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Heart rate and heart rate variability in pregnant warmblood and Shetland mares as well as their fetuses
Animal Reproduction Science 127 (2011) 183–187
Contents lists available at SciVerse ScienceDirect
Animal Reproduction Science journal homepage: www.elsevier.com/locate/anireprosci
Heart rate and heart rate variability in pregnant warmblood and Shetland mares as well as their fetuses Christina Nagel a,∗ , Jörg Aurich b , Franziska Palm b , Christine Aurich c a b c
Section for Artificial Insemination and Embryo Transfer, University of Veterinary Science, 1210 Vienna, Austria Section for Obstetrics, Gynecology and Andrology, University of Veterinary Science, 1210 Vienna, Austria Graf Lehndorff Institute for Equine Science, 16845 Neustadt (Dosse), Germany
a r t i c l e
i n f o
Article history: Received 22 December 2010 Received in revised form 19 July 2011 Accepted 23 July 2011 Available online 22 August 2011 Keywords: Horse Size Fetus Heart rate Heart rate variability
a b s t r a c t Heart rate (HR) is an important parameter of fetal well-being. In horses, HR and heart rate variability (HRV) can be determined by fetomaternal electrocardiography (ECG) from midpregnancy to foaling. Normal values for physiological parameters in larger breeds are often used as reference values in ponies. However, HR increases with decreasing size of the animal and in ponies is higher than in warmblood horses. It is not known if fetal HR is affected by breed and if values obtained in larger breeds can be used to assess Shetland fetuses. We have determined fetomaternal beat-to-beat (RR) interval (inversely correlated to HR) and HRV in warmblood (n = 6) and Shetland pregnancies (n = 7) at days 280 and 300 of gestation by ECG. Maternal RR interval was lower in pony than in warmblood mares (day 280: Shetland: 958 ± 110, warmblood: 1489 ± 126 ms, p < 0.01) The SDRR (standard deviation of RR interval) and the RMSSD (root mean square of successive RR differences) did not differ between breeds at any time. Also RR interval as well as HRV did not differ between warmblood and pony fetuses (RR interval day 280: Shetland: 606 ± 39, warmblood: 589 ± 38 ms). In conclusion, although maternal RR interval is clearly higher in Shetland than in warmblood mares, fetal RR interval in the two breeds is on the same level. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Fetal monitoring in late pregnant horse mares is mostly based on transabdominal and transrectal ultrasonography (Adams-Brendemuehl and Pipers, 1987; Curran and Ginther, 1995; Reef et al., 1995, 1996; Renaudin et al., 1997; Bucca et al., 2005). The benefit of ultrasonography is the visualisation of fetal membranes, fetal fluid and the fetal heart. However, in small breeds such as Shetland ponies, transrectal examination is limited by physical size of the mare. Furthermore ultrasonographic examination is hin-
∗ Corresponding author. Tel.: +43 1 25077 6408; fax: +43 1 25077 5490. E-mail addresses: [email protected] (C. Nagel), [email protected] (J. Aurich), [email protected] (F. Palm), [email protected] (C. Aurich). 0378-4320/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.anireprosci.2011.07.021
dered by movements of the mare and the fetus and thus evaluation over longer time periods may be difficult. Fetal cardiac action usually can only be judged by visual assessment. Fetal heart rate (HR) is an important parameter of fetal well-being (Adams-Brendemuehl and Pipers, 1987; Reef et al., 1995, 1996). For the fetus, the only way to react to reductions in placental oxygen supply, which is common in mares suffering from placentitis, is a reduction of cardiac action and decrease in heart rate. Also a lack of episodic heart rate increases can be seen. Subsequently, decompensation through loss of central regulatory mechanisms leads to persistent tachycardia and finally bradycardia and cardiac arrest (Bocking, 2003; Manning, 2002). Cardiac action can be recorded as the interval between the R-spikes of successive QRS complexes (RR interval). Heart rate variability (HRV), i.e. short-term fluctuations in heart rate, reflects the balance of sympathetic and parasympathetic
184
C. Nagel et al. / Animal Reproduction Science 127 (2011) 183–187
tone and has been used to assess the stress response of the autonomic nervous system in adult horses (Schmidt et al., 2010a,b). Decreases in the values of the HRV variables standard deviation of beat-to-beat (RR) interval (SDRR) and root mean square of successive RR differences (RMSSD) reflect a shift towards more sympathetic dominance, while increased values indicate a shift towards parasympathetic dominance (Von Borell et al., 2007). In a recent study, we have reported normal values for fetal heart rate and heart rate variability from day 170 of gestation to parturition in healthy warmblood sport horse mares with singleton pregnancies (Nagel et al., 2010). However, HR increases with decreasing size of the animal and in adult ponies is higher than in adult warmblood horses (Speirs, 1997). To apply the technique of fetomaternal electrocardiography in small breeds such as Shetland ponies it has to be determined whether not only maternal but also fetal HR and HRV is affected by breed size and if normal values obtained in larger breeds can be used to assess pony fetuses. The aim of this study was to determine, if fetal HR and HRV values obtained in warmblood horses can be transferred to smaller breeds and are thus valid for the assessment of fetal well-being in ponies. For this reason we compared warmblood sport horse mares and their fetuses and Shetland pony mares and their fetuses at two different time points during late pregnancy. 2. Materials and methods 2.1. Animals Two groups of mares with singleton pregnancies were available for this study. One group consisted of warmblood mares (n = 6) and the other of Shetland mares (n = 7). The warmblood mares were between eight and 20 years of age (10.8 ± 4.7 years). Housing, feeding and management were as described (Nagel et al., 2010). The Shetland mares were between 3 and 17 years of age (10.3 ± 4.4 years). They were housed in group stables on straw and had access to an outdoor paddock for several hours per day. Shetland mares were fed hay twice daily. Mineral supplements and water were freely available at all times. 2.2. Experimental design Fetomaternal ECG recordings in warmblood mares were made repeatedly from day 270 until parturition as described, including days 280 and 300 of pregnancy (Nagel et al., 2010). Recordings were made twice daily for 1 h, starting at 6 a.m. and 4 p.m., respectively. In the Shetland mares, ECG recordings were obtained on days 280 and 300 of pregnancy. Recordings started at noon and lasted for 1 h. 2.3. Fetomaternal electrocardiography In both groups of mares, fetomaternal ECG measurements were made with the Televet 100 recording system (Version 4.1.3, Kruuse, Marslev, Denmark) as described (Nagel et al., 2010). The ECGs were recorded from the abdominal wall of the mare. This ECG device uses a filter
allowing to display and analyse maternal and fetal cardiac action both combined and separately and to amplify the fetal signal for evaluation. The Televet 100 was fixed onto an elastic girth around the thorax of the mare and the electrodes were put on the horses’ coat with self-adhesive pads. For all measurements the green electrode (left leg) was placed on the left side of the neck. The yellow electrode (left arm) was in the left flank region approximately at the height of the hip. The black electrode (neutral) was put on the mares’ left croup. The red electrode (right arm), was positioned at the right side of the animal’s abdomen at the height of the knee. In both groups the same positions for electrodes were used. During the recordings, mares remained in their normal surroundings. 2.4. Heart rate variability analysis For HRV analysis the Kubios HRV Software (Biomedical Signal Analysis Group, Department of Applied Physics, University of Kuopio, Finland) was used. From all recordings the first 5-min time interval was used for the determination of HR and HRV variables. To remove trend components, data were detrended and, in addition, an artefact correction was made as in previous studies on horses (Schmidt et al., 2010a,b) following established procedures (Tarvainen, 2002; Tarvainen and Niskanen et al., 2008). From the recorded RR intervals, the HRV variables SDRR (standard deviation of the RR interval) and RMSSD (root mean square of successive RR differences) were calculated for mares and fetuses. In the warmblood mares, two recordings per day were made. Because we found no influence of daytime on HRV parameters (Nagel et al., 2010), the mean of the day was used for further comparisons. Therefore, in the Shetland mares ECG recordings were made once per day and these data were used for analysis. 2.5. Heart rate accelerations und decelerations In addition to HRV variables, transient accelerations and decelerations in fetal HR were calculated. As described (Nagel et al., 2010) all RR intervals deviating from the mean RR interval by more than two standard deviations were marked. An acceleration or deceleration was defined as at least 5 successive RR intervals higher or lower than twice the standard deviation of all RR intervals for a given recording time of 60 min (Colles et al., 1978). All acceleration and deceleration periods were counted and calculated as events per 1 h. The length of these periods was also noted and expressed as number of beats. RR intervals differing by more than two standard deviations from the mean RR interval and separated by only one RR interval not meeting these criteria from an acceleration and deceleration were included for calculation of the length of accelerations and decelerations. 2.6. Statistical analysis Statistical analysis was made with the PASW statistics package (SPSS, Chicago, IL, USA). As not all data
C. Nagel et al. / Animal Reproduction Science 127 (2011) 183–187
(a) RR intervall (msec)
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Fig. 1. RR interval and heart rate variability parameters in warmblood mares (n = 6) and their fetuses and Shetland pony mares (n = 7) and their fetuses on days 280 and 300 of gestation. (a) RR interval, (b) standard deviation of RR interval, (c) root mean square of successive RR interval differences. a, b, c: values with the same superscript letter differ significantly (p < 0.05) and (d): p = 0.063.
were equally distributed, non-parametric tests were used throughout. Comparisons between groups were made by Mann–Whitney-U-test. Comparisons between data obtained from the same animals at days 280 and 300 of pregnancy were compared within groups by Wilcoxon test. A p-value <0.05 was considered significant. All data given are means ± standard deviation (SD).
3. Results The comparison of Shetland and warmblood mares on days 280 and 300 of pregnancy shows that the RR interval in the Shetland mares was significantly lower than in warmblood mares (day 280: Shetland: 958 ± 110 ms, warmblood: 1489 ± 126 ms, p < 0.01, Fig. 1a). The SDRR and RMSSD showed no difference between these two breeds on day 280 as well as on day 300 of gestation (Fig. 1b and c). Fetuses of Shetland mares and warmblood mares showed no significant differences in RR interval on days 280 and 300 of pregnancy (day 280: Shetland fetus: 606 ± 39 ms, warmblood fetus: 589 ± 38 ms, Fig. 1a). There was also no difference in SDRR and RMSSD on days 280 and 300 of gestation (day 280: SDRR Shetland fetus: 11.4 ± 6.9 ms, warmblood fetus: 11.9 ± 2.1 ms; RMSSD Shetland fetus: 13.7 ± 7.6 ms, warmblood fetus: 14.5 ± 3.7 ms, Fig. 1b and c). Comparing days 280 and 300 of pregnancy, maternal RR interval and HRV variables neither in Shetland nor in warmblood mares differed between times. Fetal RR interval increased slightly between days 280 and 300. This increase was significant in warmblood fetuses (from 589 ± 38 to
639 ± 54 ms, p < 0.05) and nearly reached statistical significance in Shetland fetuses (from 606 ± 39 to 663 ± 95 ms, p = 0.063). In the fetuses, there were no differences in HRV variables between days 280 and 300 (Fig. 1a–c). The number of accelerations in heart rate per hour did not differ between fetuses of the two breeds studied and also was not different between days 280 and 300 of gestation (day 280: Shetland fetuses: 25 ± 20 acceleration/h, warmblood fetuses: 19 ± 15 accelerations/h, Fig. 2a). The duration of heart rate accelerations was significantly longer in Shetland fetuses on day 300 of pregnancy (Shetland fetuses: 48 ± 20 heartbeats, warmblood fetuses: 29 ± 9 heartbeats; p < 0.05) but did not show differences between the two breeds on day 280 (Fig. 2b). Also the number and the duration of heart rate decelerations showed no differences between days 280 and 300 of pregnancy and did not differ between Shetland fetuses and warmblood fetuses (day 280: Shetland fetuses: 26 ± 11 decelerations/h with a duration of 38 ± 21 heartbeats, warmblood fetuses: 15 ± 7 decelerations/h with a duration of 52 ± 26 heartbeats; Fig. 2c and d).
4. Discussion Equine fetal electrocardiography was introduced into veterinary medicine nearly 100 years ago (Nörr, 1921) and its use for monitoring of pregnancy has been reported occasionally since (Parkes and Colles, 1977; Buss et al., 1980). Only recently, modern ECG recorders have become available which allow reliable fetomaternal ECG record-
C. Nagel et al. / Animal Reproduction Science 127 (2011) 183–187
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Fig. 2. Number of accelerations in heart rate per hour (n/h), duration of accelerations (heartbeats), number of decelerations per hour (n/h) and duration of decelerations (heartbeats) in warmblood (n = 6) and Shetland (n = 7) fetuses on days 280 and 300; a: values with the same superscript letter differ significantly (p < 0.05).
ings from approximately day 170 of gestation until foaling (Nagel et al., 2010). Although fetal ECGs in horses have been obtained for some time already, there is still little information on physiological heart rate values in equine fetuses and in the mare during ongoing gestation. The available data have been obtained from warmblood horses or other full-size breeds. However, mares with pregnancies at risk from various breeds, including small ponies are presented for veterinary diagnostics. Physiological heart rate usually increases with decreasing size of the animal and thus is higher in adult ponies than in adult warmblood horses (Speirs, 1997). As could be expected, this finding was confirmed by comparing maternal heart rate in warmblood and Shetland mares in our study. In contrast to maternal heart rate, fetal heart rate did not differ between the breeds investigated. Also the evaluation of the accelerations and decelerations of heart rate per hour did not reveal differences between the two horse breeds. Thus, the normal values determined in a previous study for the warmblood fetus (Nagel et al., 2010) can be transferred to a much smaller breed such as the Shetland pony. With regard to gestation, Shetland ponies and warmblood horses differ primarily in fetal size and not in the physiology of pregnancy. However, confounding breed differences independent from size cannot not totally be excluded. We suggest that fetal size–at least for the time window studied – is without major effects on heart rate in the equine fetus. The decrease in fetal heart rate (i.e. increase in RR interval) with ongoing pregnancy (Nagel et al., 2010) was confirmed in the current study. It does not depend on breed or breed size but reflects primarily maturation of regulatory pathways such as the fetal autonomous nervous system.
While fetal heart rate determination in horses has been performed for a long time, maternal and fetal heart rate variability have been reported only recently (Nagel et al., 2010). Decreasing heart rate variability in adult horses has been associated with stress (Schmidt et al., 2010a,b). A decrease in HRV indicates increased sympathetic activity, decreased parasympathic activity or a combination of both (Von Borell et al., 2007). However no differences in HRV variables between the two breeds studied were found. There is thus no evidence for consistent differences in HRV between horse breeds in late pregnancy. Fetuses of Shetland and warmblood mares show no significant differences in heart rate variability indicating that not only heart rate but also heart rate variability data obtained in physiological pregnancies of a larger horse breed such as the warmblood are also largely valid for Shetland ponies. However, because transient heart rate accelerations at day 300 of gestation in Shetland ponies were of longer duration than in warmblood fetuses minor differences in HRV may exist. Because we did not compare heart rate and HRV between warmblood and Shetland mares during parturition, differences between breeds at foaling or in the immediate prepartum period cannot be excluded.
5. Conclusions Heart rate and heart rate variability values determined in healthy warmblood fetuses can be used as reference values for a much smaller breed, the Shetland pony. Gestational age has to be taken into account because heart rate and heart rate variability changes with ongoing maturation of the autonomous nervous system in the fetus.
C. Nagel et al. / Animal Reproduction Science 127 (2011) 183–187
Acknowledgement The authors are grateful to Klaus Engel, Rösch and Associates, Frankfurt (Main), Germany for continuous technical support. References Adams-Brendemuehl, C., Pipers, F.S., 1987. Antepartum evaluations of the equine fetus. J. Reprod. Fertil. Suppl. 35, 565–573. Bocking, A.D., 2003. Assessment of fetal heart rate and fetal movements in detecting oxygen deprivation in-utero. Eur. J. Obstet. Gynecol. Reprod. Biol. 110 (Suppl. 1), 108–112. Bucca, S., Fogarty, U., Collins, A., Small, V., 2005. Assessment of fetoplacental well-being in the mare from mid-gestation to term: transrectal and transabdominal ultrasonographic features. Theriogenology 64, 542–557. Buss, D.D., Asbury, A.C., Chevalier, L., 1980. Limitations in equine fetal electrocardiography. J. Am. Vet. Med. Assoc. 177, 174–176. Colles, C.M., Parkes, R.D., May, C.J., 1978. Foetal electrocardiography in the mare. Equine Vet. J. 10, 32–37. Curran, S., Ginther, O.J., 1995. M-mode ultrasonic assessment of equine fetal heart rate. Theriogenology 44, 609–617. Manning, F.A., 2002. Fetal biophysical profile: a critical appraisal. Clin. Obstet. Gynecol. 45, 975–985. Nagel, C., Aurich, J., Aurich, C., 2010. Determination of heart rate and heart rate variability in the equine fetus by fetomaternal electrocardiography. Theriogenology 73, 973–983. Nörr, J., 1921. Fötale Elektrokardiogramme vom Pferd. Z. Biol. 73, 123–128. Parkes, R.D., Colles, C.M., 1977. Fetal electrocardiography in the mare as a practical aid to diagnosing singleton and twin pregnancy. Vet. Rec. 100, 25–26.
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Reef, V.B., Vaala, W.E., Worth, L.T., Spencer, P.A., Hammett, B., 1995. Ultrasonographic evaluation of the fetus and intrauterine environment in healthy mares during late gestation. Vet. Radiol. Ultrasound. 36, 533–541. Reef, V.B., Vaala, W.E., Worth, L.T., Sertich, P.L., Spencer, P.A., 1996. Ultrasonographic assessment of fetal well-being during late gestation—development of an equine biophysical profile. Equine Vet. J. 28, 200–208. Renaudin, C.D., Troedsson, M.H., Gillis, C.L., King, V.L., Bodena, A., 1997. Ultrasonographic evaluation of the equine placenta by transrectal and transabdominal approach in the normal pregnant mare. Theriogenology 47, 559–573. Schmidt, A., Biau, S., Möstl, E., Becker-Birck, M., Morillon, B., Aurich, J., Faure, J.M., Aurich, C., 2010a. Changes in cortisol release and heart rate variability in sport horses during long-distance road transport. Domest. Anim. Endocrinol. 38, 179–189. Schmidt, A., Möstl, E., Wehnert, C., Aurich, J., Müller, J., Aurich, C., 2010b. Cortisol release and heart rate variability in horses during road transport. Horm. Behav. 57, 209–215. Speirs, V.C.,1997. The cardiovascular system. In: Clinical Examination of the Horse. Saunders, Philadelphia, London, Toronto, pp. 173–196. Tarvainen, M.P., Ranta-Aho, P.O., Karjalainen, P.A., 2002. An advanced detrending method with application to HRV analysis. IEEE Trans. Biomed. Eng. 49, 172–175. Tarvainen, M.P., Niskanen, J.P., Kubios, 2008. HRV Version 2.0 User’s Guide. Department of Physics, University of Kuopio, Kuopio, Finland. Von Borell, E., Langbein, J., Despres, G., Hansen, S., Leterrier, C., MarchandForde, J., Marchand-Fonde, R., Minero, M., Mohr, E., Prunier, A., Valance, D., Veissier, I., 2007. Heart rate variability as a measure of autonomic regulation of cardiac activity for assessing stress and welfare in farm animals—a review. Physiol. Behav. 92, 293–316.
Contents lists available at SciVerse ScienceDirect
Animal Reproduction Science journal homepage: www.elsevier.com/locate/anireprosci
Heart rate and heart rate variability in pregnant warmblood and Shetland mares as well as their fetuses Christina Nagel a,∗ , Jörg Aurich b , Franziska Palm b , Christine Aurich c a b c
Section for Artificial Insemination and Embryo Transfer, University of Veterinary Science, 1210 Vienna, Austria Section for Obstetrics, Gynecology and Andrology, University of Veterinary Science, 1210 Vienna, Austria Graf Lehndorff Institute for Equine Science, 16845 Neustadt (Dosse), Germany
a r t i c l e
i n f o
Article history: Received 22 December 2010 Received in revised form 19 July 2011 Accepted 23 July 2011 Available online 22 August 2011 Keywords: Horse Size Fetus Heart rate Heart rate variability
a b s t r a c t Heart rate (HR) is an important parameter of fetal well-being. In horses, HR and heart rate variability (HRV) can be determined by fetomaternal electrocardiography (ECG) from midpregnancy to foaling. Normal values for physiological parameters in larger breeds are often used as reference values in ponies. However, HR increases with decreasing size of the animal and in ponies is higher than in warmblood horses. It is not known if fetal HR is affected by breed and if values obtained in larger breeds can be used to assess Shetland fetuses. We have determined fetomaternal beat-to-beat (RR) interval (inversely correlated to HR) and HRV in warmblood (n = 6) and Shetland pregnancies (n = 7) at days 280 and 300 of gestation by ECG. Maternal RR interval was lower in pony than in warmblood mares (day 280: Shetland: 958 ± 110, warmblood: 1489 ± 126 ms, p < 0.01) The SDRR (standard deviation of RR interval) and the RMSSD (root mean square of successive RR differences) did not differ between breeds at any time. Also RR interval as well as HRV did not differ between warmblood and pony fetuses (RR interval day 280: Shetland: 606 ± 39, warmblood: 589 ± 38 ms). In conclusion, although maternal RR interval is clearly higher in Shetland than in warmblood mares, fetal RR interval in the two breeds is on the same level. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Fetal monitoring in late pregnant horse mares is mostly based on transabdominal and transrectal ultrasonography (Adams-Brendemuehl and Pipers, 1987; Curran and Ginther, 1995; Reef et al., 1995, 1996; Renaudin et al., 1997; Bucca et al., 2005). The benefit of ultrasonography is the visualisation of fetal membranes, fetal fluid and the fetal heart. However, in small breeds such as Shetland ponies, transrectal examination is limited by physical size of the mare. Furthermore ultrasonographic examination is hin-
∗ Corresponding author. Tel.: +43 1 25077 6408; fax: +43 1 25077 5490. E-mail addresses: [email protected] (C. Nagel), [email protected] (J. Aurich), [email protected] (F. Palm), [email protected] (C. Aurich). 0378-4320/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.anireprosci.2011.07.021
dered by movements of the mare and the fetus and thus evaluation over longer time periods may be difficult. Fetal cardiac action usually can only be judged by visual assessment. Fetal heart rate (HR) is an important parameter of fetal well-being (Adams-Brendemuehl and Pipers, 1987; Reef et al., 1995, 1996). For the fetus, the only way to react to reductions in placental oxygen supply, which is common in mares suffering from placentitis, is a reduction of cardiac action and decrease in heart rate. Also a lack of episodic heart rate increases can be seen. Subsequently, decompensation through loss of central regulatory mechanisms leads to persistent tachycardia and finally bradycardia and cardiac arrest (Bocking, 2003; Manning, 2002). Cardiac action can be recorded as the interval between the R-spikes of successive QRS complexes (RR interval). Heart rate variability (HRV), i.e. short-term fluctuations in heart rate, reflects the balance of sympathetic and parasympathetic
184
C. Nagel et al. / Animal Reproduction Science 127 (2011) 183–187
tone and has been used to assess the stress response of the autonomic nervous system in adult horses (Schmidt et al., 2010a,b). Decreases in the values of the HRV variables standard deviation of beat-to-beat (RR) interval (SDRR) and root mean square of successive RR differences (RMSSD) reflect a shift towards more sympathetic dominance, while increased values indicate a shift towards parasympathetic dominance (Von Borell et al., 2007). In a recent study, we have reported normal values for fetal heart rate and heart rate variability from day 170 of gestation to parturition in healthy warmblood sport horse mares with singleton pregnancies (Nagel et al., 2010). However, HR increases with decreasing size of the animal and in adult ponies is higher than in adult warmblood horses (Speirs, 1997). To apply the technique of fetomaternal electrocardiography in small breeds such as Shetland ponies it has to be determined whether not only maternal but also fetal HR and HRV is affected by breed size and if normal values obtained in larger breeds can be used to assess pony fetuses. The aim of this study was to determine, if fetal HR and HRV values obtained in warmblood horses can be transferred to smaller breeds and are thus valid for the assessment of fetal well-being in ponies. For this reason we compared warmblood sport horse mares and their fetuses and Shetland pony mares and their fetuses at two different time points during late pregnancy. 2. Materials and methods 2.1. Animals Two groups of mares with singleton pregnancies were available for this study. One group consisted of warmblood mares (n = 6) and the other of Shetland mares (n = 7). The warmblood mares were between eight and 20 years of age (10.8 ± 4.7 years). Housing, feeding and management were as described (Nagel et al., 2010). The Shetland mares were between 3 and 17 years of age (10.3 ± 4.4 years). They were housed in group stables on straw and had access to an outdoor paddock for several hours per day. Shetland mares were fed hay twice daily. Mineral supplements and water were freely available at all times. 2.2. Experimental design Fetomaternal ECG recordings in warmblood mares were made repeatedly from day 270 until parturition as described, including days 280 and 300 of pregnancy (Nagel et al., 2010). Recordings were made twice daily for 1 h, starting at 6 a.m. and 4 p.m., respectively. In the Shetland mares, ECG recordings were obtained on days 280 and 300 of pregnancy. Recordings started at noon and lasted for 1 h. 2.3. Fetomaternal electrocardiography In both groups of mares, fetomaternal ECG measurements were made with the Televet 100 recording system (Version 4.1.3, Kruuse, Marslev, Denmark) as described (Nagel et al., 2010). The ECGs were recorded from the abdominal wall of the mare. This ECG device uses a filter
allowing to display and analyse maternal and fetal cardiac action both combined and separately and to amplify the fetal signal for evaluation. The Televet 100 was fixed onto an elastic girth around the thorax of the mare and the electrodes were put on the horses’ coat with self-adhesive pads. For all measurements the green electrode (left leg) was placed on the left side of the neck. The yellow electrode (left arm) was in the left flank region approximately at the height of the hip. The black electrode (neutral) was put on the mares’ left croup. The red electrode (right arm), was positioned at the right side of the animal’s abdomen at the height of the knee. In both groups the same positions for electrodes were used. During the recordings, mares remained in their normal surroundings. 2.4. Heart rate variability analysis For HRV analysis the Kubios HRV Software (Biomedical Signal Analysis Group, Department of Applied Physics, University of Kuopio, Finland) was used. From all recordings the first 5-min time interval was used for the determination of HR and HRV variables. To remove trend components, data were detrended and, in addition, an artefact correction was made as in previous studies on horses (Schmidt et al., 2010a,b) following established procedures (Tarvainen, 2002; Tarvainen and Niskanen et al., 2008). From the recorded RR intervals, the HRV variables SDRR (standard deviation of the RR interval) and RMSSD (root mean square of successive RR differences) were calculated for mares and fetuses. In the warmblood mares, two recordings per day were made. Because we found no influence of daytime on HRV parameters (Nagel et al., 2010), the mean of the day was used for further comparisons. Therefore, in the Shetland mares ECG recordings were made once per day and these data were used for analysis. 2.5. Heart rate accelerations und decelerations In addition to HRV variables, transient accelerations and decelerations in fetal HR were calculated. As described (Nagel et al., 2010) all RR intervals deviating from the mean RR interval by more than two standard deviations were marked. An acceleration or deceleration was defined as at least 5 successive RR intervals higher or lower than twice the standard deviation of all RR intervals for a given recording time of 60 min (Colles et al., 1978). All acceleration and deceleration periods were counted and calculated as events per 1 h. The length of these periods was also noted and expressed as number of beats. RR intervals differing by more than two standard deviations from the mean RR interval and separated by only one RR interval not meeting these criteria from an acceleration and deceleration were included for calculation of the length of accelerations and decelerations. 2.6. Statistical analysis Statistical analysis was made with the PASW statistics package (SPSS, Chicago, IL, USA). As not all data
C. Nagel et al. / Animal Reproduction Science 127 (2011) 183–187
(a) RR intervall (msec)
1800 1500
a
1200 900 600
c
a b
(d)
(d)
b
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185
50 25
300 0
0 280 300 Days of gestation
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280 300 Days of gestation
100 75 50 25 0
280 300 Days of gestation
Fig. 1. RR interval and heart rate variability parameters in warmblood mares (n = 6) and their fetuses and Shetland pony mares (n = 7) and their fetuses on days 280 and 300 of gestation. (a) RR interval, (b) standard deviation of RR interval, (c) root mean square of successive RR interval differences. a, b, c: values with the same superscript letter differ significantly (p < 0.05) and (d): p = 0.063.
were equally distributed, non-parametric tests were used throughout. Comparisons between groups were made by Mann–Whitney-U-test. Comparisons between data obtained from the same animals at days 280 and 300 of pregnancy were compared within groups by Wilcoxon test. A p-value <0.05 was considered significant. All data given are means ± standard deviation (SD).
3. Results The comparison of Shetland and warmblood mares on days 280 and 300 of pregnancy shows that the RR interval in the Shetland mares was significantly lower than in warmblood mares (day 280: Shetland: 958 ± 110 ms, warmblood: 1489 ± 126 ms, p < 0.01, Fig. 1a). The SDRR and RMSSD showed no difference between these two breeds on day 280 as well as on day 300 of gestation (Fig. 1b and c). Fetuses of Shetland mares and warmblood mares showed no significant differences in RR interval on days 280 and 300 of pregnancy (day 280: Shetland fetus: 606 ± 39 ms, warmblood fetus: 589 ± 38 ms, Fig. 1a). There was also no difference in SDRR and RMSSD on days 280 and 300 of gestation (day 280: SDRR Shetland fetus: 11.4 ± 6.9 ms, warmblood fetus: 11.9 ± 2.1 ms; RMSSD Shetland fetus: 13.7 ± 7.6 ms, warmblood fetus: 14.5 ± 3.7 ms, Fig. 1b and c). Comparing days 280 and 300 of pregnancy, maternal RR interval and HRV variables neither in Shetland nor in warmblood mares differed between times. Fetal RR interval increased slightly between days 280 and 300. This increase was significant in warmblood fetuses (from 589 ± 38 to
639 ± 54 ms, p < 0.05) and nearly reached statistical significance in Shetland fetuses (from 606 ± 39 to 663 ± 95 ms, p = 0.063). In the fetuses, there were no differences in HRV variables between days 280 and 300 (Fig. 1a–c). The number of accelerations in heart rate per hour did not differ between fetuses of the two breeds studied and also was not different between days 280 and 300 of gestation (day 280: Shetland fetuses: 25 ± 20 acceleration/h, warmblood fetuses: 19 ± 15 accelerations/h, Fig. 2a). The duration of heart rate accelerations was significantly longer in Shetland fetuses on day 300 of pregnancy (Shetland fetuses: 48 ± 20 heartbeats, warmblood fetuses: 29 ± 9 heartbeats; p < 0.05) but did not show differences between the two breeds on day 280 (Fig. 2b). Also the number and the duration of heart rate decelerations showed no differences between days 280 and 300 of pregnancy and did not differ between Shetland fetuses and warmblood fetuses (day 280: Shetland fetuses: 26 ± 11 decelerations/h with a duration of 38 ± 21 heartbeats, warmblood fetuses: 15 ± 7 decelerations/h with a duration of 52 ± 26 heartbeats; Fig. 2c and d).
4. Discussion Equine fetal electrocardiography was introduced into veterinary medicine nearly 100 years ago (Nörr, 1921) and its use for monitoring of pregnancy has been reported occasionally since (Parkes and Colles, 1977; Buss et al., 1980). Only recently, modern ECG recorders have become available which allow reliable fetomaternal ECG record-
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Number (n/h)
(a) 75
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Fig. 2. Number of accelerations in heart rate per hour (n/h), duration of accelerations (heartbeats), number of decelerations per hour (n/h) and duration of decelerations (heartbeats) in warmblood (n = 6) and Shetland (n = 7) fetuses on days 280 and 300; a: values with the same superscript letter differ significantly (p < 0.05).
ings from approximately day 170 of gestation until foaling (Nagel et al., 2010). Although fetal ECGs in horses have been obtained for some time already, there is still little information on physiological heart rate values in equine fetuses and in the mare during ongoing gestation. The available data have been obtained from warmblood horses or other full-size breeds. However, mares with pregnancies at risk from various breeds, including small ponies are presented for veterinary diagnostics. Physiological heart rate usually increases with decreasing size of the animal and thus is higher in adult ponies than in adult warmblood horses (Speirs, 1997). As could be expected, this finding was confirmed by comparing maternal heart rate in warmblood and Shetland mares in our study. In contrast to maternal heart rate, fetal heart rate did not differ between the breeds investigated. Also the evaluation of the accelerations and decelerations of heart rate per hour did not reveal differences between the two horse breeds. Thus, the normal values determined in a previous study for the warmblood fetus (Nagel et al., 2010) can be transferred to a much smaller breed such as the Shetland pony. With regard to gestation, Shetland ponies and warmblood horses differ primarily in fetal size and not in the physiology of pregnancy. However, confounding breed differences independent from size cannot not totally be excluded. We suggest that fetal size–at least for the time window studied – is without major effects on heart rate in the equine fetus. The decrease in fetal heart rate (i.e. increase in RR interval) with ongoing pregnancy (Nagel et al., 2010) was confirmed in the current study. It does not depend on breed or breed size but reflects primarily maturation of regulatory pathways such as the fetal autonomous nervous system.
While fetal heart rate determination in horses has been performed for a long time, maternal and fetal heart rate variability have been reported only recently (Nagel et al., 2010). Decreasing heart rate variability in adult horses has been associated with stress (Schmidt et al., 2010a,b). A decrease in HRV indicates increased sympathetic activity, decreased parasympathic activity or a combination of both (Von Borell et al., 2007). However no differences in HRV variables between the two breeds studied were found. There is thus no evidence for consistent differences in HRV between horse breeds in late pregnancy. Fetuses of Shetland and warmblood mares show no significant differences in heart rate variability indicating that not only heart rate but also heart rate variability data obtained in physiological pregnancies of a larger horse breed such as the warmblood are also largely valid for Shetland ponies. However, because transient heart rate accelerations at day 300 of gestation in Shetland ponies were of longer duration than in warmblood fetuses minor differences in HRV may exist. Because we did not compare heart rate and HRV between warmblood and Shetland mares during parturition, differences between breeds at foaling or in the immediate prepartum period cannot be excluded.
5. Conclusions Heart rate and heart rate variability values determined in healthy warmblood fetuses can be used as reference values for a much smaller breed, the Shetland pony. Gestational age has to be taken into account because heart rate and heart rate variability changes with ongoing maturation of the autonomous nervous system in the fetus.
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