imaging technique that can detect airflow changes in standing healthy horses during a histamine provocation test.

OBJECTIVES

To explore EIT-calculated flow variables before and after exercise in healthy horses and horses with mild-to-moderate (MEA) and severe equine asthma (SEA).

ANIMALS

Nine healthy horses 9 horses diagnosed with MEA and 5 with SEA were prospectively included.

METHODS

Recordings were performed before and after 15 minutes of lunging. Absolute values from global and regional peak inspiratory (PIF, positive value) and expiratory (PEF, negative value) flows were calculated. Data were analyzed using a mixed model analysis followed by Bonferroni's multiple comparisons test to evaluate the impact of exercise and diagnosis on flow indices.

RESULTS

Control horses after exercise had significantly lower global PEF and PIF compared to horses with SEA (mean difference [95% confidence interval, CI]: 0.0859 arbitrary units [AU; 0.0339-0.1379], P < .001 and 0.0726 AU [0.0264-0.1188], P = .001, respectively) and horses with MEA (0.0561 AU [0.0129-0.0994], P = .007 and 0.0587 AU [0.0202-0.0973], P = .002, respectively). No other significant differences were detected.

CONCLUSIONS AND CLINICAL IMPORTANCE

Electrical impedance tomography derived PIF and PEF differed significantly between healthy horses and horses with SEA or MEA after exercise, but not before exercise. Differences between MEA and SEA were not observed, but the study population was small.

Dynamic relative regional strain visualized by electrical impedance tomography in patients suffering from COVID-19

Respiratory failure due to SARS-CoV-2 may progress rapidly. During the course of COVID-19, patients develop an increased respiratory drive, which may induce high mechanical strain a known risk factor for Patient Self-Inflicted Lung Injury (P-SILI). We developed a novel Electrical Impedance Tomography-based approach to visualize the Dynamic Relative Regional Strain (DRRS) in SARS-CoV-2 positive patients and compared these findings with measurements in lung healthy volunteers. DRRS was defined as the ratio of tidal impedance changes and end-expiratory lung impedance within each pixel of the lung region. DRRS values of the ten patients were considerably higher than those of the ten healthy volunteers. On repeated examination, patterns, magnitude and frequency distribution of DRRS were reproducible and in line with the clinical course of the patients. Lung ultrasound scores correlated with the number of pixels showing DRRS values above the derived threshold. Using Electrical Impedance Tomography we were able to generate, for the first time, images of DRRS which might indicate P-SILI in patients suffering from COVID-19.

Trial Registration This observational study was registered 06.04.2020 in German Clinical Trials Register (DRKS00021276).

Electrical impedance tomography to measure lung ventilation distribution in healthy horses and horses with left-sided cardiac volume overload

Background

Left‐sided cardiac volume overload (LCVO) can cause fluid accumulation in lung tissue changing the distribution of ventilation, which can be evaluated by electrical impedance tomography (EIT).

Objectives

To describe and compare EIT variables in horses with naturally occurring compensated and decompensated LCVO and compare them to a healthy cohort.

Animals

Fourteen adult horses, including university teaching horses and clinical cases (healthy: 8; LCVO: 4 compensated, 2 decompensated).

Methods

In this prospective cohort study, EIT was used in standing, unsedated horses and analyzed for conventional variables, ventilated right (VAR) and left (VAL) lung area, linear‐plane distribution variables (avg‐max VΔZ_Line, VΔZ_Line), global peak flows, inhomogeneity factor, and estimated tidal volume. Horses with decompensated LCVO were assessed before and after administration of furosemide. Variables for healthy and LCVO‐affected horses were compared using a Mann‐Whitney test or unpaired t‐test and observations from compensated and decompensated horses are reported.

Results

Compared to the healthy horses, the LCVO cohort had significantly less VAL (mean difference 3.02; 95% confidence interval .77‐5.2; P = .02), more VAR (−1.13; −2.18 to −.08; P = .04), smaller avg‐max VΔZL_Line (2.54; 1.07‐4.00; P = .003) and VΔZL_Line (median difference 5.40; 1.71‐9.09; P = .01). Observation of EIT alterations were reflected by clinical signs in horses with decompensated LCVO and after administration of furosemide.

Conclusions and Clinical Importance

EIT measurements of ventilation distribution showed less ventilation in the left lung of horses with LCVO and might be useful as an objective assessment of the ventilation effects of cardiogenic pulmonary disease in horses.

Can bronchoconstriction and bronchodilatation in horses be detected using electrical impedance tomography?

BACKGROUND

Electrical impedance tomography (EIT) generates images of the lungs based on impedance change and was able to detect changes in airflow after histamine challenge in horses.

OBJECTIVES

To confirm that EIT can detect histamine-provoked changes in airflow and subsequent drug-induced bronchodilatation. Novel EIT flow variables were developed and examined for changes in airflow.

METHODS

Bronchoconstriction was induced using stepwise histamine bronchoprovocation in 17 healthy sedated horses. The EIT variables were recorded at baseline, after saline nebulization (control), at the histamine concentration causing bronchoconstriction (Cmax ) and 2 and 10 minutes after albuterol (salbutamol) administration. Peak global inspiratory (PIFEIT ) and peak expiratory EIT (PEFEIT ) flow, slope of the global expiratory flow-volume curve (FVslope ), steepest FVslope over all pixels in the lung field, total impedance change (surrogate for tidal volume; VTEIT ) and intercept on the expiratory FV curve normalized to VTEIT (FVintercept /VTEIT ) were indexed to baseline and analyzed for a difference from the control, at Cmax , 2 and 10 minutes after albuterol. Multiple linear regression explored the explanation of the variance of Δflow, a validated variable to evaluate bronchoconstriction using all EIT variables.

RESULTS

At Cmax , PIFEIT , PEFEIT , and FVslope significantly increased whereas FVintercept /VT decreased. All variables returned to baseline 10 minutes after albuterol. The VTEIT did not change. Multivariable investigation suggested 51% of Δflow variance was explained by a combination of PIFEIT and PEFEIT .

CONCLUSIONS AND CLINICAL IMPORTANCE

Changes in airflow during histamine challenge and subsequent albuterol administration could be detected by various EIT flow volume variables.

Transmission of Oscillatory Volumes into the Preterm Lung during Noninvasive High-Frequency Ventilation

Rationale: There is increasing evidence for a clinical benefit of noninvasive high-frequency oscillatory ventilation (nHFOV) in preterm infants. However, it is still unknown whether the generated oscillations are effectively transmitted to the alveoli.Objectives: To assess magnitude and regional distribution of oscillatory volumes (V_Osc) at the lung level.Methods: In 30 prone preterm infants enrolled in a randomized crossover trial comparing nHFOV with nasal continuous positive airway pressure, electrical impedance tomography recordings were performed. During nHFOV, the smallest amplitude to achieve visible chest wall vibration was used, and the frequency was set at 8 hertz.Measurements and Main Results: Thirty consecutive breaths during artifact-free tidal ventilation were extracted for each of the 228 electrical impedance tomography recordings. After application of corresponding frequency filters, Vt and V_Osc were calculated. There was a signal at 8 and 16 Hz during nHFOV, which was not detectable during nasal continuous positive airway pressure, corresponding to the set oscillatory frequency and its second harmonic. During nHFOV, the mean (SD) V_Osc/Vt ratio was 0.20 (0.13). Oscillations were more likely to be transmitted to the non-gravity-dependent (mean difference [95% confidence interval], 0.041 [0.025-0.058]; P < 0.001) and right-sided lung (mean difference [95% confidence interval], 0.040 [0.019-0.061]; P < 0.001) when compared with spontaneous Vt.Conclusions: In preterm infants, V_Osc during nHFOV are transmitted to the lung. Compared with the regional distribution of tidal breaths, oscillations preferentially reach the right and non-gravity-dependent lung. These data increase our understanding of the physiological processes underpinning nHFOV and may lead to further refinement of this novel technique.

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.

Investigation of selected respiratory effects of (dex)medetomidine in healthy Beagles

OBJECTIVE

To investigate the effects of sedative doses of intravenous (IV) medetomidine (MED) or dexmedetomidine (DEX) on selected respiratory variables in dogs.

STUDY DESIGN

Randomized, blinded, crossover study.

ANIMALS

A total of eight healthy adult research Beagles.

METHODS

Dogs breathing room air had an electrical impedance tomography belt placed around the chest and were maintained in right lateral recumbency. Respiratory rate (f_R) in movements minute^-1 (mpm) and changes in thoracic impedance (ΔZ) in arbitrary units (AU) were recorded for 120 seconds before (T0) and exactly 10 minutes (T10) after the administration of IV DEX (10 μg kg^-1) or MED (20 μg kg^-1), with a minimum washout period of 10 days between treatments. Minute ΔZ (ΔZ˙) was calculated by multiplying median ΔZ with f_R. Data are presented as median (interquartile range). Significance for an overall effect of drugs (DEX versus MED) or treatment (T0 versus T10) was quantified with a two-way analysis of variance for repeated measures, followed by, when appropriate, Wilcoxon's signed rank test for each factor.

RESULTS

Overall, f_R decreased from 26 (22-29) mpm at T0 to 13 (10-21) mpm at T10 (p = 0.003) and ΔZ increased from 1.133 (0.856-1.599) AU at T0 to 1.650 (1.273-2.813) AU at T10 (p = 0.007), but ΔZ˙ did not change [30.375 (23.411-32.445) AU minute^-1 at T0 and 30.581 (22.487-35.091) AU minute^-1 at T10]. There was no difference between DEX and MED. Most dogs developed a peculiar breathing pattern characterized by clusters of breaths followed by short periods of apnoea.

CONCLUSIONS AND CLINICAL RELEVANCE

Both drugs caused a change in breathing pattern, reduction in f_R and increase in ΔZ but did not affect ΔZ˙. It is likely that (dex)medetomidine resulted in reduction in f_R and increase in tidal volume without impacting minute volume.

Evaluation of histamine-provoked changes in airflow using electrical impedance tomography in horses

BACKGROUND

Electrical impedance tomography (EIT) generates thoracic impedance images of the lungs and has been used to assess ventilation in horses. This technique may have application in the detection of changes in airflow associated with equine asthma.

OBJECTIVES

The objective was to determine if histamine-induced airflow changes observed with flowmetric plethysmography (Δflow) could also be explained using global and regional respiratory gas flow signals calculated from EIT signals.

STUDY DESIGN

Experimental in vivo study.

METHODS

Six horses, sedated using detomidine were fitted with a thoracic EIT belt and flowmetric plethysmography hardware. Saline (baseline = BL) and increasing concentrations of histamine (C1-4) were nebulised into the face mask until a change in breathing pattern was clinically confirmed and Δflow increased greater or equal to 50%. After nebulisation Δflow and EIT images were recorded over 3 minutes and peak global inspiratory (InF_global ) and expiratory (ExF_global ) flow as well as peak regional expiratory and inspiratory flow for the dorsal and the ventral area of the right and left lungs were evaluated. Delta flow, InF_global and ExF_global at subsequent concentrations were indexed to baseline (y_i  = C_i /BL-1). Indexed and nonindexed variables were evaluated for a difference from baseline at sequential histamine doses (time). Multiple linear regression assessment of variance in delta flow was also investigated.

RESULTS

Consistent with histamine-provoked increases in Δflow, the global flow indices increased significantly. A significant increase in regional inspiratory flow was seen in the right and left ventral lung and dorsal right lung. Multiple regression revealed that the variance in ExF_global , and right and left ventral expiratory flow best explained the variance in Δflow (r²  = .82).

MAIN LIMITATIONS

Low number of horses and horses were healthy.

CONCLUSIONS

Standardised changes in airflow during histamine challenge could be detected using EIT gas flow variables.