• View in gallery
    Figure 1—

    Box-and-whisker plots of SDRR measured within 1 hour after admission to a referral hospital for horses with signs of acute (< 24 hours’ duration) abdominal pain (ie, colic) that survived to discharge (survivors; n = 41) or died or were euthanized (nonsurvivors; 10). For each plot, the horizontal line in the box represents the median, and the upper and lower boundaries of the box represent the 75th and 25th percentiles, respectively. Upper and lower whiskers represent the 90th and 10th percentiles, respectively, and circles represent outlier values. The black line depicts the proposed cutoff (40 milliseconds) for predicting outcome (survival vs nonsurvival).

  • View in gallery
    Figure 2—

    Box-and-whisker plots of SD2 for the horses in Figure 1. The black line depicts the proposed cutoff (42 milliseconds) for predicting outcome (survival vs nonsurvival). See Figure 1 for remainder of key.

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Prognostic value of measuring heart rate variability at the time of hospital admission in horses with colic

Valentina Vitale1Servei de Medicina Interna Equina, Departament de Medicina i Cirurgia Animals, Facultat de Veterinària, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain.

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Judit Viu1Servei de Medicina Interna Equina, Departament de Medicina i Cirurgia Animals, Facultat de Veterinària, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain.

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Lara Armengou1Servei de Medicina Interna Equina, Departament de Medicina i Cirurgia Animals, Facultat de Veterinària, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain.
2Unitat Equina, Fundació Hospital Clínic Veterinari, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain.

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José Ríos3Unitat de Bioestadistica, Facultat de Medicina, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain.

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Eduard Jose-Cunilleras1Servei de Medicina Interna Equina, Departament de Medicina i Cirurgia Animals, Facultat de Veterinària, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain.
2Unitat Equina, Fundació Hospital Clínic Veterinari, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain.

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Abstract

OBJECTIVE

To evaluate the prognostic value of measuring heart rate variability (HRV) in horses with colic at the time of admission to a referral hospital.

ANIMALS

51 horses > 1 year of age with colic (41 that survived [survivors] and 10 that died or were euthanized [nonsurvivors]).

PROCEDURES

HRV was recorded within 1 hour after admission by use of heart rate sensors with horses restrained in stocks. A 5-minute recording period was analyzed to obtain HRV measurements (eg, SD of R-R intervals [SDRR], root mean square of successive differences between R-R intervals [RMSSD], and geometric SDs determined from Poincaré plots [SD1 and SD2]). Variables associated with outcome (survival vs nonsurvival) were identified. Measurements were compared among diagnostic categories for colic (obstructive, inflammatory, or ischemic).

RESULTS

SDRR and RMSSD were significantly higher in survivors (median [25th to 75th percentile], 91.0 milliseconds [78.9 to 114.6 milliseconds] and 64.8 milliseconds [40.9 to 78.4 milliseconds], respectively) than in nonsurvivors (50.7 milliseconds [29.1 to 69.2 milliseconds] and 33.4 milliseconds [12.6 to 47.9 milliseconds], respectively). Similarly, SD1 and SD2 were significantly higher in survivors (48.3 milliseconds [28.9 to 60.9 milliseconds] and 111.3 milliseconds [93.0 to 146.6 milliseconds], respectively) than in nonsurvivors (23.7 milliseconds [8.9 to 33.9 milliseconds] and 65.1 milliseconds [33.7 to 91.9 milliseconds], respectively). The SDRR and SD2 were significantly higher for horses with obstructive colic than for horses with ischemic colic.

CONCLUSIONS AND CLINICAL RELEVANCE

Analysis of HRV in horses with colic may provide information on the underlying cause and be helpful in identifying horses less likely to survive.

Abstract

OBJECTIVE

To evaluate the prognostic value of measuring heart rate variability (HRV) in horses with colic at the time of admission to a referral hospital.

ANIMALS

51 horses > 1 year of age with colic (41 that survived [survivors] and 10 that died or were euthanized [nonsurvivors]).

PROCEDURES

HRV was recorded within 1 hour after admission by use of heart rate sensors with horses restrained in stocks. A 5-minute recording period was analyzed to obtain HRV measurements (eg, SD of R-R intervals [SDRR], root mean square of successive differences between R-R intervals [RMSSD], and geometric SDs determined from Poincaré plots [SD1 and SD2]). Variables associated with outcome (survival vs nonsurvival) were identified. Measurements were compared among diagnostic categories for colic (obstructive, inflammatory, or ischemic).

RESULTS

SDRR and RMSSD were significantly higher in survivors (median [25th to 75th percentile], 91.0 milliseconds [78.9 to 114.6 milliseconds] and 64.8 milliseconds [40.9 to 78.4 milliseconds], respectively) than in nonsurvivors (50.7 milliseconds [29.1 to 69.2 milliseconds] and 33.4 milliseconds [12.6 to 47.9 milliseconds], respectively). Similarly, SD1 and SD2 were significantly higher in survivors (48.3 milliseconds [28.9 to 60.9 milliseconds] and 111.3 milliseconds [93.0 to 146.6 milliseconds], respectively) than in nonsurvivors (23.7 milliseconds [8.9 to 33.9 milliseconds] and 65.1 milliseconds [33.7 to 91.9 milliseconds], respectively). The SDRR and SD2 were significantly higher for horses with obstructive colic than for horses with ischemic colic.

CONCLUSIONS AND CLINICAL RELEVANCE

Analysis of HRV in horses with colic may provide information on the underlying cause and be helpful in identifying horses less likely to survive.

The time intervals between successive heartbeats are not of equal duration, and these short-term changes are known as HRV.1 Analysis of HRV provides a noninvasive indication of activity of the sympathetic and parasympathetic components of the autonomic nervous system. A decrease in HRV is interpreted as evidence of increased sympathetic activity, decreased parasympathetic activity, or a combination of the two.2

In human patients with sepsis, various measures of HRV have been shown to be prognostic markers for adverse outcomes, such as multiple organ failure.3,4 In horses, acute abdominal pain (ie, colic) may be associated with severe manifestations of systemic disease,5 including endotoxemia, poor tissue perfusion, inflammation, disseminated intravascular coagulation, and multiple organ failure, that are often associated with a fatal outcome.6,7 Thus, identifying prognostic markers that could be used early in the course of treatment for horses with colic would be useful for clinicians. In a previous study,8 low HRV was strongly associated with the presence of ischemic lesions and nonsurvival in horses that underwent exploratory laparotomy because of severe gastrointestinal disease. To our knowledge, however, HRV at the time of hospital admission in horses with colic has not been described previously.

The objective of the study reported here was to evaluate the prognostic value of measuring HRV at the time of admission to a referral hospital in horses with signs of colic. We hypothesized that horses that died or were euthanized before hospital discharge (nonsurvivors) would have decreased HRV on admission, compared with horses that survived to hospital discharge (survivors). We also hypothesized that horses with diagnostic evidence of ischemic lesions would have decreased HRV on admission, compared with horses with evidence of obstructive lesions.

Materials and Methods

The study was designed as a prospective cohort study. Client-owned horses > 1 year of age that were evaluated at the Fundació Hospital Clínic Veterinari of the Universitat Autònoma de Barcelona, Spain, between September 2016 and February 2018 because of signs of acute (< 24 hours’ duration) abdominal pain (ie, colic) were considered for inclusion in the study. Horses with any cardiac arrhythmias (as detected by auscultation and confirmed by ECG) were excluded from the study. In addition, horses that had received hyoscine-N-butylbromide in the 24 hours prior to hospital admission were also excluded from the study because the anticholinergic effects of this drug could have altered HRV.9 Informed consent for participation in the study was obtained from all owners at the time of hospital admission. The study protocol was approved by the institutional review board.

For all horses included in the study, age, breed, sex, and heart rate determined by the attending clinician at the time of admission were recorded. Heart rate variability was recorded within 1 hour after admission with horses restrained in stocks. Recordings were obtained over a 15-minute period by use of heart rate sensorsa attached to an elastic surcingle. Whenever possible, HRV recordings were obtained prior to administration of any drugs and without subjecting the patient to any other diagnostic procedures or treatments (eg, rectal palpation or IV catheter placement) during the recording period. Because patients were admitted on an emergency basis, the welfare and health of the horses were priorities. Accordingly, nasogastric intubation for gastric decompression was always performed prior to obtaining HRV recordings, and the stomach tube was usually maintained in place during the recording period. Abdominal ultrasonography and abdominocentesis were typically performed after HRV recordings were obtained.

Heart rate variability was recorded as described.1 Standard softwareb was used to extract R-R interval data as text files that were subsequently analyzed with an open-source software program.c After application of the artifact removal option (medium setting), the central 5-minute period of the 15-minute HRV recording was selected for analysis, as previously recommended.8,10,11 Time-domain (SDRR and RMSSD), frequency-domain (LF peak, HF peak, LF power, HF power, and the ratio of LF power to HF power [LF:HF]), and nonlinear (SD1 and SD2) variables were calculated.

For all horses, the underlying cause of colic was classified as obstructive, inflammatory, or ischemic, as previously described,12,13 on the basis of history, physical examination findings, results of rectal palpation and nasogastric intubation, results of ancillary diagnostic tests (CBC, serum biochemical analysis, blood gas analysis, abdominal ultrasonography, and peritoneal fluid analysis), and, when available, results of abdominal radiography and exploratory laparotomy. The degree of dehydration at the time of hospital admission was assessed by evaluating mucous membrane color and capillary refill time and measuring PCV (reference range, 30% to 45%) and total protein concentration (reference range, 5.5 to 7.5 g/dL). Dehydration status was classified as mild (PCV > 40% to ≤ 50% or total protein concentration ≤ 7.5 g/dL), moderate (PCV > 50% to ≤ 65% or total protein concentration > 7.5 to ≤ 8.6 g/dL), or severe (PCV > 65% or total protein concentration > 8.6 g/dL). Finally, horses were classified on the basis of outcome as survivors (survived to hospital discharge) or nonsurvivors (died or were euthanized before discharge).

Statistical analysis

All statistical analyses were performed with commercially available software.d Two-sided values of P < 0.05 were considered significant.

Age, heart rate, and frequency distributions of sex and administration of analgesia were compared between survivors and nonsurvivors and among diagnostic categories of colic by means of the Fisher exact test or, when > 2 categories were present, the Mann-Whitney U test.

The Mann-Whitney U test was used to test for associations between individual HRV variables and outcome (survival vs nonsurvival), with ROC curves created for those variables significantly (P < 0.05) associated with outcome. Area under the ROC curve was calculated, and the positive likelihood ratio (sensitivity/[1 – specificity]) at various cutoffs for individual HRV variables was calculated. Variables were considered potentially useful for differentiating survivors from nonsurvivors if the positive likelihood ratio was > 5, and the cutoff that provided the highest positive likelihood ratio was reported. Other cutoff values for RMSSD and SDRR from a previous study were also evaluated8 by means of the Fisher exact test or, when > 2 categories were present, the Mann-Whitney U test.

The Kruskal-Wallis test was used to test for associations between individual HRV variables and the diagnostic category of colic (obstructive vs inflammatory vs ischemic), with Mann-Whitney U tests used for pairwise comparisons if the P value for the global test of significance was < 0.10. The effect of dehydration status (mild, moderate, or severe) on associations between HRV variables and outcome or diagnostic category was assessed by means of adjusted ANOVA with a nonparametric approach after a rank transformation was applied to all dependent variables.

Results

A total of 81 horses > 1 year of age were evaluated because of colic during the study period. Thirty horses were excluded because an arrythmia was identified (n = 2) or hyoscine-N-butylbromide was administered ≤ 24 hours prior to hospital admission (1) or because of technical issues or lack of available staff at the time of admission (27). Owners of the remaining 51 horses agreed to enroll their horses in the study.

Of the 51 horses included in the study, 22 were mares, 17 were geldings, and 12 were stallions. Age ranged from 1 to 30 years (mean ± SD, 13.6 ± 6.7 years). There were 18 crossbred horses, 15 Andalusians, 6 Arabians, 6 warmbloods, 3 Friesians, 1 pony, 1 Standardbred, and 1 Thoroughbred. Twenty-eight horses had obstructive colic, 12 had inflammatory colic, and 11 had ischemic colic. Thirty-one horses were treated medically, and 20 underwent exploratory laparotomy. Of the 20 horses that underwent laparotomy, 10 had ischemic colic (3 required small intestinal resection), 7 had obstructive colic, and 3 had inflammatory colic.

Forty-one horses survived to hospital discharge (survivors); mean ± SD hospitalization time was 4.8 ± 3.4 days. The remaining 10 horses were euthanized or died prior to discharge (nonsurvivors). Six of the 10 nonsurviving horses had inflammatory colic, 3 had ischemic colic, and 1 had obstructive colic. Four of the nonsurviving horses were euthanized intraoperatively. Nineteen of the 51 horses were administered an α2-adrenoceptor agonist, with or without butorphanol, before or during the HRV recording period because of signs of severe pain. Ten of these horses had obstructive colic, 6 had ischemic colic, and 3 had inflammatory colic. Although outcome (survival vs nonsurvival) was not significantly associated with administration of analgesic agents (yes vs no), horses with ischemic colic were significantly (P = 0.047) more likely to receive analgesics at the time of hospital admission than were horses with obstructive colic. In the 24 hours prior to hospital admission, 20 of the 51 horses received flunixin meglumine and none received hyoscine-N-butylbromide.

Heart rate at the time of admission did not differ between surviving and nonsurviving horses or among horses with obstructive, inflammatory, or ischemic colic (Tables 1 and 2). There were significant differences between surviving and nonsurviving horses with respect to SDRR (P = 0.001), RMSSD (P = 0.01), SD1 (P = 0.01), SD2 (P = 0.003), LF peak (P = 0.001), and HF peak (P = 0.01). However, LF:HF ratio, LF power, and HF power did not differ between survivors and nonsurvivors.

Table 1—

Measurements of HRV obtained within 1 hour after admission to a referral hospital for horses with signs of acute (< 24 hours’ duration) abdominal pain (ie, colic) that survived to discharge (survivors; n = 41) or died or were euthanized (nonsurvivors; 10).

 Outcome 
VariableSurvivorsNonsurvivorsP value
Heart rate (beats/min)53.36 (48.39–61.17)63.33 (47.61–77.25)0.31
SDRR (ms)91.0 (78.9–114.6)50.7 (29.1–69.2)0.001
SDRR > 40 ms38 (93)5 (50)0.004
SDRR ≤ 40 ms3 (7)5 (50)
RMSSD (ms)64.8 (40.9–78.4)33.4 (12.6–47.9)0.01
SD1 (ms)48.3 (28.9–60.9)23.7 (8.9–33.9)0.01
SD2 (ms)111.3 (93.0–146.6)65.1 (33.7–91.9)0.003
SD2 > 42 ms38 (93)6 (60)0.021
SD2 ≤ 42 ms3 (7)4 (40)
LF peak (Hz)2,104.0 (1,185.0–3,563.0)271.5 (177.0–938.0)0.001
HF peak (Hz)2,041.0 (1,240.0–3,843.0)508.0 (118.0–960.0)0.01
LF power (nu)50.6 (37.1–71.3)51.6 (30.0–62.3)0.90
HF power (nu)49.4 (30.0–67.0)48.3 (37.7–70.0)0.90
LF:HF1.02 (0.59–2.48)1.07 (0.43–1.65)0.90

Values are reported as median (interquartile [25th to 75th percentile] range) or as number (%) of horses.

Recordings were obtained over a 15-minute period while horses were restrained in stocks by use of heart rate sensorsa that were attached to an elastic surcingle; the central 5-minute period of the recording was selected for analysis.

LF:HF = Ratio of LF power to HF power. nu = Normalized units. — = Not applicable.

Table 2—

Measurements of HRV for the horses in Table 1 grouped by underlying cause of colic.

 Underlying cause of colic 
VariableObstructive (n = 28)Ischemic (n = 11)Inflammatory (n = 12)P value
Heart rate (beats/min)55.62 (51.81–62.24)48.39 (43.39–63.18)62.33 (41.28–77.87)0.32
SDRR (ms)100.8 (86.7–115.7)a46.0 (29.7–67.6)b67.1 (36.0–89.6)a,b0.001
RMSSD (ms)69.4 (42.7–95.1)47.9 (12.6–65.9)34.6 (18.5–75.1)0.08
SD1 (ms)51.5 (30.3–67.4)33.9 (8.9–46.6)24.5 (13.0–53.0)0.06
SD2 (ms)119.8 (102.6–152.6)a59.1 (36.8–84.0)b91.7 (50.8–114.5)a,b0.001
LF peak (Hz)2,303.5 (1,766.0–3,913.0)a577.0 (137.0–1,271.0)b862.0 (311.0–2,998.0)a,b0.004
HF peak (Hz)2,229.5 (1,363.0–4,834.0)a632.0 (45.0–2,643.0)b973.0 (442.0–3,349.0)a,b0.03
LF power (nu)57.0 (29.4–71.7)51.7 (21.8–71.3)49.5 (30.0–74.7)0.87
HF power (nu)43.0 (28.3–70.6)48.3 (28.7–78.2)50.5 (25.3–70.0)0.87
LF:HF1.38 (0.42–2.53)1.07 (0.28–2.48)0.98 (0.43–2.95)0.87

Within a row, values with different superscript letters differ significantly (P < 0.05).

See Table 1 for remainder of key.

Analysis of the ROC curves indicated that only SDRR and SD2 yielded positive likelihood ratios > 5. Area under the ROC curve for SDRR was 82% (95% confidence interval, 70% to 94%). The optimal cutoff for SDRR was 40 milliseconds, with a positive likelihood ratio at this cutoff of 6.8. A cutoff of 40 milliseconds for SDRR correctly identified the outcome for 38 of the 41 (93%; P = 0.004) surviving horses (Table 1; Figure 1). Area under the ROC curve for SD2 was 80% (95% confidence interval, 66% to 94%). The optimal cutoff for SD2 was 42 milliseconds, with a positive likelihood ratio at this cutoff of 8.16. A cutoff of 42 milliseconds for SD2 correctly identified the outcome for 38 of the 41 (93%; P = 0.021) surviving horses (Table 1; Figure 2).

Figure 1—
Figure 1—

Box-and-whisker plots of SDRR measured within 1 hour after admission to a referral hospital for horses with signs of acute (< 24 hours’ duration) abdominal pain (ie, colic) that survived to discharge (survivors; n = 41) or died or were euthanized (nonsurvivors; 10). For each plot, the horizontal line in the box represents the median, and the upper and lower boundaries of the box represent the 75th and 25th percentiles, respectively. Upper and lower whiskers represent the 90th and 10th percentiles, respectively, and circles represent outlier values. The black line depicts the proposed cutoff (40 milliseconds) for predicting outcome (survival vs nonsurvival).

Citation: American Journal of Veterinary Research 81, 2; 10.2460/ajvr.81.2.147

Figure 2—
Figure 2—

Box-and-whisker plots of SD2 for the horses in Figure 1. The black line depicts the proposed cutoff (42 milliseconds) for predicting outcome (survival vs nonsurvival). See Figure 1 for remainder of key.

Citation: American Journal of Veterinary Research 81, 2; 10.2460/ajvr.81.2.147

There were significant differences between horses with obstructive colic and horses with ischemic colic for SDRR (P = 0.001), SD2 (P = 0.001), LF peak (P = 0.004), and HF peak (P = 0.03; Table 2). None of the HRV variables differed significantly between horses with obstructive versus inflammatory colic or between horses with ischemic versus inflammatory colic.

Adjustment for dehydration status did not affect any of the significant associations between HRV variables and outcome or between HRV variables and diagnostic category (data not shown).

Discussion

Results of the study reported here suggested that, in horses with colic, measuring HRV at the time of admission may be of prognostic value and could potentially be helpful, in connection with other findings, in differentiating obstructive from ischemic colic. Specifically, horses in the present study with an SDRR < 40 milliseconds or an SD2 < 42 milliseconds were substantially more likely to die or be euthanized than were horses with higher values.

A 5-minute period of the HRV recording was used for analysis, consistent with previous recommendations.8,10,11 The 15-minute recording period was sufficient to collect usable data, yet allowed for minimal interference with diagnostic and clinical management of the horses.

In the present study, SDRR, RMSSD, SD1, SD2, LF peak, and HF peak were significantly lower in nonsurviving horses, compared with values for surviving horses. However, the lack of significant differences between surviving and nonsurviving horses with respect to certain frequency-domain measurements (ie, LF:HF, LF power, and HF power) called into question the relevance of the differences detected for LF peak and HF peak. Values for LF peak and HF peak can have high inter- and intraindividual variability because total power (LF + HF) is not always consistently defined.14,15 Thus, LF power and HF power provide more standardized measurements.11 Furthermore, frequency-domain measurements are influenced by respiratory rate and blood pressure,16,17 both of which are frequently altered in horses with colic.18 The SDRR and SD2, which are both influenced by sympathetic and parasympathetic activity,11 were useful for differentiating between nonsurviving and surviving horses in the present study. Indeed, cutoffs of 40 milliseconds for SDRR and 42 milliseconds for SD2 each correctly identified the outcome for 38 of the 41 (93%) surviving horses. Although the RMSSD and SD1 were significantly lower for nonsurviving horses, compared with surviving horses, no useful cutoff values were identified for these variables.

McConachie et al8 proposed a cutoff of 17.2 milliseconds for RMSSD as an ideal value for predicting nonsurvival in horses that underwent exploratory laparotomy because of acute gastrointestinal disease. In the present study, this cutoff was not useful for distinguishing surviving from nonsurviving horses because 43 of 51 (84%) horses had RMSSD values > 17.2 milliseconds. In a study by Vitale et al,10 the mean ± SD value of RMSSD over a 5-minute HRV recording period for horses restrained in stocks was 53.9 ± 21.0 milliseconds. In comparison, mean RMSSD for horses in the present study, which were also restrained in stocks, was 68.6 ± 44.3 milliseconds and 31.7 ± 20.3 milliseconds for surviving and nonsurviving horses, respectively. The RMSSD primarily represents vagal activity,2 and low vagal tone would be expected in the horses of the present study (particularly in nonsurviving horses) as a result of restraint, stress, and pain.10,11,19,20 The sympathovagal imbalance of nonsurviving horses could have resulted from dysfunction associated with withdrawal of both branches of the autonomic nervous system or abnormalities of cardiac pacemaker cells.8,21

When comparing horses among diagnostic groups in the present study, SDRR, SD2, LF peak, and HF peak were significantly lower for horses with ischemic colic, compared with values for horses with obstructive colic. These differences could have been due to the fact that, of the 10 nonsurviving horses, 3 had ischemic colic, only 1 had obstructive colic, and the rest had inflammatory colic. The low SDRR and SD2 values for horses with ischemic colic were consistent with the findings of McConachie et al.8 As previously suggested,8 the observed HRV alterations in horses with ischemic gastrointestinal disease may have important pathophysiologic and clinical implications related to cardiovascular function. Therefore, HRV analysis in horses admitted with signs of acute abdominal pain may be a useful adjunct to physical and diagnostic examination findings for classification of colic and earlier identification of horses more likely to die or be euthanized.8,18

There were several limitations to the present study. Because horses ≤ 1 year of age, those identified with arrythmias on admission, and those for which recordings could not be performed correctly were excluded, the sample size was small. Heart rate variability follows circadian rhythms, with increased parasympathetic activity at night and increased sympathetic activity during the day.16,22 Nevertheless, horses with colic can require emergency intervention at any time of the day or night. Accordingly, attempts to include only horses admitted during certain hours of the day would have further limited the sample size in the present study. Although administration of analgesic agents prior to or during the HRV recording period was not associated with the outcome (survival vs nonsurvival), we could not exclude the possibility that these drugs had a direct pharmacological influence on the HRV measurements. The administration of flunixin meglumine likely had no direct effect on the HRV measurements but could have indirectly altered measurements through reduction in pain.16

For practical reasons, horses were restrained in stocks during the HRV recording period. Stress associated with prolonged restraint could have affected HRV measurements through increased sympathetic activity.10,23 However, because restraint conditions were similar for all horses in the present study, it is unlikely that restraint influenced our findings in a differential manner. In addition, nasogastric intubation, which could have potentially influenced sympathovagal balance, was usually performed before the HRV recording period. Finally, the degree of dehydration could have affected cardiovascular function, which would have influenced HRV measurements. However, adjustment for dehydration status did not alter associations between HRV measurements and outcome or diagnostic group.

In conclusion, measuring HRV is a noninvasive test that can be performed rapidly and economically in equine referral hospitals.8 Measurement of HRV requires a heart rate monitor with HRV features and software for HRV analysis, for which open-source versions are available. For horses with signs of acute abdominal pain (ie, colic), measuring HRV provides indicators of cardiovascular health and autonomic nervous system function that might be useful for clinical monitoring.3,4,8 In the present study, certain HRV variables obtained at the time of admission differed between horses that did and did not survive to hospital discharge. Additional studies, ideally with a larger patient population, are needed to confirm our findings and to validate our proposed cutoff values for SDRR and SD2.

Acknowledgments

The study was performed at the Unitat Equina of the Fundació Hospital Clínic Veterinari, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain.

No external funding was used in this study. The authors declare that there were no conflicts of interest.

Presented as a poster at the 34th World Veterinary Association Congress, Barcelona, Spain, May 2018.

ABBREVIATIONS

HF

High frequency (0.07 to 0.6 Hz)

HRV

Heart rate variability

LF

Low frequency (0.01 to 0.07 Hz)

RMSSD

Root mean square of successive differences between R-R intervals

ROC

Receiver operating characteristic

SD1

Geometric SD perpendicular to the line of identity for a Poincaré plot of R-R interval versus the preceding R-R interval

SD2

Geometric SD along the line of identity for a Poincaré plot of R-R interval versus the preceding R-R interval

SDRR

SD of R-R intervals

Footnotes

a.

Polar V800, Polar Electro Oy, Kempele, Finland.

b.

FlowSync, version 3.0, Polar Electro Oy, Kempele, Finland. Available at: www.flow.polar.com/start.

c.

Kubios HRV standard, version 2.2, Biosignal analysis and medical imaging group, Department of Applied Physics, University of Eastern Finland, Kuopio, Finland. Available at: kubios.com.

d.

SPSS Statistics, version 20.0, IBM Corp, Armonk, NY.

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Contributor Notes

Dr. Vitale's present address is University Teaching Hospital, Sydney School of Veterinary Science, University of Sydney, Camden, NSW, Australia.

Dr. Viu's present address is Hospital Veterinario, Sierra de Madrid, 28750 San Agustín de Guadalix, Madrid, Spain.

Address correspondence to Dr. Jose-Cunilleras (eduard.jose.cunilleras@uab.cat).