Does respiratory sinus arrhythmia occur in fishes?

Abstract

The hypothesis that respiratory modulation of heart rate variability (HRV) or respiratory sinus arrhythmia (RSA) is restricted to mammals was tested on four Antarctic and four sub-Antarctic species of fish, that shared close genotypic or ecotypic similarities but, due to their different environmental temperatures, faced vastly different selection pressures related to oxygen supply. The intrinsic heart rate (fH) for all the fish species studied was approximately 25% greater than respiration rate (fV), but vagal activity successively delayed heart beats, producing a resting fH that was synchronized with fV in a progressive manner. Power spectral statistics showed that these episodes of relative bradycardia occurred in a cyclical manner every 2-4 heart beats in temperate species but at >4 heart beats in Antarctic species, indicating a more relaxed selection pressure for cardio-respiratory coupling. This evidence that vagally mediated control of fH operates around the ventilatory cycle in fish demonstrates that influences similar to those controlling RSA in mammals operate in non-mammalian vertebrates.

Figure 1

(a) Probability distribution of 750 consecutive R–R intervals grouped into 30 ms bins for the temperate fish P. angustata (n=6). Log-normal distribution curve fitted to data (Shapiro-Wilks test=P<0.1; standard skewdness=3.13). (b) Probability distribution of 750 consecutive R–R intervals in 30 ms bins for the Antarctic T. bernachii (n=6). Normal distribution curve fitted to the data (Shapiro-Wilks test=P<0.1; standard skewdness=1.7). Vertical dashed line indicates length of intrinsic heart rate, and the dotted line the ventilation cycle.

Figure 2

Power spectra calculated from the resting instantaneous e.c.g. from four Antarctic and four temperate fish species. Raw e.c.g. traces were examined and sections of 3×512 consecutive R–R intervals without ectopics or artefacts were selected, tested for stationarity and underwent fast Fourier transformation (FFT) analysis using a Hanning window to reduce spectral leakage, with the resulting spectra being plotted graphically (see methods Altimiras 1999). Each spectrum length is set by the Nyquist criterion, which is in turn determined by the individual fishes’ actual heart rate. Antarctic species: 14.3 bpm=T. bernacchii (n=5), T. pennelli (n=5), 14.6 bpm=T. hansoni (n=5), 15.3 bpm=T. nicolai (n=5); temperate species: 20.6 bpm=M. scorpius (n=5), 27.2 bpm=B. variegatus (n=5), 28.9 bpm=P. angustata (n=6). (For clarity 46 bpm=the temperate K. stewarti (n=5) was omitted).