Johannes P. Schramel

Measurement of tidal volume using Respiratory Ultrasonic Plethysmography in anaesthetized, mechanically ventilated horses

OBJECTIVE To compare tidal volume estimations obtained from Respiratory Ultrasonic Plethysmography (RUP) with simultaneous spirometric measurements in anaesthetized, mechanically ventilated horses. STUDY DESIGN Prospective randomized experimental study. ANIMALS Five experimental horses. METHODS Five horses were anaesthetized twice (1 week apart) in random order in lateral and in dorsal recumbency. Nine ventilation modes (treatments) were scheduled in random order (each lasting 4 minutes) applying combinations of different tidal volumes (8, 10, 12 mL kg(-1)) and positive end-expiratory pressures (PEEP) (0, 10, 20 cm H(2)O). Baseline ventilation mode (tidal volume=15 mL kg(-1), PEEP=0 cm H(2)O) was applied for 4 minutes between all treatments. Spirometry and RUP data were downloaded to personal computers. Linear regression analyses (RUP versus spirometric tidal volume) were performed using different subsets of data. Additonally RUP was calibrated against spirometry using a regression equation for all RUP signal values (thoracic, abdominal and combined) with all data collectively and also by an individually determined best regression equation (highest R(2)) for each experiment (horse versus recumbency) separately. Agreement between methods was assessed with Bland-Altman analyses. RESULTS The highest correlation of RUP and spirometric tidal volume (R(2)=0.81) was found with the combined RUP signal in horses in lateral recumbency and ventilated without PEEP. The bias ±2 SD was 0±2.66 L when RUP was calibrated for collective data, but decreased to 0±0.87 L when RUP was calibrated with individual data. CONCLUSIONS AND CLINICAL RELEVANCE A possible use of RUP for tidal volume measurement during IPPV needs individual calibration to obtain limits of agreement within ±20%.

Effects of infrared camera angle and distance on measurement and reproducibility of thermographically determined temperatures of the distolateral aspects of the forelimbs in horses

OBJECTIVE To assess effects of camera angle and distance on measurement and reproducibility of thermographically determined temperatures of the distolateral aspect of the forelimbs in horses. DESIGN Evaluation study. ANIMALS 10 adult horses. PROCEDURES Thermographic images of both forelimbs were obtained at 3 times during the day (replicates 1, 2, and 3); maximum surface temperature over 1 region (distolateral aspect of the third metacarpal bone and metacarpophalangeal joint) was measured. Standard images were obtained every 5 minutes for 1 hour with the camera positioned at an angle of 90° and a distance of 1.0 m from the forelimb; additional images were obtained at changed (± 20°) angles or at a 1.5-m distance. At the end of each replicate, 4 sets of additional images were obtained at 2-minute intervals to assess short-term reproducibility. RESULTS Mean ± SD temperature difference between left and right forelimbs was 0.32° ± 0.27°C (0.58° ± 0.49°F) in standard images. Temperatures measured via standard images were highly correlated with those measured with the camera positioned at changed angles or distance. Mean ± SD differences between temperatures measured via standard images and those measured from changed angles or distance were considered small (≤ 0.22° ± 0.18°C [0.40° ± 0.32°F] for all comparisons). The degree of short-term reproducibility was high. CONCLUSIONS AND CLINICAL RELEVANCE Thermographically determined temperatures were unaffected by 20° changes in camera angle or a 0.5-m increase in camera distance from the forelimb. Minor temperature differences between left and right forelimbs were detected in the study and should be considered during diagnostic investigations.

Distribution of ventilation in pregnant Shetland ponies measured by Electrical Impedance Tomography.

The regional distribution of ventilation in conscious standing pregnant Shetland pony mares was investigated by Electrical Impedance Tomography (EIT). Six ponies were repeatedly examined a minimum of four weeks prior to (antepartum, AP) until three weeks after parturition (postpartum, PP). From the cross-sectional ventilation image the ventral to dorsal (V/D), left to right (L/R) ventilation distribution ratio and the relative ventilation in four horizontal regions of interest (ROI) placed symmetrically in the chest was analyzed. Antepartum V/D was 0.74 ± 0.09 on day -28 ± 3 (AP28) and decreased to 0.68 ± 0.10 on day -3 ± 2 (AP3). Postpartum V/D increased significantly (p<0.05) to 0.96 ± 0.08 on day 7 ± 2 (PP7). The L/R ventilation distribution remains unaffected. Ventilation in the most ventral ROI was significantly lower on days AP28 and AP3 compared to PP7. These results suggest that in Shetland ponies late pregnancy compromises the ventilation in ventral (dependent) lung regions. We demonstrated the feasibility of a repeated EIT measurement in standing conscious ponies.

Assessment of distribution of ventilation and regional lung compliance by electrical impedance tomography in anaesthetized horses undergoing alveolar recruitment manoeuvres

OBJECTIVE To examine changes in the distribution of ventilation and regional lung compliances in anaesthetized horses during the alveolar recruitment manoeuvre (ARM). STUDY DESIGN Experimental study in which a series of treatments were administered in a fixed order on one occasion. ANIMALS Five adult Warmblood horses. METHODS Animals were anaesthetized (xylazine, midazolam-ketamine, isoflurane), placed in dorsal recumbency and ventilated with 100% oxygen using peak inspiratory pressure (PIP) and positive end-expiratory pressure (PEEP) of 20 cmH2O and 0 cmH2O, respectively. Thoracic electrical impedance tomography (EIT), spirometry and routine anaesthesia monitoring were performed. At 90 minutes after induction of anaesthesia, PIP and PEEP were increased in steps of 5 cmH2O to 50 cmH2O and 30 cmH2O, respectively, and then decreased to baseline values. Each step lasted 10 minutes. Data were recorded and functional EIT images were created using three breaths at the end of each step. Arterial blood samples were analysed. Values for left-to-right and sternal-to-dorsal centre of ventilation (COV), lung compliances and Bohr dead space were calculated. RESULTS Distribution of ventilation drifted leftward and dorsally during recruitment. Mean±standard deviation (SD) values at baseline and highest airway pressures, respectively, were 49.9±0.7% and 48.0±0.6% for left-to-right COV (p=0.009), and 46.3±2.0% and 54.6±2.0% for sternal-to-dorsal COV (p=0.0001). Compliance of dependent lung regions and PaO2 increased, whereas compliance of non-dependent lung regions decreased during ARM and then returned to baseline (p<0.001). Bohr dead space decreased after ARM (p=0.007). Interestingly, PaO2 correlated to the compliance of the dependent lung (r2=0.71, p<0.001). CONCLUSIONS AND CLINICAL RELEVANCE The proportion of tidal volume distributed to dependent and left lung regions increased during ARM, presumably as a result of opening atelectasis. Monitoring compliance of the dependent lung with EIT may substitute PaO2 measurements during ARM to identify an optimal PEEP.

A novel flow partition device for spirometry during large animal anaesthesia

OBJECTIVE We describe and test a novel device for large animal anaesthesia monitoring that uses standard human medicine spirometry sensors. STUDY DESIGN In-vitro study. METHODS The device consists of two adapters that enable the flow to be split evenly into four tubes in parallel, each tube containing a D-lite sensor. The performance of this flow partitioning device (FPD) over a range of flows from 100 to 700 L minute⁻¹ was determined and the pressure versus flow relation, resistance and dead space was compared with a Horse-lite (Moens 2010). RESULTS Equipped with four D-lite sensors, and a flow of 700 L minute⁻¹ the pressure drop of the FPD was 13.5 cm H₂O, resistance 1.17 cm H₂O second L⁻¹ and volume (potential dead space) 182 mL, compared to 2.8 cm H₂O, 0.24 cm H₂O second L⁻¹ and 54 mL respectively for the Horse-lite. The predicted value of the flow partition of ¼ could be confirmed. Limits of agreement were found to be 4.2% in inspiratory direction and 7.1% in expiratory direction. CONCLUSIONS AND CLINICAL RELEVANCE The FPD is an affordable device that extends the specification of any commercially available human spirometry sensors to large animal applications. However, an increase in total resistance and dead space has to be taken into account. Therefore, the new device could be useful during equine anaesthesia.

In vitro validation of a new respiratory ultrasonic plethysmograph

OBJECTIVE The in-vitro validation of a novel Respiratory Ultrasonic Plethysmography (RUP) system designed to detect circumference changes of rib cage and abdominal compartments in large and small animals. STUDY DESIGN Experimental in vitro study. METHODS The experimental system includes two compliant fluid-filled rubber tubes functioning as ultrasonic waveguides. Each has an ultrasonic transmitter and a detector at the opposing ends. Sensor length can be individually adapted in the range of 0.15-2 m. Data are downloaded to a computer at a sampling rate of 10 or 100 Hz. Measurements have a resolution of 0.3 mm. Baseline stability, linearity and repeatability were investigated with dedicated experiments. The base line drift was tested measuring a fixed distance for 2 hours continuously and then 18 hours later. A hand-operated horse thorax dummy (elliptically shaped, circumference 1.73 m) was used to compare waveforms of RUP with a respiratory inductive plethysmograph (RIP). The electromagnetic interference was tested by approaching metallic objects. RESULTS Baseline drift and repeatability (10 repeated steps of 1.6% and 6.6% elongations and contractions) were within ± 0.3 mm. The response of the system for tube stretching up to 11% of total length was linear with a coefficient of determination for linearity of 0.998. In contrast to RIP, electromagnetic interference could not be observed with RUP. CONCLUSIONS AND CLINICAL RELEVANCE The low baseline drift and the lack of electromagnetic interference favours the use of RUP compared to an RIP device when studying the breathing pattern and end expiratory lung volume changes in conscious and anaesthetized animals.

Assessment of distribution of ventilation by electrical impedance tomography in standing horses

The aim was to evaluate the feasibility of using electrical impedance tomography (EIT) in horses. Thoracic EIT was used in nine horses. Thoracic and abdominal circumference changes were also measured with respiratory ultrasound plethysmography (RUP). Data were recorded during baseline, rebreathing of CO2 and sedation. Three breaths were selected for analysis from each recording. During baseline breathing, horses regularly took single large breaths (sighs), which were also analysed. Functional EIT images were created using standard deviations (SD) of pixel signals and correlation coefficients (R) of each pixel signal with a reference respiratory signal. Left-to-right ratio, centre-of-ventilation and global-inhomogeneity-index were calculated. RM-ANOVA and Bonferroni tests were used (P < 0.05). Distribution of ventilation shifted towards right during sighs and towards dependent regions during sighs, rebreathing and sedation. Global-inhomogeneity-index did not change for SD but increased for R images during sedation. The sum of SDs for the respiratory EIT signals correlated well with thoracic (r(2) = 0.78) and abdominal (r(2) = 0.82) tidal circumferential changes. Inverse respiratory signals were identified on the images at sternal location and based on reviewing CT images, seemed to correspond to location of gas filled intestines. Application of EIT in standing non-sedated horses is feasible. EIT images may provide physiologically useful information even in situations, such as sighs, that cannot easily be tested by other methods.

In vitro and in vivo evaluation of a new large animal spirometry device using mainstream CO2 flow sensors

REASONS FOR PERFORMING STUDY A spirometry device equipped with mainstream CO2 flow sensor is not available for large animal anaesthesia. OBJECTIVES To measure the resistance of a new large animal spirometry device and assess its agreement with reference methods for volume measurements. STUDY DESIGN In vitro experiment and crossover study using anaesthetised horses. METHODS A flow partitioning device (FPD) equipped with 4 human CO2 flow sensors was tested. Pressure differences were measured across the whole FPD and across each sensor separately using air flows (range: 90-720 l/min). One sensor was connected to a spirometry monitor for in vitro volume (3, 5 and 7 l) measurements. These measurements were compared with a reference method. Five anaesthetised horses were used for tidal volume (VT) measurements using the FPD and a horse-lite sensor (reference method). Bland-Altman analysis, ANOVA and linear regression analysis were used for data analysis. RESULTS Pressure differences across each sensor were similar suggesting equal flow partitioning. The resistance of the device increased with flow (range: 0.3-1.5 cmH2 O s/l) and was higher than that of the horse-lite. The limits of agreement for volume measurements were within -1 and 2% in vitro and -12 and 0% in vivo. Nine of 147 VT measurements in horses were outside of the ± 10% limits of acceptance but most of these erroneous measurements occurred with VTs lower than 4 l. The determined correction factor for volume measurements was 3.97 ± 0.03. CONCLUSIONS The limits of agreement for volume measurements by the new device were within ± 10% using clinically relevant range of volumes. The new spirometry device can be recommended for measurement of VT in adult Warmblood horses.

Breathing pattern and thoracoabdominal asynchrony in horses with chronic obstructive and inflammatory lung disease.

The aim of the study was to show that changes in thoracoabdominal asynchrony (TAA) between quiet breathing and CO2-induced hyperpnoea can be used to differentiate between horses with healthy airways and those suffering from inflammatory airway disease (IAD) or recurrent airway obstruction (RAO). The level of TAA was displayed by the Pearsons correlation coefficient (PCC) of thoracic and abdominal signals, generated by respiratory ultrasonic plethysmography (RUP) during quiet breathing and hyperpnoea. Changes in TAA were expressed as the quotient of the PCCs (PCCQ) during normal breathing and hyperpnoea. Horses with RAO and IAD showed significant higher median PCCQ than healthy horses. Median PCCQ of horses with RAO and IAD was not significantly different. Horses affected by a pulmonary disorder showed lower TAA compared to the control group. This study suggests that TAA provides a useful parameter to differentiate horses with RAO and IAD from healthy horses.

Physiologic Factors Influencing the Arterial-To-End-Tidal CO2 Difference and the Alveolar Dead Space Fraction in Spontaneously Breathing Anesthetised Horses

The arterial to end-tidal CO2 difference (P(a-ET)CO2) and alveolar dead space fraction (VDalvfrac = P(a-ET)CO2/PaCO2), are used to estimate Enghoff’s “pulmonary dead space” (V/QEng), a factor which is also influenced by venous admixture and other pulmonary perfusion abnormalities and thus is not just a measure of dead space as the name suggests. The aim of this experimental study was to evaluate which factors influence these CO2 indices in anesthetized spontaneously breathing horses. Six healthy adult horses were anesthetized in dorsal recumbency breathing spontaneously for 3 h. Data to calculate the CO2 indices (response variables) and dead space variables were measured every 30 min. Bohr’s physiological and alveolar dead space variables, cardiac output (CO), mean pulmonary pressure (MPP), venous admixture (Q˙s/Q˙t), airway dead space, tidal volume, oxygen consumption, and slope III of the volumetric capnogram were evaluated (explanatory variables). Univariate Pearson correlation was first explored for both CO2 indices before V/QEng and the explanatory variables with rho were reported. Multiple linear regression analysis was performed on P(a-ET)CO2 and VDalvfrac assessing which explanatory variables best explained the variance in each response. The simplest, best-fit model was selected based on the maximum adjusted R2 and smallest Mallow’s p (Cp). The R2 of the selected model, representing how much of the variance in the response could be explained by the selected variables, was reported. The highest correlation was found with the alveolar part of V/QEng to alveolar tidal volume ratio for both, P(a-ET)CO2 (r = 0.899) and VDalvfrac (r = 0.938). Venous admixture and CO best explained P(a-ET)CO2 (R2 = 0.752; Cp = 4.372) and VDalvfrac (R2 = 0.711; Cp = 9.915). Adding MPP (P(a-ET)CO2) and airway dead space (VDalvfrac) to the models improved them only marginally. No “real” dead space variables from Bohr’s equation contributed to the explanation of the variance of the two CO2 indices. P(a-ET)CO2 and VDalvfrac were closely associated with the alveolar part of V/QEng and as such, were also influenced by variables representing a dysfunctional pulmonary perfusion. Neither P(a-ET)CO2 nor VDalvfrac should be considered pulmonary dead space, but used as global indices of V/Q mismatching under the described conditions.