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Measurement of pulmonary transit time in healthy cats by use of ultrasound contrast media “Sonovue®”: Feasibility, reproducibility, and values in 42 cats
Journal of Veterinary Cardiology (2013) 15, 181e187
www.elsevier.com/locate/jvc
Measurement of pulmonary transit time in healthy cats by use of ultrasound contrast media “Sonovue”: Feasibility, reproducibility, and values in 42 cats Andrea Streitberger, Mag. med. vet.*, Verena Hocke, Dr. med. vet. , Peter Modler, Dr. med. vet. Traunkreis Vet Clinic, Veterina¨rstraße 2, 4642 Sattledt, Austria Received 3 July 2012; received in revised form 1 May 2013; accepted 6 May 2013
KEYWORDS Pulmonary blood volume; Cardiac performance; Contrast echocardiography
Abstract Objective: To evaluate the feasibility of measuring pulmonary transit time (PTT) in healthy cats by transthoracic echocardiography using the ultrasound contrast agent Sonovue. To determine normalized PTT (nPTT) values in 42 healthy cats and to estimate the interobserver variability and the within-day repeatability of nPTT measurements. Animals: Forty-two privately owned healthy cats of different breeds, gender and age presented for cardiac examination. Methods: A bolus injection of contrast agent (Sonovue) was administered intravenously. The right parasternal short axis echocardiographic view was used to record the contrast agent’s transit time from the pulmonary artery to the left atrium. Pulmonary transit time and nPTT were determined independently by three examiners with different levels of experience. Results: Normalized PTT was 4.12 1.0 (mean SD) in our population. The median interobserver variability across our population was 6.8%, the median within-day variability for the three observers were 13.1%, 12.7% and 13%. No effect of the observer’s experience on nPTT measurement was identified. Age, sex and body weight did not significantly influence nPTT. Conclusions: This study demonstrates that nPTT measurement is feasible in cats using ultrasound and the blood pool contrast media Sonovue. Measurements of
* Corresponding author. E-mail addresses: [email protected], [email protected] (A. Streitberger). 1760-2734/$ - see front matter ª 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jvc.2013.05.001
182
A. Streitberger et al. nPTT can be performed in a clinical setting. Normalized PTT values in healthy cats are comparable with those reported in healthy dogs. ª 2013 Elsevier B.V. All rights reserved.
Abbreviations CHF CO ECG MMVD FPRNA HCM mRR nPTT PBV PTT TA
TP
congestive heart failure cardiac output electrocardiogram myxomatous mitral valve disease first-pass radionuclide angiocardiography hypertrophic cardiomyopathy mean ReR interval normalized pulmonary transit time pulmonary blood volume pulmonary transit time displayed time at which the contrast agent is first visible within the left atrium displayed time at which the contrast agent is passing the pulmonic valve
Introduction Pulmonary transit time (PTT) is the time for a sample of blood to pass through the pulmonary circulation. It depends on pulmonary blood volume (PBV) and cardiac output (CO) according to the formula PTT ¼ PBV/CO. Usually it is normalized to the heart rate (nPTT ¼ PTT/mRR; nPTT ¼ normalized PTT, mRR ¼ mean ReR interval). Thus, nPTT represents the number of stroke volumes that are necessary to drive a sample of blood from the pulmonic valve to the left atrium.1,2 Most studies on nPTT in humans and animals have used first-pass radionuclide angiography (FPRNA).1,3,4 It has been shown in humans that nPTT is independent of body size and loading conditions,5e7 and that it changes only minimally with exercise.3,8,9 In dogs, significantly elevated nPTT was observed with compensated myxomatous mitral valve disease (MMVD), and substantial lengthening of nPTT was found in congestive heart failure. These increases in nPTT were due to increased pulmonary blood volume rather than decreased forward cardiac output.1,10,11
Recently, nPTT was measured in dogs using second-generation ultrasound contrast media.12 a,b Two of these studies used Sonovue as the contrast agent.a,b,c The interobserver and within-day variability of PTT and nPTT measurements were within reasonable limits for clinical use, and nPTT was not influenced by age, body weight, gender or heart rate.c,d The contrast agent Sonovue is based on stabilized sulfur hexafluoride (SF6) microbubbles surrounded by a phospholipid shell. Sulfur hexafluoride is an inorganic, colorless and odorless gas with a low solubility in blood. The agent is supplied as a lyophilisate, which is mixed with saline to form a white suspension. Because 90% of the bubbles are smaller than 8 mm, they are able to access the entire pulmonary blood volume, which makes Sonovue highly suitable for measuring PTT. In human medicine, Sonovue has been shown to be safe in both abdominal and cardiac applications. In a study with a sample size of 23,188 procedures, the severe adverse event rate was 0.0086%.13 Diagnosis and staging of feline cardiomyopathies, as well as risk assessment in affected cats, are currently based on thoracic radiographs, echocardiography and biomarkers. Echocardiographic diagnosis and evaluation of left ventricular systolic and diastolic function are mainly estimated by measuring anatomic dimensions and performing Doppler recordings, which are usually sensitive to changes in loading conditions. Even though left atrial enlargement is easy to recognize on ultrasound and is associated with reduced survival time,14,15 a considerable overlap of left atrial size between cats with CHF and asymptomatic cats has
a
Kresken JG, Wendt RT, Ha ¨ggstro ¨m J. Echocardiographical estimate of pulmonary transit time (PTT, nPTT) in dogs using the echocardiographic contrast media Sonovue. Proceedings, 19th ECVIM-CA Congress, 2009, Porto, Portugal. b Wendt RT, Kresken JG, Ha ¨ggstro ¨m J. Repeatability and interobserver variability of pulmonary transit time (PTT, nPTT) in dogs using the echocardiographic contrast media Sonovue. Proceedings, 20th ECVIM-CA Congress, 2010, Toulouse, Frankreich. c Sonovue, Bracco Diagnostics, Milano, Italy. d Esaote MyLab 70, Milano, Italy.
Pulmonary transit time in cats been reported.14 Therefore, nPTT would assist with staging by adding information about pulmonary blood volume and cardiac output. It might be useful for assessing the risk of congestive heart failure and for evaluating the effects of therapeutic drugs. And finally, it may offer insights into the pathophysiology of feline cardiomyopathies. The aims of the present study were to assess the feasibility of PTT and nPTT measurements in healthy cats by ultrasound using the contrast agent Sonovue and to describe nPTT values in a population of healthy cats. Methodologically, it was intended to assess the interobserver variability, within-day variability of individual observers, repeatability of nPTT measurement and the influence of the experience level of the observer with related techniques.
183 was not used because this was not feasible for the ultrasound probe being used. A timer module with a resolution of 1/100 s was started, and a bolus of 0.0023e0.0032 mg/kg of contrast agent microbubbles (Sonovue) was injected into the cephalic vein through a venous catheter, while the ultrasound data was recorded as a digital loop. Lower dosages (0.0009 mg/kg, 0.00135 mg/kg and 0.0018 mg/kg of sulfur hexafluoride) were tried in the first 3 cats but did not yield sufficient echocontrast and the recordings were repeated using the higher dosage. Each cat underwent this procedure once. In addition, in 5 cats a contrast study was performed 5 times each, in the course of one day, for evaluation of within-day variability.
Measurement of nPTT
Animals, materials and methods The study was designed as a prospective data analysis. It was approved by the ethics committee of the regional government of Upper Austria and by the animal welfare commission of the University of Veterinary Medicine, Vienna.
Study population The study population consisted of 42 cats that were presented for routine cardiac examination. The median age was 3 years (range: 0.5e11.0) and the median body weight was 3.95 kg (range: 2.3e6.5). Seventeen cats were male and 25 were female. A complete clinical examination was performed on all the cats. Cats were included if they were clinically normal or had only minimal clinical disease (e.g. dental calculus formation was considered minimal). Patients with heart murmurs were excluded, regardless of the origin of the heart murmur. No blood samples or thoracic radiographs were taken. All cats underwent a complete echocardiographic examination with concomitant ECG recording and were determined to be free of cardiac disease.
Contrast study procedure All contrast studies were performed by the same examiner (PM) with the cats positioned in right lateral recumbency. No sedation was used. A right parasternal short axis view of the heart base was displayed using a 7.5e10.0 MHz phased array probe and the ECG was recorded simultaneously.d Frame rates ranged between 120 and 140 frames per seconds. A low mechanical index software
The measurements of PTT were done by offline analysis of the ultrasound recordings, by 3 veterinarians with different levels of experience in echocardiographic diagnostics (low, medium, high), independently of each other. “Low” corresponded to a novice in cardiology, the “medium”experienced observer was an assistant veterinarian in cardiology and the “high” level of experience was a certified cardiologist. The first individual frame on which the contrast agent was initially visible just behind the pulmonic valve was selected (Fig. 1). The corresponding time (TP) displayed on the timer was recorded. The clip was then carefully reviewed and the frame on which the contrast media was first visible inside the left atrium was identified (Fig. 2). The time was again recorded (TA). The pulmonary transit time was then calculated (TA-TP). To generate the nPTT value, the PTT was divided by the mean ReR interval (nPTT ¼ PTT/mRR). The mean ReR interval was calculated from the heart rates that were measured by the ECG during the contrast studies.
Data analysis Data were analyzed using commercial statistics software (SPSS and MedCalc).e,f For all statistical tests used, values of P < 0.05 were considered statistically significant. To test for a significant effect of the observer’s experience on nPTT values, Bland-Altman plots
e f
SPSS Statistics 21 Demo Version, IBM, US. MedCalc Software, Ostend, Belgium.
184
A. Streitberger et al. coefficients of variation. The median coefficient of variation for each observer across the 5 cats was used to estimate the within-day variability for each observer. To identify potential effects of age and body weight on nPTT, correlation analysis was used. Univariate and multivariate analysis were performed to determine whether nPTT could be reliably predicted by age and/or body weight using a linear regression model. A potential influence of sex on nPTT values was assessed by means of a two-sample t-Test.
Results Figure 1 Echocardiographic image when the contrast agent has just passed the pulmonic valve and is visible within the right ventricular outflow tract and the main pulmonary artery.
were generated. Interobserver variability was described on the basis of the three different nPTT values measured by the three different observers on each cat. Coefficients of variation were computed for each cat; these 42 coefficients of variation were then presented as median and interquartile ranges. The averages of the 3 nPTT values of each cat were then used to describe the nPTT values in our study population. They were tested for normal distribution using a KolmogoroveSmirnov test and presented as mean and standard deviation. The within-day variability of the nPTT-results was calculated based on the 5 cats in each of which 5 repeat contrast studies had been performed on one day. Each observer generated 5 nPTT values per cat (one for each contrast study), which were then used to calculate
Quality of contrast studies and tolerability Using the dosage of Sonovue mentioned above, good image quality was achieved in all cats. All cats tolerated the injection of Sonovue very well; neither ECG changes nor adverse reactions were observed.
Reproducibility and nPTT values BlandeAltman analysis did not reveal any significant influence of the observer’s experience on nPTT measurements (Fig. 3A, B, C). The median inter-observer variability across our population was 6.8% with an interquartile range of 4.3%e 12.3%. The calculated values of nPTT in this population were 4.12 1.0 (mean SD). The median within-day variability for the observer with high, medium and low experience was 13.1%, 12.7% and 13%, respectively (average 12.9%).
Impact of age, weight, and sex on PTT No correlation between age and nPTT (r ¼ 0.24; P ¼ 0.12) or between body weight and nPTT (r ¼ 0.1; P ¼ 0.54) could be identified. nPTT values could not be reliably predicted by age (R2 ¼ 0.058), body weight (R2 ¼ 0.01) or both (R2 ¼ 0.1) using a linear regression model. There was no significant difference in nPTT between male and female cats.
Discussion
Figure 2 Echocardiographic image showing the contrast agent reaching the left atrium.
This study demonstrates that PTT/nPTT measurement is feasible in cats using transthoracic echocardiography and the blood pool contrast agent Sonovue.
Pulmonary transit time in cats
Figure 3 Bland-Altman-Plots showing non-significant effects of observer’s experience on nPTT measurements. (Low, A; Medium, B; and High, C refer to the observer’s level of experience.)
The dosage of Sonovue needed to achieve good images was 0.0023e0.0032 mg/kg. This is higher than previously reported in dogs (0.0014e0.0023 mg/kg).a The reason for this difference could be the injection technique. In the study on dogs, the
185 injection of contrast agent was followed by an injection of saline.a This was considered not to be practical in cats since the contrast agent reached the pulmonary artery within 1e2 s, so there was not enough time to exchange the syringes. A double-injection technique could probably be achieved using a three-way stopcock with an extension set. Another possible explanation is that unlike in previous studies in dogs,a,b we did not use a special low mechanical index software, since this was not available for a high frequency probe. Still, the dosage we used was not higher than the maximum dosage used in human medicine (0.0014e0.0032 mg/kg),13,16,17 g and it was lower than the dosage of Sonovue that was previously reported for contrast-enhanced abdominal ultrasound in cats.18 However, the use of low mechanical index software in combination with high frequency probes and a double-injection technique remain as potential ways of reducing the dosage and might be worth following up. Adverse events after the injection of Sonovue were not observed in any cat of our study population using clinical observation, heart ultrasound or concomitant ECG recordings. This agrees with a recent retrospective study, which did not find any immediate or delayed adverse events in 77 cats, 43 of which were clinically ill.19 In that study, 98.1% of the examinations required more than one bolus injection of the contrast agent. No effect of contrast ultrasound on mortality of clinically ill cats was observed. The authors concluded that contrast-enhanced ultrasound is a safe method in clinically ill dogs and cats. In a human study with 23,188 procedures, the overall reporting rate of serious adverse events was 0.0086%.13 These serious events were related to the respiratory system (bronchospasm). Other mild and infrequent sideeffects observed in humans include mild hypotension, sinus tachycardia, leukocytosis, headache, vomiting, supraventricular tachycardia, tingling sensation in a hand, facial flushing and single ventricular extrasystoles.17,20e23 Sonovue was also well tolerated in patients with severe heart failure and pulmonary hypertension.20 Chronic toxicological studies were performed with Sonovue at doses up to 0.225 mg/kg in rats and monkeys, and no specific event or safety issues were observed.16 Certainly, we might have missed minor side effects or major effects that did not appear clinically. We did not perform blood
g Personal data, Dr. Med. Univ. Ralph Stephan von Bardeleben, University of Mainz, Germany.
186 pressure measurements after the contrast passage, nor did we take blood samples for troponin measurements, CBC or blood chemistry. Thus, further prospective studies assessing the safety of Sonovue in cats are necessary. There was no significant influence of the observers’ experience on the nPTT measuring results. This suggests that the procedure is simple and robust and does not need to be limited to veterinarians with a high level of experience in echocardiography. The degree of inter-observer variability we observed was comparable to interobserver variability reported generally for echocardiographic measurements24,25 and must be seen in the context of some technical limitations. The large sector angle used to simultaneously display the pulmonary artery and the left atrium leads to a decrease in sampling rates, which may influence the inter-observer and within-day variability. A lower frame rate can make it easier for the observers to choose the same frame for TP and TA, but the selection of different frames would lead to larger differences in the time reading. In this case it was not feasible to evaluate the influence of frame rate on the variability. Also, since all contrast study recordings were performed by the same observer, the effect of examiner-dependent technical adjustments and views may be underestimated. The calculated median within-day variability are comparable to those reported for other echocardiographic indices of cardiac function.26 Of course these data have to be interpreted with caution since only 5 cats were used for this analysis. The within-day variation could be caused by technical influences (different image settings, different image quality, slight differences in the imaging plane) or by some biological variability (potential physiological changes in pulmonary transit times). The effect of a specific interobserver and within-day variability on the usefulness of a clinical test can only be accurately assessed if the purpose of the test is clearly defined and differences between values in normal and abnormal individuals are known. In the present study, we collected data only from healthy individuals. However, data reported for dogs with compensated and decompensated degenerative mitral valve disease indicate a relatively small impact of interobserver and within-day variability on nPTT measurements.1,3 This is the first report on the measurement of pulmonary transit times in cats. The nPTT values we observed in our study population (4.12 1.0; mean SD) were comparable with those reported
A. Streitberger et al. in dogs using radionuclide angiography (4.6 0.6; mean SD)3 or contrast echocardiography (4.3 0.2; mean SD).a The evaluation of physiological indicators like PTT/nPTT can potentially provide additional insight into the pathophysiology and progression of feline cardiomyopathies, especially in terms of cardiac performance and/or pulmonary blood volume. Results using this technique may assist with understanding the progression of this heterogeneous group of cardiac diseases. Since the use of the contrast agent and ultrasound is easily applicable in clinical practice, it may have the potential to become an additional tool for the evaluation of feline cardiac patients with special regard to prognosis and risk assessment. It may also be useful for understanding the effects of pharmacologic treatment on cardiac function in cats. Further studies are necessary to compare these results to those measured in cats in different stages of diastolic dysfunction and also to nPTT values of cats in congestive heart failure.
Conclusions It is possible to measure nPTT in cats under clinical conditions using the ultrasound contrast media Sonovue. The values in our study population are comparable to those published in dogs.
Conflict of interest statement The authors have no conflict of interest.
Acknowledgments The authors would like to thank Dr. med. vet. Jan Gerd Kresken and Dr. med. vet. Ralph Wendt for their support, and interesting discussions and Robert O’Brien, DVM, DACVR, for his help with our manuscript.
References 1. Lord P, Eriksson A, Ha ¨ggstro ¨m J, Ja ¨rvinen AK, Kvart C, Hansson K, Maripuu E, Ma ¨kela ¨ O. Increased pulmonary transit times in asymptomatic dogs with mitral regurgitation. J Vet Intern Med 2003;17:824e829. 2. Giuntini C, Lewis ML, Luis AS, Harvey RM. A study of the pulmonary blood volume in man by quantitative radiocardiography. J Clin Invest 1963;42:1589e1605.
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3. Eriksson A, Hansson K, Ha ¨ggstro ¨m J, Ja ¨rvinen AK, Lord P. Pulmonary blood volume in mitral regurgitation in Cavalier King Charles Spaniels. J Vet Intern Med 2010;24:1393e1399. 4. Leitl GP, Buchanan JW, Wagner HN. Monitoring cardiac function with nuclear techniques. Am J Cardiol 1980;46: 1125e1132. 5. Mu ¨ller HM, Tripolt MB, Rehak PH, Groell R, Rienmu ¨ller R, Tscheliessnigg KH. Noninvasive measurement of pulmonary vascular resistances by assessment of cardiac output and pulmonary transit time. Invest Radiol 2000;35:727e731. 6. Franc ¸ois CJ, Shors SM, Bonow RO, Finn JP. Analysis of cardiopulmonary transit times at contrast material-enhanced MR imaging in patients with heart disease. Radiology 2003;227:447e452. 7. Shors SM, Cotts WG, Pavlovic-Surjancev B, Franc ¸ois CJ, Gheorghiade M, Finn JP. Heart failure: evaluation of cardiopulmonary transit times with time-resolved MR angiography. Radiology 2003;229:743e748. 8. Van Der Walt JJ, Van Rooyen JM, Cilliers GD, Van Ryssen JCJ, Van Aarde MN. Ratio of cardiopulmonary blood volume to stroke volume as an index of cardiac function in animals and in man. Cardiovasc Res 1981;15:580e587. 9. Kuikka J, Timisjarvi J, Tuominen M. Quantitative radiocardiographic evaluation of cardiac dynamics in man at rest and during exercise. Scand J Clin Lab Invest 1979;39: 423e434. 10. Carlsson C, Ha ¨ggstro ¨m J, Eriksson A, Ja ¨rvinen AK, Kvart C, Lord P. Size and shape of right heart chambers in mitral valve regurgitation in small-breed dogs. J Vet Intern Med 2009;23:1007e1013. 11. Mu ¨ller HM, Rehak PH, Puchinger M, Wagner D, Marte W, Tscheliessnigg KH. Measurement of cardiac output and pulmonary transit time for assessment of pulmonary vascular resistance in domestic piglets. Exp Physiol 2009;94(6): 659e664. 12. Crosara S, Ljungvall I, Margiocco ML, Ha ¨ggstro ¨m J, Tarducci A, Borgarelli M. Use of contrast echocardiography for quantitative and qualitative evaluation of myocardial perfusion and pulmonary transit time in healthy dogs. Am J Vet Res 2012;73:194e201. 13. Piscaglia F, Bolondi L. The safety of sonovue in abdominal applications: retrospective analysis of 23188 investigations. Ultrasound Med Biol 2006;32:1369e1375. 14. Rush JE, Freeman LM, Fenollosa NK, Brown DJ. Population and survival characteristics of cats with hypertrophic cardiomyopathy: 260 cases (1990e1999). J Am Vet Med Assoc 2002;220:202e207. 15. Payne J, Luis Fuentes V, Boswood A, Connolly D, Koffas H, Brodbelt D. Population characteristics and survival in 127 referred cats with hypertrophic cardiomyopathy (1997 to 2005). J Small Anim Pract 2010;51:540e547.
16. Schneider M. Sono Vue, a new ultrasound contrast agent. Eur Radiol 1999;9:347e348. 17. Hoffmann R, von Bardeleben S, ten Cate F, Borges AC, Kasprzak J, Firschke C, Lafitte S, Al-Saadi N, KuntzHehner S, Engelhardt M, Becher H, Vanoverschelde JL. Assessment of systolic left ventricular function: a multicentre comparison of cineventriculography, cardiac magnetic resonance imaging, unenhanced and contrastenhanced echocardiography. Eur Heart J 2005;26:607e616. 18. Diana A, Specchi S, Toaldo MB, Chinocchietti R, Laghi A, Cipone M. Contrast-enhanced ultrasonography of the small bowel in healthy cats. Vet Radiol Ultrasound 2011;52: 555e559. 19. Seiler GS, Brown JC, Reetz JA, Taeymans O, Bucknoff M, Rossi F, Ohlerth S, Alder D, Rademacher N, Drost T, Pollard RE, Travetti O, Pey P, Saunders JH, Shanaman MM, ´ Brien RT, Gaschen L. Safety of contrastOliveira CR, O enhanced ultrasonography in dogs and cats: 488 cases (2002-2011). J Am Vet Med Assoc 2013;242:1255e1259. 20. Galema TW, Geleijnse ML, Vletter WB, Laat L, Michels M, ten Cate FJ. Clinical usefulness of SonoVue contrast echocardiography: the thoraxcentre experience. Net Heart J 2007;15:55e60. 21. Bokor D. Diagnostic efficacy of SonoVue. Am J Cardiol 2000; 86:19Ge24G. 22. Bokor D, Chambers JB, Rees PJ, Mant TGK, Luzzani F, Spinazzi A. Clinical safety of SonoVue, a new contrast agent for ultrasound imaging, in healthy volunteers and in patients with chronic obstructive pulmonary disease. Invest Radiol 2001;36:104e109. 23. Gaibazzi N, Reverberi C, Squeri A, De Iaco G, Ardissino D, Gherli T. Contrast stress echocardiography for the diagnosis of coronary artery disease in patients with chest pain but without acute coronary syndrome: incremental value of myocardial perfusion. J Am Soc Echocardiogr 2009;22: 404e410. 24. Chetboul V, Concordet D, Pouchelon JL, Athanassiadis N, Muller C, Benigni L, Munari AC, Lefebvre HP. Effects of inter- and intra-observer variability on echocardiographic measurements in awake cats. J Vet Med A 2003;50: 326e331. 25. Chetboul V, Athanassiadis N, Concordet D, Nicolle A, Tessier D, Castagnet M, Pouchelon JL, Lefebvre HP. Observer-dependent variability of quantitative clinical endpoints: the example of canine echocardiography. J Vet Pharmacol Therap 2004;27:49e56. 26. Wess G, Keller LJM, Klausnitzer M, Killich M, Hartmann K. Comparison of longitudinal myocardial tissue velocity, strain, and strain rate measured by two-dimensional speckle tracking and by color tissue Doppler imaging in healthy dogs. J Vet Cardiol 2011;13:31e43.
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www.elsevier.com/locate/jvc
Measurement of pulmonary transit time in healthy cats by use of ultrasound contrast media “Sonovue”: Feasibility, reproducibility, and values in 42 cats Andrea Streitberger, Mag. med. vet.*, Verena Hocke, Dr. med. vet. , Peter Modler, Dr. med. vet. Traunkreis Vet Clinic, Veterina¨rstraße 2, 4642 Sattledt, Austria Received 3 July 2012; received in revised form 1 May 2013; accepted 6 May 2013
KEYWORDS Pulmonary blood volume; Cardiac performance; Contrast echocardiography
Abstract Objective: To evaluate the feasibility of measuring pulmonary transit time (PTT) in healthy cats by transthoracic echocardiography using the ultrasound contrast agent Sonovue. To determine normalized PTT (nPTT) values in 42 healthy cats and to estimate the interobserver variability and the within-day repeatability of nPTT measurements. Animals: Forty-two privately owned healthy cats of different breeds, gender and age presented for cardiac examination. Methods: A bolus injection of contrast agent (Sonovue) was administered intravenously. The right parasternal short axis echocardiographic view was used to record the contrast agent’s transit time from the pulmonary artery to the left atrium. Pulmonary transit time and nPTT were determined independently by three examiners with different levels of experience. Results: Normalized PTT was 4.12 1.0 (mean SD) in our population. The median interobserver variability across our population was 6.8%, the median within-day variability for the three observers were 13.1%, 12.7% and 13%. No effect of the observer’s experience on nPTT measurement was identified. Age, sex and body weight did not significantly influence nPTT. Conclusions: This study demonstrates that nPTT measurement is feasible in cats using ultrasound and the blood pool contrast media Sonovue. Measurements of
* Corresponding author. E-mail addresses: [email protected], [email protected] (A. Streitberger). 1760-2734/$ - see front matter ª 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jvc.2013.05.001
182
A. Streitberger et al. nPTT can be performed in a clinical setting. Normalized PTT values in healthy cats are comparable with those reported in healthy dogs. ª 2013 Elsevier B.V. All rights reserved.
Abbreviations CHF CO ECG MMVD FPRNA HCM mRR nPTT PBV PTT TA
TP
congestive heart failure cardiac output electrocardiogram myxomatous mitral valve disease first-pass radionuclide angiocardiography hypertrophic cardiomyopathy mean ReR interval normalized pulmonary transit time pulmonary blood volume pulmonary transit time displayed time at which the contrast agent is first visible within the left atrium displayed time at which the contrast agent is passing the pulmonic valve
Introduction Pulmonary transit time (PTT) is the time for a sample of blood to pass through the pulmonary circulation. It depends on pulmonary blood volume (PBV) and cardiac output (CO) according to the formula PTT ¼ PBV/CO. Usually it is normalized to the heart rate (nPTT ¼ PTT/mRR; nPTT ¼ normalized PTT, mRR ¼ mean ReR interval). Thus, nPTT represents the number of stroke volumes that are necessary to drive a sample of blood from the pulmonic valve to the left atrium.1,2 Most studies on nPTT in humans and animals have used first-pass radionuclide angiography (FPRNA).1,3,4 It has been shown in humans that nPTT is independent of body size and loading conditions,5e7 and that it changes only minimally with exercise.3,8,9 In dogs, significantly elevated nPTT was observed with compensated myxomatous mitral valve disease (MMVD), and substantial lengthening of nPTT was found in congestive heart failure. These increases in nPTT were due to increased pulmonary blood volume rather than decreased forward cardiac output.1,10,11
Recently, nPTT was measured in dogs using second-generation ultrasound contrast media.12 a,b Two of these studies used Sonovue as the contrast agent.a,b,c The interobserver and within-day variability of PTT and nPTT measurements were within reasonable limits for clinical use, and nPTT was not influenced by age, body weight, gender or heart rate.c,d The contrast agent Sonovue is based on stabilized sulfur hexafluoride (SF6) microbubbles surrounded by a phospholipid shell. Sulfur hexafluoride is an inorganic, colorless and odorless gas with a low solubility in blood. The agent is supplied as a lyophilisate, which is mixed with saline to form a white suspension. Because 90% of the bubbles are smaller than 8 mm, they are able to access the entire pulmonary blood volume, which makes Sonovue highly suitable for measuring PTT. In human medicine, Sonovue has been shown to be safe in both abdominal and cardiac applications. In a study with a sample size of 23,188 procedures, the severe adverse event rate was 0.0086%.13 Diagnosis and staging of feline cardiomyopathies, as well as risk assessment in affected cats, are currently based on thoracic radiographs, echocardiography and biomarkers. Echocardiographic diagnosis and evaluation of left ventricular systolic and diastolic function are mainly estimated by measuring anatomic dimensions and performing Doppler recordings, which are usually sensitive to changes in loading conditions. Even though left atrial enlargement is easy to recognize on ultrasound and is associated with reduced survival time,14,15 a considerable overlap of left atrial size between cats with CHF and asymptomatic cats has
a
Kresken JG, Wendt RT, Ha ¨ggstro ¨m J. Echocardiographical estimate of pulmonary transit time (PTT, nPTT) in dogs using the echocardiographic contrast media Sonovue. Proceedings, 19th ECVIM-CA Congress, 2009, Porto, Portugal. b Wendt RT, Kresken JG, Ha ¨ggstro ¨m J. Repeatability and interobserver variability of pulmonary transit time (PTT, nPTT) in dogs using the echocardiographic contrast media Sonovue. Proceedings, 20th ECVIM-CA Congress, 2010, Toulouse, Frankreich. c Sonovue, Bracco Diagnostics, Milano, Italy. d Esaote MyLab 70, Milano, Italy.
Pulmonary transit time in cats been reported.14 Therefore, nPTT would assist with staging by adding information about pulmonary blood volume and cardiac output. It might be useful for assessing the risk of congestive heart failure and for evaluating the effects of therapeutic drugs. And finally, it may offer insights into the pathophysiology of feline cardiomyopathies. The aims of the present study were to assess the feasibility of PTT and nPTT measurements in healthy cats by ultrasound using the contrast agent Sonovue and to describe nPTT values in a population of healthy cats. Methodologically, it was intended to assess the interobserver variability, within-day variability of individual observers, repeatability of nPTT measurement and the influence of the experience level of the observer with related techniques.
183 was not used because this was not feasible for the ultrasound probe being used. A timer module with a resolution of 1/100 s was started, and a bolus of 0.0023e0.0032 mg/kg of contrast agent microbubbles (Sonovue) was injected into the cephalic vein through a venous catheter, while the ultrasound data was recorded as a digital loop. Lower dosages (0.0009 mg/kg, 0.00135 mg/kg and 0.0018 mg/kg of sulfur hexafluoride) were tried in the first 3 cats but did not yield sufficient echocontrast and the recordings were repeated using the higher dosage. Each cat underwent this procedure once. In addition, in 5 cats a contrast study was performed 5 times each, in the course of one day, for evaluation of within-day variability.
Measurement of nPTT
Animals, materials and methods The study was designed as a prospective data analysis. It was approved by the ethics committee of the regional government of Upper Austria and by the animal welfare commission of the University of Veterinary Medicine, Vienna.
Study population The study population consisted of 42 cats that were presented for routine cardiac examination. The median age was 3 years (range: 0.5e11.0) and the median body weight was 3.95 kg (range: 2.3e6.5). Seventeen cats were male and 25 were female. A complete clinical examination was performed on all the cats. Cats were included if they were clinically normal or had only minimal clinical disease (e.g. dental calculus formation was considered minimal). Patients with heart murmurs were excluded, regardless of the origin of the heart murmur. No blood samples or thoracic radiographs were taken. All cats underwent a complete echocardiographic examination with concomitant ECG recording and were determined to be free of cardiac disease.
Contrast study procedure All contrast studies were performed by the same examiner (PM) with the cats positioned in right lateral recumbency. No sedation was used. A right parasternal short axis view of the heart base was displayed using a 7.5e10.0 MHz phased array probe and the ECG was recorded simultaneously.d Frame rates ranged between 120 and 140 frames per seconds. A low mechanical index software
The measurements of PTT were done by offline analysis of the ultrasound recordings, by 3 veterinarians with different levels of experience in echocardiographic diagnostics (low, medium, high), independently of each other. “Low” corresponded to a novice in cardiology, the “medium”experienced observer was an assistant veterinarian in cardiology and the “high” level of experience was a certified cardiologist. The first individual frame on which the contrast agent was initially visible just behind the pulmonic valve was selected (Fig. 1). The corresponding time (TP) displayed on the timer was recorded. The clip was then carefully reviewed and the frame on which the contrast media was first visible inside the left atrium was identified (Fig. 2). The time was again recorded (TA). The pulmonary transit time was then calculated (TA-TP). To generate the nPTT value, the PTT was divided by the mean ReR interval (nPTT ¼ PTT/mRR). The mean ReR interval was calculated from the heart rates that were measured by the ECG during the contrast studies.
Data analysis Data were analyzed using commercial statistics software (SPSS and MedCalc).e,f For all statistical tests used, values of P < 0.05 were considered statistically significant. To test for a significant effect of the observer’s experience on nPTT values, Bland-Altman plots
e f
SPSS Statistics 21 Demo Version, IBM, US. MedCalc Software, Ostend, Belgium.
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A. Streitberger et al. coefficients of variation. The median coefficient of variation for each observer across the 5 cats was used to estimate the within-day variability for each observer. To identify potential effects of age and body weight on nPTT, correlation analysis was used. Univariate and multivariate analysis were performed to determine whether nPTT could be reliably predicted by age and/or body weight using a linear regression model. A potential influence of sex on nPTT values was assessed by means of a two-sample t-Test.
Results Figure 1 Echocardiographic image when the contrast agent has just passed the pulmonic valve and is visible within the right ventricular outflow tract and the main pulmonary artery.
were generated. Interobserver variability was described on the basis of the three different nPTT values measured by the three different observers on each cat. Coefficients of variation were computed for each cat; these 42 coefficients of variation were then presented as median and interquartile ranges. The averages of the 3 nPTT values of each cat were then used to describe the nPTT values in our study population. They were tested for normal distribution using a KolmogoroveSmirnov test and presented as mean and standard deviation. The within-day variability of the nPTT-results was calculated based on the 5 cats in each of which 5 repeat contrast studies had been performed on one day. Each observer generated 5 nPTT values per cat (one for each contrast study), which were then used to calculate
Quality of contrast studies and tolerability Using the dosage of Sonovue mentioned above, good image quality was achieved in all cats. All cats tolerated the injection of Sonovue very well; neither ECG changes nor adverse reactions were observed.
Reproducibility and nPTT values BlandeAltman analysis did not reveal any significant influence of the observer’s experience on nPTT measurements (Fig. 3A, B, C). The median inter-observer variability across our population was 6.8% with an interquartile range of 4.3%e 12.3%. The calculated values of nPTT in this population were 4.12 1.0 (mean SD). The median within-day variability for the observer with high, medium and low experience was 13.1%, 12.7% and 13%, respectively (average 12.9%).
Impact of age, weight, and sex on PTT No correlation between age and nPTT (r ¼ 0.24; P ¼ 0.12) or between body weight and nPTT (r ¼ 0.1; P ¼ 0.54) could be identified. nPTT values could not be reliably predicted by age (R2 ¼ 0.058), body weight (R2 ¼ 0.01) or both (R2 ¼ 0.1) using a linear regression model. There was no significant difference in nPTT between male and female cats.
Discussion
Figure 2 Echocardiographic image showing the contrast agent reaching the left atrium.
This study demonstrates that PTT/nPTT measurement is feasible in cats using transthoracic echocardiography and the blood pool contrast agent Sonovue.
Pulmonary transit time in cats
Figure 3 Bland-Altman-Plots showing non-significant effects of observer’s experience on nPTT measurements. (Low, A; Medium, B; and High, C refer to the observer’s level of experience.)
The dosage of Sonovue needed to achieve good images was 0.0023e0.0032 mg/kg. This is higher than previously reported in dogs (0.0014e0.0023 mg/kg).a The reason for this difference could be the injection technique. In the study on dogs, the
185 injection of contrast agent was followed by an injection of saline.a This was considered not to be practical in cats since the contrast agent reached the pulmonary artery within 1e2 s, so there was not enough time to exchange the syringes. A double-injection technique could probably be achieved using a three-way stopcock with an extension set. Another possible explanation is that unlike in previous studies in dogs,a,b we did not use a special low mechanical index software, since this was not available for a high frequency probe. Still, the dosage we used was not higher than the maximum dosage used in human medicine (0.0014e0.0032 mg/kg),13,16,17 g and it was lower than the dosage of Sonovue that was previously reported for contrast-enhanced abdominal ultrasound in cats.18 However, the use of low mechanical index software in combination with high frequency probes and a double-injection technique remain as potential ways of reducing the dosage and might be worth following up. Adverse events after the injection of Sonovue were not observed in any cat of our study population using clinical observation, heart ultrasound or concomitant ECG recordings. This agrees with a recent retrospective study, which did not find any immediate or delayed adverse events in 77 cats, 43 of which were clinically ill.19 In that study, 98.1% of the examinations required more than one bolus injection of the contrast agent. No effect of contrast ultrasound on mortality of clinically ill cats was observed. The authors concluded that contrast-enhanced ultrasound is a safe method in clinically ill dogs and cats. In a human study with 23,188 procedures, the overall reporting rate of serious adverse events was 0.0086%.13 These serious events were related to the respiratory system (bronchospasm). Other mild and infrequent sideeffects observed in humans include mild hypotension, sinus tachycardia, leukocytosis, headache, vomiting, supraventricular tachycardia, tingling sensation in a hand, facial flushing and single ventricular extrasystoles.17,20e23 Sonovue was also well tolerated in patients with severe heart failure and pulmonary hypertension.20 Chronic toxicological studies were performed with Sonovue at doses up to 0.225 mg/kg in rats and monkeys, and no specific event or safety issues were observed.16 Certainly, we might have missed minor side effects or major effects that did not appear clinically. We did not perform blood
g Personal data, Dr. Med. Univ. Ralph Stephan von Bardeleben, University of Mainz, Germany.
186 pressure measurements after the contrast passage, nor did we take blood samples for troponin measurements, CBC or blood chemistry. Thus, further prospective studies assessing the safety of Sonovue in cats are necessary. There was no significant influence of the observers’ experience on the nPTT measuring results. This suggests that the procedure is simple and robust and does not need to be limited to veterinarians with a high level of experience in echocardiography. The degree of inter-observer variability we observed was comparable to interobserver variability reported generally for echocardiographic measurements24,25 and must be seen in the context of some technical limitations. The large sector angle used to simultaneously display the pulmonary artery and the left atrium leads to a decrease in sampling rates, which may influence the inter-observer and within-day variability. A lower frame rate can make it easier for the observers to choose the same frame for TP and TA, but the selection of different frames would lead to larger differences in the time reading. In this case it was not feasible to evaluate the influence of frame rate on the variability. Also, since all contrast study recordings were performed by the same observer, the effect of examiner-dependent technical adjustments and views may be underestimated. The calculated median within-day variability are comparable to those reported for other echocardiographic indices of cardiac function.26 Of course these data have to be interpreted with caution since only 5 cats were used for this analysis. The within-day variation could be caused by technical influences (different image settings, different image quality, slight differences in the imaging plane) or by some biological variability (potential physiological changes in pulmonary transit times). The effect of a specific interobserver and within-day variability on the usefulness of a clinical test can only be accurately assessed if the purpose of the test is clearly defined and differences between values in normal and abnormal individuals are known. In the present study, we collected data only from healthy individuals. However, data reported for dogs with compensated and decompensated degenerative mitral valve disease indicate a relatively small impact of interobserver and within-day variability on nPTT measurements.1,3 This is the first report on the measurement of pulmonary transit times in cats. The nPTT values we observed in our study population (4.12 1.0; mean SD) were comparable with those reported
A. Streitberger et al. in dogs using radionuclide angiography (4.6 0.6; mean SD)3 or contrast echocardiography (4.3 0.2; mean SD).a The evaluation of physiological indicators like PTT/nPTT can potentially provide additional insight into the pathophysiology and progression of feline cardiomyopathies, especially in terms of cardiac performance and/or pulmonary blood volume. Results using this technique may assist with understanding the progression of this heterogeneous group of cardiac diseases. Since the use of the contrast agent and ultrasound is easily applicable in clinical practice, it may have the potential to become an additional tool for the evaluation of feline cardiac patients with special regard to prognosis and risk assessment. It may also be useful for understanding the effects of pharmacologic treatment on cardiac function in cats. Further studies are necessary to compare these results to those measured in cats in different stages of diastolic dysfunction and also to nPTT values of cats in congestive heart failure.
Conclusions It is possible to measure nPTT in cats under clinical conditions using the ultrasound contrast media Sonovue. The values in our study population are comparable to those published in dogs.
Conflict of interest statement The authors have no conflict of interest.
Acknowledgments The authors would like to thank Dr. med. vet. Jan Gerd Kresken and Dr. med. vet. Ralph Wendt for their support, and interesting discussions and Robert O’Brien, DVM, DACVR, for his help with our manuscript.
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