Measurement of cardiac output by transoesophageal Doppler echocardiography in anaesthetized horses: comparison with thermodilution

Novel Intracardiac Ultrasound Images Developed on a Cardiac Ultrasound Simulator and Validated in Live Horses

INTRODUCTION

Ultrasonographic guidance of catheter-based interventions in horses is based primarily on transthoracic echocardiography (TTE). Intracardiac echocardiography (ICE) has the potential to provide detailed imaging of specific cardiac regions. Insight and training in echocardiographic guidance can be acquired using an echocardiography simulator.

HYPOTHESIS/OBJECTIVES

Use an echocardiography simulator for horses to determine specific ICE views for catheter-based interventions and validate these in live horses.

ANIMALS

Six adult healthy experimental horses.

METHODS

Observational study. An echocardiographic phantom based on a three-dimensional computer model of the equine heart was used. This phantom was positioned in a water tank, allowing simultaneous TTE and ICE catheter introduction. Novel ICE images from within the thoracic inlet and right atrium were determined on the ultrasound simulator, with TTE as back-up modality to determine ICE catheter position in the simulator if necessary. Images were validated in six horses, with adaptations to catheter manipulations where needed.

RESULTS

Novel ICE images developed on the ultrasound simulator could be replicated in live horses, with no changes in catheter manipulations. These views allowed visualization of the tributaries of the cranial vena cava, both atria, pulmonary veins, aorta, and pulmonary artery.

CONCLUSIONS AND CLINICAL IMPORTANCE

The ultrasound simulator was useful in developing additional ICE images in order to understand echocardiographic anatomy. This simulator creates possibilities for ICE diagnosis of specific cardiac conditions and further development of ICE-guided catheter-based interventions in horses. The ultrasound simulator can be helpful for providing echocardiographic training and reduction of experimental animal use.

Measuring the accuracy of cardiac output using POCUS: the introduction of artificial intelligence into routine care

Background

Shock management requires quick and reliable means to monitor the hemodynamic effects of fluid resuscitation. Point-of-care ultrasound (POCUS) is a relatively quick and non-invasive imaging technique capable of capturing cardiac output (CO) variations in acute settings. However, POCUS is plagued by variable operator skill and interpretation. Artificial intelligence may assist healthcare professionals obtain more objective and precise measurements during ultrasound imaging, thus increasing usability among users with varying experience. In this feasibility study, we compared the performance of novice POCUS users in measuring CO with manual techniques to a novel automation-assisted technique that provides real-time feedback to correct image acquisition for optimal aortic outflow velocity measurement.

Methods

28 junior critical care trainees with limited experience in POCUS performed manual and automation-assisted CO measurements on a single healthy volunteer. CO measurements were obtained using left ventricular outflow tract (LVOT) velocity time integral (VTI) and LVOT diameter. Measurements obtained by study subjects were compared to those taken by board-certified echocardiographers. Comparative analyses were performed using Spearman’s rank correlation and Bland–Altman matched-pairs analysis.

Results

Adequate image acquisition was 100% feasible. The correlation between manual and automated VTI values was not significant (p = 0.11) and means from both groups underestimated the mean values obtained by board-certified echocardiographers. Automated measurements of VTI in the trainee cohort were found to have more reproducibility, narrower measurement range (6.2 vs. 10.3 cm), and reduced standard deviation (1.98 vs. 2.33 cm) compared to manual measurements. The coefficient of variation across raters was 11.5%, 13.6% and 15.4% for board-certified echocardiographers, automated, and manual VTI tracing, respectively.

Conclusions

Our study demonstrates that novel automation-assisted VTI is feasible and can decrease variability while increasing precision in CO measurement. These results support the use of artificial intelligence-augmented image acquisition in routine critical care ultrasound and may have a role for evaluating the response of CO to hemodynamic interventions. Further investigations into artificial intelligence-assisted ultrasound systems in clinical settings are warranted.

Feasibility of the echocardiographic subcostal view in newborn foals: two-dimensional and Doppler aortic findings

BACKGROUND

In dogs, due to better alignment with the aortic outflow, the subcostal (SC) transducer site provides greater Doppler-derived velocities than those obtained from the left parasternal view. The feasibility of this imaging approach has never been described in equine echocardiography.

OBJECTIVES

The objective of this study was to evaluate the feasibility of the SC view in newborn foals and compare aortic two-dimensional and Doppler-derived velocity measurements with those of standard parasternal long-axis (LAX) views.

STUDY DESIGN

A prospective observational study.

METHODS

Twenty-three newborn healthy Standardbred and Warmblood foals, aged from 7 h to 6 days, underwent transthoracic two-dimensional (2DE), M-mode and Doppler echocardiography that was performed in lateral recumbency. Right and left parasternal long-axis (R-LAX, L-LAX) and SC views were obtained to perform 2DE and Doppler assessments of the aortic valve (AoV). Aortic diameter at the sinus of Valsalva (AoS D) was measured from R-LAX and SC images. Aortic maximal velocity (AoV Vel_max ), velocity time integral (AoV VTI) and pressure gradient (AoV PG) were obtained by pulsed wave Doppler spectra from SC and L-LAX views.

RESULTS

The SC view was feasible in all foals. No significant difference was found in AoS D between different views (P = 0.06), and no significant correlation was detected for bodyweight (BW). AoV Vel_max , VTI and PG obtained from the SC view were greater than from the L-LAX view (P<0.0001, P = 0.0001 and P<0.0001 respectively), especially in foals with lower BW.

MAIN LIMITATIONS

The order of the transducer site was not randomised among foals, and the observer was not blinded during offline measurement.

CONCLUSIONS

The SC view can be easily obtained in recumbent newborn foals and provides optimal alignment with aortic outflow, leading to more reliable Doppler flow velocity than the conventional L-LAX view. Further investigations of the potential use of this view in Doppler estimation of aortic outflow and cardiac assessment of sick foals are recommended.

Evaluation of the non-calibrated pulse contour cardiac output monitor FloTrac/Vigileo against thermodilution in standing horses

OBJECTIVE

To evaluate the non-calibrated, minimally invasive cardiac output (CO) monitor FloTrac/Vigileo (FloTrac) against thermodilution (TD) CO in standing horses.

STUDY DESIGN

Prospective, experimental trial.

ANIMALS

Nine adult horses weighing a median (range) of 535 (470-602) kg.

METHODS

Catheters were placed in the right atrium, pulmonary artery and carotid artery under local anaesthesia. CO was measured 147 times by TD and FloTrac and indexed to body weight. Changes in CO were achieved with romifidine or xylazine and dobutamine constant rate infusions. Bland-Altman analysis, concordance and polar plot analysis were used to assess agreement and ability to track changes in CO.

RESULTS

Mean ± standard deviation COTD of 48 ± 16 mL kg(-1) minute(-1) (range: 19-93 mL kg(-1) minute(-1) ) and mean COF loTrac of 9 ± 3 mL kg(-1) minute(-1) (range: 5-21 mL kg(-1) minute(-1) ) were measured. Low agreement with a large mean bias of 39 mL kg(-1) minute(-1) and wide limits of agreement of 8-70 mL kg(-1) minute(-1) were found. The percentage error of 108% and precision of TD of ± 18% resulted in an estimated precision of FloTrac of ± 106%. Comparison of changes in COF loTrac with changes in COTD gave a concordance rate of 52% in the four-quadrant plot, and a mean polar angle of -11° with radial limits of agreement of ± 61 ° in the polar plot. Mean arterial pressure (MAP) and COF loTrac were positively correlated (r = 0.5, p < 0.0001). No correlation of MAP with COTD was observed.

CONCLUSIONS AND CLINICAL RELEVANCE

The FloTrac system, originally designed for use in humans, neither measured absolute CO in standing horses accurately nor tracked relative changes in CO measured by TD correctly. The false dependence of COF loTrac on arterial blood pressure further discourages the use of this technique in horses.

Cardiopulmonary Effects of Laparoscopic Ovariectomy of Variable Duration in Cats

OBJECTIVE

To evaluate the cardiopulmonary effects of low-pressure (6 mmHg) peritoneal insufflation of varying duration in healthy cats during ovariectomy (OVE).

STUDY DESIGN

Prospective, randomized study.

ANIMALS

Female cats (n = 24).

METHODS

After anesthesia induction, cats had short (Short LAP; n = 8) or long duration (Long LAP; n = 8) laparoscopic ovariectomy, or Open OVE (Open; n = 8) for comparison. Hemodynamic and pulmonary measurements were recorded after induction of anesthesia (T0), 5 minutes after abdominal insufflation had reached 6 mmHg of pressure (T1), after the 2nd ovary had been resected (T2), after abdominal decompression (T3), and at the end of anesthesia, after abdominal closure (T4). Hemodynamic and pulmonary variables were compared between groups.

RESULTS

Low-pressure abdominal insufflation caused cardiopulmonary changes in cats. At T1 and T2, Long LAP and Short LAP caused a significant change in PvCO2 and RC when compared with Open. During T3, RC was lower only in Long LAP. At T2, there was decrease in SV, but not CO for Long LAP when compared with Open.

CONCLUSIONS

Duration of insufflation was associated with worsening of negative cardiopulmonary effects; however, these effects were reversible and resolved by the end of the procedure.

Comparison between lithium dilution and pulse contour analysis techniques for cardiac output measurement in isoflurane anaesthetized ponies: influence of different inotropic drugs

OBJECTIVE

To compare cardiac output () measurements using lithium dilution (LiDCO) and pulse contour analysis (PulseCO) techniques in isoflurane-anaesthetized ponies before and during the administration of different inotropic/vasoactive drugs.

STUDY DESIGN

Prospective randomized experimental cross-over trial.

ANIMALS

Six ponies aged 5.0 +/- 1.6 (4-6.5) years and weighing 286 +/- 53 (212-368) kg.

METHODS

After sedation (romifidine) and induction (midazolam + ketamine), anaesthesia was maintained with isoflurane in oxygen. After 90 minutes (= T0), one of four treatments was administered: saline 0.1 mL kg(-1) (S), enoximone 0.5 mg kg(-1) IV (E), enoximone followed by dobutamine (0.5 microg kg(-1) minute(-1) for 120 minutes) (ED) or enoximone followed by a calcium chloride infusion (0.5 mg kg(-1) minute(-1) for 10 minutes) (EC). Data were recorded for 120 minutes after T0. The PulseCO (recorded from carotid artery) was calibrated before T0, no further recalibrations were performed. was determined with LiDCO ((LiDCO)) and PulseCO ((PulseCO)) simultaneously at T5, T10, T20, T40, T60, T80, T100 and T120. Systemic vascular resistances (SVR(LiDCO) and SVR(PulseCO)) were calculated.

RESULTS

In the saline group, (PulseCO) was 4.9 +/- 12.3% lower than LiDCO (p < 0.01), whereas SVR(PulseCO) was 6.9 +/- 14.4% higher than SVR(LiDCO) (p < 0.01). These differences increased over time (mean +/- SEM), by 0.06 +/- 0.03% minute(-1) (p = 0.042) and SVR by 0.08 +/- 0.03% minute(-1) (p = 0.018). (PulseCO) was higher than (LiDCO) in the EC group (1.8 +/- 23.3%), but lower than (LiDCO) in groups E (-11.7 +/- 20.4%) and ED (-10.0 +/- 25.9%) (significant difference between treatments, p < 0.01). The differences in SVR in groups E (20.4 +/- 32.0%) and ED (20.7 +/- 35.3%) were significantly higher than in groups S (6.9 +/- 14.4%) and EC (3.1 +/- 22.2%) (p < 0.01).

CONCLUSIONS AND CLINICAL RELEVANCE

Pulse contour analysis values deviated significantly from LiDCO measurements in isoflurane-anaesthetized ponies. This difference was influenced by inotropic/vasoactive drugs.

Continuous, non-invasive techniques to determine cardiac output in children after cardiac surgery: evaluation of transesophageal Doppler and electric velocimetry

BACKGROUND

Continuous and non-invasive measurement of cardiac output (CO) may contribute helpful information to the care and treatment of the critically ill pediatric patient. Different methods are available but their clinical verification is still a major problem.

AIM

Comparison of reliability and safety of two continuous non-invasive methods with transthoracic echocardiography (TTE) for CO measurement: electric velocimetry technique (EV, Aesculon) and transesophageal Doppler (TED, CardioQP). METHODS/MATERIAL: In 26 infants and children who had undergone corrective cardiac surgery at a median age of 3.5 (1-17) years CO and stroke volume (SV) were obtained by EV, TED and TTE. Each patient had five measurements on the first day after surgery, during mechanical ventilation and sedation.

RESULTS

Values for CO and SV from TED and EV correlated well with those of TTE (r = 0.85 and r = 0.88), but mean values were significantly lower than the values of TTE for TED (P = 0.02) and EV (P = 0.001). According to Bland-Altman analysis, bias was 0.36 l/min with a precision of 1.67 l/min for TED vs. TTE and 0.87 l/min (bias) with a precision of 3.26 l/min for EV vs. TTE. No severe adverse events were observed and the handling of both systems was easy in the sedated child.

CONCLUSIONS

In pediatric patients non-invasive measurement of CO and SV with TED and EV is useful for continuous monitoring after heart surgery. Both new methods seem to underestimate cardiac output in terms of absolute values. However, TED shows tolerable bias and precision and may be helpful for continuous CO monitoring in a deeply sedated and ventilated pediatric patient, e.g. in the operating room or intensive care unit.

Comparison of Fick and thermodilution cardiac output determinations in standing horses

The Fick and thermodilution (TD) methods are two currently popular techniques for determination of cardiac output (CO) in adult horses. To our knowledge, a comparison of these two techniques has not been reported. Six healthy, resting, fit, adult horses of either sex and weighing 516.5+/-33.2 kg (mean+/-SD) were instrumented to enable measurement of cardiac output. Resting CO was determined by the Fick method and by thermodilution while the horses stood quietly in the stocks. Fick and thermodilution CO measurements were repeated under conditions of increased cardiac output achieved with the use of a dobutamine infusion (5 microg kg(-1) min(-1), IV), and again under conditions of decreased CO induced by administration of xylazine (0.5 mg/kg, IV). Fick and thermodilution cardiac outputs were compared using Bland-Altman analysis for repeated measures. The mean of the differences+/-1.96SD (bias and precision) between the two techniques was 1.88+/-24.17 L/min. Variability between measurements with the two techniques was decreased to 3.41+/-46.78 mL kg(-1) min(-1) when CO was normalized for body size by calculation of cardiac index.

Evaluation of cardiovascular effects of total intravenous anesthesia with propofol or a combination of ketamine-medetomidine-propofol in horses

OBJECTIVE

To evaluate the cardiovascular effects of total IV anesthesia with propofol (P-TIVA) or ketamine-medetomidine-propofol (KMP-TIVA) in horses.

ANIMALS

5 Thoroughbreds.

PROCEDURES

Horses were anesthetized twice for 4 hours, once with P-TIVA and once with KMP-TIVA. Horses were medicated with medetomidine (0.005 mg/kg, IV) and anesthetized with ketamine (2.5 mg/kg, IV) and midazolam (0.04 mg/kg, IV). After receiving a loading dose of propofol (0.5 mg/kg, IV), anesthesia was maintained with a constant rate infusion of propofol (0.22 mg/kg/min) for P-TIVA or with a constant rate infusion of propofol (0.14 mg/kg/min), ketamine (1 mg/kg/h), and medetomidine (0.00125 mg/kg/h) for KMP-TIVA. Ventilation was artificially controlled throughout anesthesia. Cardiovascular measurements were determined before medication and every 30 minutes during anesthesia, and recovery from anesthesia was scored.

RESULTS

Cardiovascular function was maintained within acceptable limits during P-TIVA and KMP-TIVA. Heart rate ranged from 30 to 40 beats/min, and mean arterial blood pressure was > 90 mm Hg in all horses during anesthesia. Heart rate was lower in horses anesthetized with KMP-TIVA, compared with P-TIVA. Cardiac index decreased significantly, reaching minimum values (65% of baseline values) at 90 minutes during KMP-TIVA, whereas cardiac index was maintained between 80% and 90% of baseline values during P-TIVA. Stroke volume and systemic vascular resistance were similarly maintained during both methods of anesthesia. With P-TIVA, some spontaneous limb movements occurred, whereas with KMP-TIVA, no movements were observed.

CONCLUSIONS AND CLINICAL RELEVANCE

Cardiovascular measurements remained within acceptable values in artificially ventilated horses during P-TIVA or KMP-TIVA. Decreased cardiac output associated with KMP-TIVA was primarily the result of decreases in heart rate.

Minimally Invasive Determination of Cardiac Output by Transthoracic Bioimpedance, Partial Carbon Dioxide Rebreathing, and Transesophageal Doppler Echocardiography in Beagle Dogs

Minimally invasive cardiac output was determined using transthoracic bioimpedance (BICO), partial carbon dioxide rebreathing (NICO) and transesophageal Doppler echocardiography (TEECO) and compared to thermodilution (TDCO) in 6 beagle dogs. The dogs were 2 years old, weigh between 9.1-13.0 kg and were anesthetized with nitrous oxide-oxygen-sevoflurane. All dogs were administered a neuromuscular blocking drug and artificially ventilated during anesthesia. Thirty paired measurements of TDCO and each non-invasive method were collected during low, intermediate, and high values of cardiac output achieved by varying the depth of anesthesia and the administration of dobutamine. Cardiac output values ranged from 1.10-2.50 L/min for BICO compared to 0.81-4.88 L/min for TDCO; 0.70-2.60 L/min for NICO compared to 0.89-4.45 L/min for TDCO; and 0.59-4.37 L/min for TEECO compared to 0.57-4.15 L/min for TDCO. The limits of agreement and percentage error were -0.58 +/- 1.56 L/min and +/- 75.4% for BICO, -1.04 +/- 1.08 L/min and +/- 56.0% for NICO, and 0.03 +/- 0.26 L/min and +/- 12.3% for TEECO compared to TDCO. In conclusion, TEECO provided the best agreement to TDCO in sevoflurane anesthetized beagle dogs.

Skeletal muscle blood flow in anaesthetized horses. Part I: measurement techniques

OBJECTIVE

The objective of this review was to describe the methodology and limitations of techniques that have been used to measure skeletal muscle blood flow in anaesthetized horses.

DATABASE USED

Pubmed, personal files.

CONCLUSION

Numerous techniques have been used in horses to study skeletal muscle blood flow during anaesthesia and after the administration of vasoactive agents. Of the available techniques, blood flow measurements are limited to either microvascular flow (radioactive xenon, laser Doppler flowmetry) or total blood flow (radioactive microspheres, electromagnetic flowmetry, Doppler ultrasonography). None of the techniques currently available are able to fully assess the distribution of flow throughout the skeletal muscle. Near-infrared spectroscopy has the potential to assess the adequacy of oxygenation within muscles; however, this technique is not without limitations, and more work is required to assess its suitability. Understanding the limitations of these techniques is an important prerequisite to the critical evaluation of the information currently available on the effects of anaesthesia and vasoactive drugs on skeletal muscle blood flow.

The effects of halothane and isoflurane on cardiovascular function in laterally recumbent horses

BACKGROUND

Experimental studies in adult horses have shown that general anaesthesia maintained with isoflurane is associated with less depression of cardiovascular function compared with halothane anaesthesia. Adverse effects of intermittent positive-pressure ventilation (IPPV) have also been demonstrated. Nevertheless, the haemodynamic effects of these agents and the effects of differing modes of ventilation have not been assessed during clinical anaesthesia in horses undergoing surgery.

METHODS

The haemodynamic effects of isoflurane or halothane anaesthesia during spontaneous or IPPV were studied non-invasively in 32 laterally recumbent horses undergoing elective surgery. Indices of cardiac function and measurements of femoral arterial blood flow and resistance were recorded using transoesophageal and transcutaneous Doppler echocardiography, respectively. Arterial pressure was measured directly using a facial artery catheter.

RESULTS

Cardiac index (CI) was significantly higher during isoflurane anaesthesia than during halothane anaesthesia and was also higher during spontaneous ventilation with isoflurane. CI decreased significantly over time and an inverse relationship was observed between CI and mean arterial pressure (MAP). Horses with higher MAP had a significantly lower CI. During isoflurane anaesthesia, femoral arterial blood flow was significantly higher in both pelvic limbs compared with halothane anaesthesia, and flow in the lower limb was significantly higher during spontaneous ventilation than during IPPV. No significant change in femoral blood flow was observed over time.

CONCLUSION

The effects of anaesthetics and mode of ventilation on cardiovascular function recorded under surgical conditions in horses are similar to those reported under experimental conditions. However, in contrast with previous experimental studies, CI progressively decreased over time regardless of agent used or mode of ventilation employed.

Use of lithium dilution and pulse contour analysis cardiac output determination in anaesthetized horses: a clinical evaluation

OBJECTIVE

To assess the suitability of a human algorithm for calculation of continuous cardiac output from the arterial pulse waveform, in anaesthetized horses.

STUDY DESIGN

Prospective clinical study.

ANIMALS

Twenty-four clinical cases undergoing anaesthesia for various conditions.

MATERIALS AND METHODS

Cardiac output (Qt), measured by lithium dilution (QtLiDCO), was compared with a preceding, calibrated Qt measured from the pulse waveform (QtPulse). These comparisons were repeated every 20-30 minutes. Positive inotropes or vasopressors were administered when clinically indicated. Cardiac indices from 30.7 to 114.9 mL kg(-1) minute(-1) were recorded. Unusually shaped QtLiDCO curves were rejected and the measurement was repeated immediately.

RESULTS

Eighty-nine comparisons were made between QtLiDCO and QtPulse. The bias between the mean (+/-SD) of the two methods (QtLiDCO - QtPulse) was -0.07 L minute(-1)(+/-3.08) (0.24 +/- 6.48 mL kg(-1) minute(-1)). The limits of agreement were -12.72 and 13.2 mL kg(-1) minute(-1) (Bland & Altman 1986; Mantha et al. 2000). Linear regression analysis demonstrated a correlation coefficient (r2) of 0.89. Cardiac output in individual patients varied from 49.1 to 183% of the initial measurement at the time of calibration. Linear regression of log-transformed Qt variation for each method found a mean difference of 9% with limits of agreement of -4.1 to 22.1%.

CONCLUSIONS AND CLINICAL RELEVANCE

This method of pulse contour analysis is a relatively noninvasive and reliable way of monitoring continuous Qt in the horse under anaesthesia. The ability to easily monitor Qt might decrease morbidity and mortality in the anaesthetized horse.

Comparison of thermal dilution and electrical impedance dilution methods for measurement of cardiac output in standing and exercising horses

OBJECTIVE

To compare cardiac output measured in the pulmonary artery and a carotid artery by use of thermal and electrical impedance dilution. Animals-7 fit, clinically normal Standardbreds between 2 and 5 years of age.

PROCEDURE

Transient changes in electrical impedance and temperature of blood were induced by bolus injections of ice-cold saline hypertonic (6% and 9% NaCl) solutions. Cardiac output was calculated by applying Stewart-Hamilton principles to the indicator dilution transients. Measurements were made during sequential exercise episodes on a level treadmill over approximately an 8-fold range of cardiac output values.

RESULTS

We detected agreement between cardiac output determined by use of electrical impedance dilution at the pulmonary artery and carotid artery. Cardiac output from thermal dilution measured at the carotid artery exceeded that measured at the pulmonary artery. Cardiac output from the thermal dilution technique exceeded cardiac output from the electrical impedance dilution technique at both locations.

CONCLUSIONS AND CLINICAL RELEVANCE

The electrical impedance indicator is conserved on first transit; therefore cardiac output measured by electrical impedance dilution at the carotid artery is reliable over a large range of values. Thermal dilution provides a larger estimate of cardiac output, compared with the electrical impedance dilution technique, probably because of a loss of indicator. The transpulmonary electrical impedance dilution technique may have potential for clinical application, particularly in animals in which catheterization of the pulmonary artery is not appropriate or blood loss must be minimized.

Cardiac output technologies with special reference to the horse

Critical illness, anesthesia, primary cardiovascular disease, and exercise may result in marked hemodynamic alterations. Measuring cardiac output (CO) is central to defining these alterations for both clinician and researcher. In the past 10 years, several new methods of measuring CO have been developed for the human medical market. Some of these methods are now validated in the horse and are in clinical use. The Fick method has been used in equine research for more than a century. It depends on simultaneous measurement of mixed venous (pulmonary arterial) and peripheral arterial oxygen content and oxygen uptake by the lungs. The technique is technically demanding, which restricts its clinical use. Indicator dilution techniques, with indocyanine green, cold (thermodilution), or lithium as the marker, have also been widely used in the horse. The indocyanine technique is cumbersome, and thermodilution requires right heart catheterization, which is not a benign procedure, making both of these methods less than ideal for clinical use. Lithium dilution requires catheterization of a peripheral artery and a jugular vein. It has recently been validated in anesthetized adult horses and neonatal foals. Doppler echocardiography is a noninvasive ultrasound-based technique. More accurate measurements are obtained with transesophageal than with transthoracic measurements; however, both methods require considerable technical expertise. Bioimpedance and pulse contour analysis are 2 new methods that have yet to be validated in the horse. With the currently available technology, lithium dilution appears to be the method of measuring CO best suited to the equine clinic.

Echo-Doppler findings of a carotid-jugular fistula in a foal

A seven-month-old male Standardbred was referred with a 4-month history of left jugular groove distension. On physical examination severely dilated left maxillary, linguofacial and jugular veins were appreciable. Blood gas analysis results indicated higher degree of oxygenation in the left jugular vein. Bi-directional flow signals, with normal systolic and early diastolic flow, and late diastolic retrograde flow, were noted on pulsed-wave Doppler interrogation and color flow mapping of the left jugular blood flow. A systolic-diastolic signal with spectral broadening was detected when positioning the sample gate near the internal jugular wall, in the distal left cervical region. Color Doppler imaging of the same region showed blood flow passage between the left common carotid artery and the left jugular vein through a small carotid-jugular fistula. Markedly increased blood flow velocities and volumetric flow, and decreased resistive index were recorded in the left compared to the right common carotid artery.

Comparison of the cardiopulmonary effects of anesthesia maintained by continuous infusion of romifidine, guaifenesin, and ketamine with anesthesia maintained by inhalation of halothane in horses

OBJECTIVE

To compare cardiopulmonary responses during anesthesia maintained with halothane and responses during anesthesia maintained by use of a total intravenous anesthetic (TIVA) regimen in horses.

ANIMALS

7 healthy adult horses (1 female, 6 geldings).

PROCEDURE

Each horse was anesthetized twice. Romifidine was administered IV, and anesthesia was induced by IV administration of ketamine. Anesthesia was maintained for 75 minutes by administration of halothane (HA) or IV infusion of romifidine, guaifenesin, and ketamine (TIVA). The order for TIVA or HA was randomized. Cardiopulmonary variables were measured 40, 60, and 75 minutes after the start of HA orTIVA.

RESULTS

Systolic, diastolic, and mean carotid arterial pressures, velocity time integral, and peak acceleration of aortic blood flow were greater, and systolic, diastolic, and mean pulmonary arterial pressure were lower at all time points for TIVA than for HA. Pre-ejection period was shorter and ejection time was longer for TIVA than for HA. Heart rate was greater for HA at 60 minutes. Minute ventilation and alveolar ventilation were greater and inspiratory time was longer for TIVA than for HA at 75 minutes. The PaCO2 was higher at 60 and 75 minutes for HA than forTIVA.

CONCLUSIONS AND CLINICAL RELEVANCE

Horses receiving a constant-rate infusion of romifidine, guaifenesin, and ketamine maintained higher arterial blood pressures than when they were administered HA. There was some indication that left ventricular function may be better during TIVA, but influences of preload and afterload on measured variables could account for some of these differences.

Pharmacokinetics and toxic effects of lithium chloride after intravenous adminstration in conscious horses

OBJECTIVE

To determine the pharmacokinetics and toxic effects associated with IV administration of lithium chloride (LiCl) to conscious healthy horses.

ANIMALS

6 healthy Standardbred horses.

PROCEDURE

Twenty 3-mmol boluses of LiCl (0.15 mmol/L) were injected IV at 3-minute intervals (total dose, 60 mmol) during a 1-hour period. Blood samples for measurement of serum lithium concentrations were collected before injection and up to 24 hours after injection. Behavioral and systemic toxic effects of LiCl were also assessed.

RESULTS

Lithium elimination could best be described by a 3-compartment model for 5 of the 6 horses. Mean peak serum concentration was 0.561 mmol/L (range, 0.529 to 0.613 mmol/L), with actual measured mean serum value of 0.575 mmol/L (range, 0.52 to 0.67 mmol/L) at 2.5 minutes after administration of the last bolus. Half-life was 43.5 hours (range, 32 to 84 hours), and after 24 hours, mean serum lithium concentration was 0.13+/-0.05 mmol/L (range, 0.07 to 0.21 mmol/L). The 60-mmol dose of LiCl did not produce significant differences in any measured hematologic or biochemical variables, gastrointestinal motility, or ECG variables evaluated during the study period.

CONCLUSIONS AND CLINICAL RELEVANCE

Distribution of lithium best fit a 3-compartment model, and clearance of the electrolyte was slow. Healthy horses remained unaffected by LiCl at doses that exceeded those required for determination of cardiac output. Peak serum concentrations were less than steady-state serum concentrations that reportedly cause toxic effects in other species.

Comparison of detomidine and romifidine as premedicants before ketamine and halothane anesthesia in horses undergoing elective surgery

OBJECTIVE

To compare detomidine hydrochloride and romifidine as premedicants in horses undergoing elective surgery.

ANIMALS

100 client-owned horses.

PROCEDURE

After administration of acepromazine (0.03 mg/kg, IV), 50 horses received detomidine hydrochloride (0.02 mg/kg of body weight, IV) and 50 received romifidine (0.1 mg/kg, IV) before induction and maintenance of anesthesia with ketamine hydrochloride (2 mg/kg) and halothane, respectively. Arterial blood pressure and blood gases, ECG, and heart and respiratory rates were recorded. Induction and recovery were timed and graded.

RESULTS

Mean (+/- SD) duration of anesthesia for all horses was 104 +/- 28 minutes. Significant differences in induction and recovery times or grades were not detected between groups. Mean arterial blood pressure (MABP) decreased in both groups 30 minutes after induction, compared with values at 10 minutes. From 40 to 70 minutes after induction, MABP was significantly higher in detomidine-treated horses, compared with romifidine-treated horses, although more romifidine-treated horses received dobutamine infusions. In all horses, mean respiratory rate ranged from 9 to 11 breaths/min, PaO2 from 200 to 300 mm Hg, PaCO2 from 59 to 67 mm Hg, arterial pH from 7.33 to 7.29, and heart rate from 30 to 33 beats/min, with no significant differences between groups.

CONCLUSIONS AND CLINICAL RELEVANCE

Detomidine and romifidine were both satisfactory premedicants. Romifidine led to more severe hypotension than detomidine, despite administration of dobutamine to more romifidine-treated horses. Both detomidine and romifidine are acceptable alpha2-adrenoceptor agonists for use as premedicants before general anesthesia in horses; however, detomidine may be preferable when maintenance of blood pressure is particularly important.

Effect of a 30-minute infusion of dobutamine hydrochloride on hind limb blood flow and hemodynamics in halothane-anesthetized horses

OBJECTIVE

To evaluate the hemodynamic effects of dobutamine hydrochloride (0.5 microg/kg of body weight/min) in halothane-anesthetized horses.

ANIMALS

6 adult Thoroughbred horses.

PROCEDURE

Anesthesia was induced by use of romifidine (100 microg/kg) and ketamine (2.2 mg/kg), IV. Anesthesia was maintained by halothane (end-tidal concentration 0.9 to 1.0%). Aortic, left ventricular, and right atrial pressures were measured, using catheter-mounted strain gauge transducers. Cardiac output (CO), velocity time integral, maximal aortic blood flow velocity and acceleration, and left ventricular preejection period and ejection time were measured from aortic velocity waveforms obtained by transesophageal Doppler echocardiography. Velocity waveforms were recorded from the femoral vessels, using Doppler ultrasonography. The time-averaged mean velocity and early diastolic deceleration slope (EDDS) were measured. Pulsatility index (PI) and volumetric flow were calculated. Microvascular perfusion was measured in the semimembranosus muscles by laser Doppler flowmetry. Data were recorded 60 minutes after induction of anesthesia (control) and at 15 and 30 minutes after start of an infusion of dobutamine (0.5 microg/kg/min).

RESULTS

Aortic pressures were significantly increased during the infusion of dobutamine. No change was observed in the indices of left ventricular systolic function including CO. Femoral arterial flow significantly increased, and the PI and EDDS decreased. No change was observed in the femoral venous flow or in microvascular perfusion.

CONCLUSIONS AND CLINICAL RELEVANCE

At this dosage, dobutamine did not alter left ventricular systolic function. Femoral blood flow was preferentially increased as the result of local vasodilatation. The lack of effect of dobutamine on microvascular perfusion suggests that increased femoral flow is not necessarily associated with improved perfusion of skeletal muscles.

Cardiac output measured by lithium dilution, thermodilution, and transesophageal Doppler echocardiography in anesthetized horses

OBJECTIVE

To assess the suitability of lithium dilution as a method for measuring cardiac output in anesthetized horses, compared with thermodilution and transesophageal Doppler echocardiography.

ANIMALS

6 horses (3 Thoroughbreds, 3 crossbreeds).

PROCEDURE

Cardiac output was measured in 6 anesthetized horses as lithium dilution cardiac output (LiDCO), thermodilution cardiac output (TDCO), and transesophageal Doppler echocardiographic cardiac output (DopplerCO). For the LiDCO measurements, lithium chloride was administered i.v., and cardiac output was derived from the arterial lithium dilution curve. Sodium nitroprusside, phenylephrine hydrochloride, and dobutamine hydrochloride were used to alter cardiac output. Experiments were divided into 4 periods. During each period, 3 LiDCO measurements, 3 DopplerCO measurements, and 3 sets of 3 TDCO measurements were obtained.

RESULTS

70 comparisons were made between LiDCO, DopplerCO, and triplicate TDCO measurements over a range of 10 to 43 L/min. The mean (+/- SD) of the differences of LiDCO - TDCO was -0.86 +/- 2.80 L/min; LiDCO = -1.90 + 1.05 TDCO (r = 0.94). The mean of the differences of DopplerCO - TDCO was 1.82 +/- 2.67 L/min; DopplerCO = 2.36 + 0.98 TDCO (r = 0.94). The mean of the differences of LiDCO - DopplerCO was -2.68 +/- 3.01 L/min; LiDCO = -2.53 + 0.99 DopplerCO (r = 0.93).

CONCLUSIONS AND CLINICAL RELEVANCE

These results indicate that lithium dilution is a suitable method for measuring cardiac output in horses. As well as being accurate, it avoids the need for pulmonary artery catheterization and is quick and safe to use. Monitoring cardiac output during anesthesia in horses may help reduce the high anesthetic mortality in this species.

Repeatability of Doppler ultrasound measurements of hindlimb blood flow in halothane anaesthetised horses

The purpose of this study was to determine the repeatability of femoral blood flow recorded using Doppler ultrasound in anaesthetised horses. Doppler ultrasound of the femoral artery and vein was performed in 6 horses anaesthetised with halothane and positioned in left lateral recumbency. Velocity spectra, recorded using low pulse repetition frequency, were used to calculate time-averaged mean velocity (TAV), velocity of component a (TaVa), velocity of component b (TaVb), volumetric flow, early diastolic deceleration slope (EDDS) and pulsatility index (PI). Within-patient variability was determined for sequential Doppler measurements recorded during a single standardised anaesthetic episode. Within-patient variability was also determined for Doppler and cardiovascular measurements recorded during 4 separate standardised anaesthetic episodes performed at intervals of at least one month. Within-patient variation during a single anaesthetic episode was small. Coefficients of variation (cv) were <12.5% for arterial measurements and <17% for venous measurements. Intraclass correlation coefficient was >0.75 for all measurements. No significant change was observed in measurements of cardiovascular function suggesting that within-patient variation observed during a single anaesthetic episode was due to measurement error. In contrast, within-patient variation during 4 separate anaesthetic episodes was marked (cv>17%) for most Doppler measurements obtained from arteries and veins. Variation in measurements of cardiovascular function were marked (cv>20%), suggesting that there is marked biological variation in central and peripheral observed. Further studies are warranted to determine the ability of this technique to detect differences in blood flow during administration of different anaesthetic agents.

Doppler ultrasonography and single-fiber laser Doppler flowmetry for measurement of hind limb blood flow in anesthetized horses

OBJECTIVE

To use Doppler ultrasonography and single-fiber laser Doppler flowmetry (LDF) to evaluate blood flow in the dependent and nondependent hind limbs of anesthetized horses and to evaluate changes in femoral arterial blood flow and microvascular skeletal muscle perfusion in response to administration of phenylephrine hydrochloride or dobutamine hydrochloride.

ANIMALS

6 healthy adult horses.

PROCEDURE

Horses were anesthetized and positioned in left lateral recumbency. Doppler ultrasonography was used to measure velocity and volumetric flow in the femoral vessels. Single-fiber LDF was used to measure relative microvascular perfusion at a single site in the semimembranosus muscles. Phenylephrine or dobutamine was then administered to decrease or increase femoral arterial blood flow, and changes in blood flow and microvascular perfusion were recorded.

RESULTS

Administration of phenylephrine resulted in significant decreases in femoral arterial and venous blood flows and cardiac output and significant increases in mean aortic blood pressure, systemic vascular resistance, and PCV. Administration of dobutamine resulted in significant increases in femoral arterial blood flow, mean aortic blood pressure, and PCV. Significant changes in microvascular perfusion were not detected.

CONCLUSION AND CLINICAL RELEVANCE

Results suggest that Doppler ultrasonography and single-fiber LDF can be used to study blood flows in the hind limbs of anesthetized horses. However, further studies are required to determine why changes in femoral arterial blood flows were not associated with changes in microvascular perfusion.

Measurements of hindlimb blood flow recorded using Doppler ultrasound during administration of vasoactive agents in halothane-anesthetized horses

The purpose of the study was to determine the ability of Doppler ultrasound to detect changes in femoral blood flow during pharmacologic manipulation of arterial blood pressure. Doppler ultrasonography was performed in the femoral vessels of six halothane-anesthetized horses before and during administration of phenylephrine HCI and sodium nitroprusside. The time-averaged mean velocity and volumetric flow were calculated. The contour of the velocity waveform was assessed, and the early diastolic deceleration slope (EDDS) and pulsatility index (PI) were calculated. Administration of phenylephrine HCI resulted in increased mean aortic blood pressure (MABP) by 40% (29.3-53.0%). This caused significant decrease in cardiac output (26.8 to 13.5 l/min), femoral arterial velocity (left artery 7.20 to 4.00 cm/s; right artery 5.01 to 3.39 cm/s) and volumetric flow (left artery 556 to 221 ml/min; right artery 397 to 193 ml/min) in the femoral vessels and significant increase in systemic vascular resistance (163 to 433 dyn-s/cm5), EDDS (1a: 285 to 468: ra: 250 to 481) and PI (1a: 9.38 to 20.4; ra 17.1 to 29.1). Administration of sodium nitroprusside resulted in a decreased MABP of 27.2% (22.5-33%). This increased cardiac output (20.8 to 32.4 L/min), however, no significant changes were observed in femoral blood flow. Despite obvious changes in the waveform contour, no significant change occurred in EDDS or PI. These results suggest that Doppler ultrasound may be useful for measuring femoral blood flow in anesthetized horses. However, waveform analysis appears to be limited when multiple changes occur in central and peripheral haemodynamics.

A meta-analysis of studies using bias and precision statistics to compare cardiac output measurement techniques

INTRODUCTION

Bias and precision statistics have succeeded regression analysis when measurement techniques are compared. However, when applied to cardiac output measurements, inconsistencies occur in reporting the results of this form of analysis.

METHODS

A MEDLINE search was performed, dating from 1986. Studies comparing techniques of cardiac output measurement using bias and precision statistics were surveyed. An error-gram was constructed from the percentage errors in the test and reference methods and was used to determine acceptable limits of agreement between methods.

RESULTS

Twenty-five articles were found. Presentation of statistical data varied greatly. Four different statistical parameters were used to describe the agreement between measurements. The overall limits of agreement in studies evaluating bioimpedance (n = 23) was +/-37% (15-82%) and in those evaluating Doppler ultrasound (n = 11) +/-65% (25-225%). Objective criteria used to assess outcome were given in only 44% of the articles. These were (i) limits of agreement approaching +/-15-20%, (ii) limits of agreement of less than 1 L/min, and (iii) more than 75% of bias measurements within +/-20% of the mean. Graphically, we showed that limits of agreement of up to +/-30% were acceptable.

CONCLUSIONS

When using bias and precision statistics, cardiac output, bias, limits of agreement, and percentage error should be presented. Using current reference methods, acceptance of a new technique should rely on limits of agreement of up to +/-30%.

Haemodynamic effects of a sixty minute infusion of dopamine hydrochloride in horses anaesthetised with halothane

To describe the haemodynamic effects of a 60 min infusion of dopamine 4 microg/kg bwt/min during halothane anaesthesia, 7 mature Thoroughbred horses were studied. The infusion began 1 h after induction of anaesthesia by romifidine (100 microg/kg) and ketamine (2.2 mg/kg bwt). Throughout the period of dopamine infusion and for 30 min after its discontinuation, the horses were ventilated by intermittent positive pressure to maintain PaCO2 between 4.6-5.4 KPa. Inspired halothane concentration was adjusted to maintain an end tidal halothane concentration of 0.9%. Haemodynamic variables were measured using intracardiac strain gauge transducers sited in the left and right ventricle, aorta, and pulmonary artery. Left ventricular pressure was differentiated to obtain maximal rate of increase of intraventricular pressure (LVdp/dtmax). Transoesophageal Doppler echocardiography was performed to measure maximum aortic blood flow velocity (vmax) and acceleration (dv/dtmax), left ventricular velocity time integral (vTI) and cardiac output (CO), and left ventricular pre-ejection period (PEP) and ejection time (ET). Measurements were made during the 60 min infusion, and for 30 min after the infusion was discontinued. Infusion of dopamine 4 microg/kg/min significantly decreased mean aortic pressure, while left and right ventricular end-diastolic pressure and mean pulmonary artery pressure remained unchanged. There was a small, but significant, increase in heart rate during dopamine infusion. Maximum acceleration of aortic blood flow, CO and vTI were also significantly increased by dopamine infusion. Maximal rate of increase of intraventricular pressure (LVdp/dtmax) was significantly decreased 10 min after commencing infusion, but then returned to baseline for the remainder of the study. Left ventricular pre-ejection period (PEP) decreased during dopamine infusion whilst ejection time (ET) significantly increased. All measured variables except LVET had returned to baseline values within 30 min of discontinuing the infusion. This study demonstrated beneficial effects of dopamine infusion upon left ventricular systolic function. However, the therapeutic value of the drug is likely to be limited in clinical anaesthesia due to the simultaneous falls in arterial blood pressure which accompany its administration.