Characterizing the effect of incrementally increasing dry bulb temperature on linear and nonlinear measures of heart rate variability in nonpregnant, mid-gestation, and late-gestation sows

Authors: BYRD, CHRISTOPHER - North Dakota State University; MCCONN, BETTY - Orise Fellow; GASKILL, BRIANNA - Purdue University; SCHINCKEL, ALLAN - Purdue University; GREEN-MILLER, ANGELA - University Of Illinois; Lay Jr, Donald; Johnson, Jay

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/21/2021
Publication Date: 1/10/2022
Citation: Byrd, C.J., Mcconn, B.R., Gaskill, B.N., Schinckel, A.P., Green-Miller, A.R., Lay Jr, D.C., Johnson, J.S. 2022. Characterizing the effect of incrementally increasing dry bulb temperature on linear and nonlinear measures of heart rate variability in nonpregnant, mid-gestation, and late-gestation sows. Journal of Animal Science. 100(1). Article skac004. https://doi.org/10.1093/jas/skac004.
DOI: https://doi.org/10.1093/jas/skac004

Interpretive Summary: Characterizing the sow physiological response to increased temperature is a crucial step in identifying effective mitigation strategies aimed at reducing economic losses and improving sow welfare under heat stress conditions. It is well-known that increasing ambient temperatures have a detrimental effect on sow health, productivity, and welfare. In addition, advancing gestation can increase sow body temperature sensitivity to heat stress. However, little information exists regarding the physiological responses of modern sows at differing gestational stages in response to increasing environmental temperatures. Specifically, information regarding the use of heart rate variability measures to evaluate heat stress sensitivity in gestating sows is lacking. Therefore, the study objective was to evaluate differences in the heart rate variability response of sows at three reproductive stages (non-pregnant, mid-gestation, late-gestation). In the present study, it was determined that late-gestation sows had heart rate variability measures indicative of a greater stress response when compared to non-pregnant and mid-gestation sows. However, an effect of environmental temperature on heart rate variability was not observed. These data imply that heart rate variability may not accurately determine whether sows at differing gestation stages suffer more or less from heat stress.

Technical Abstract: Characterizing the sow physiological response to an increased heat load is essential for effective heat stress mitigation. The study objective was to characterize the effects of a 400-min heating episode on sow heart rate variability (HRV) at different reproductive stages. Heart rate variability is a commonly used non-invasive proxy measure of autonomic function. Twenty-seven sows were enrolled in the study according to their gestation stage at time of selection: 1) non-pregnant (NP; n = 7), 2) mid-gestation (MID; n = 11), and 3) late-gestation (LATE; n = 8). The HRV data utilized in the study were collected from each pig as the dry bulb temperature in the room increased incrementally 19.84 ± 2.15 °C to 35.54 ± 0.43 °C (range: 17.1 – 37.5 °C) over a 400-min period. After data collection, one 5-min set of continuous heart rate data were identified per pig for each of nine temperature intervals (19-20.99, 21-22.99, 23-24.99, 25-26.99, 27-28.99, 29-30.99, 31-32.99, 33-34.99, 35-36.99 °C). Mean inter-beat interval length (RR), standard deviation of r-r intervals (SDNN), root mean of successive square differences (RMSSD), high frequency spectral power (HF), sample entropy (SampEn), short-term detrended fluctuation analysis (DFAa1), and three measures (%REC, DET, LMEAN) derived from recurrence quantification analysis were calculated for each data set. All data were analyzed using the PROC GLIMMIX procedure in SAS 9.4. Overall, LATE sows exhibited lower RR than NP sows (P < 0.01). The standard deviation of r-r intervals and RMSSD differed between each group (P < 0.01), with LATE sows exhibiting the lowest SDNN and RMSSD and NP sows exhibiting the greatest SDNN and RMSSD. Late-gestation sows exhibited lower HF than both MID and NP sows (P < 0.0001). Late-gestation sows also exhibited greater DFA values than NP sows (P = 0.05), whereas MID sows were intermediate to- but not different from NP or LATE sows (P > 0.05). Late-gestation sows exhibited greater DET compared to MID sows (P = 0.001). Late-gestation sows exhibited greater %REC and LMEAN compared to MID (P < 0.01) and NP sows (all P < 0.01). In conclusion, LATE sows exhibited indicators of greater autonomic stress throughout the heating period compared to MID and NP sows. However, temperature by treatment interactions were not detected as dry bulb increased. Future studies are needed to fully elucidate the effect of gestational stage and increasing dry bulb temperature on sow HRV.