Cardiorespiratory coupling, where breathing results in variation in the instantaneous heart rate (if _H), has been reported in seals and cetaceans (Irving, 1939; Ridgway, 1972; Kooyman, 1985; Kaczmarek et al., 2018; Fahlman et al., 2020b; Blawas et al., 2021b). In the bottlenose dolphin (Tursiops truncatus), the intermittent breathing pattern begins with a rapid exhalation followed by inhalation with a breath duration of approximately 1 s (Fahlman et al., 2017). The end of the breath, i.e., end-inspiration, results in an initial increase in if _H followed by a slow decline to a heart rate between 35 and 50 beats • min⁻¹ (Cauture et al., 2019; Fahlman et al., 2020b; Blawas et al., 2021a). During the inter-breath period there is often a sinusoidal variation in if _H with a period of varying duration of between 4 and 10 s (Cauture et al., 2019; Blawas et al., 2021a). The magnitude of this respiratory tachycardia varies both with changes in breathing frequency (f _R) and tidal volume (V _T) (Cauture et al., 2019; Blawas et al., 2021a; Blawas et al., 2021b). However, the respiratory characteristics of the bottlenose dolphin, with high respiratory flow during exhalation and a breath duration that lasts one or two heartbeat cycles, makes it difficult to separately evaluate how exhalation and inhalation affect cardiac function.

To improve our understanding of the ventilatory tachycardia, and to assess the variation in if _H during the exhalation and inspiration phases in bottlenose dolphins, we measured electrocardiograms (ECGs) using a data logger with suction cup-embedded electrodes and respiratory flow using a custom-built pneumotachometer during both normal breaths and breaths where exhalation and inhalation were separated. Our results provide evidence that normal breaths result in a respiratory tachycardia, and that exhalation and inhalation, respectively, result in a decrease and increase in if _H. Thus, the cardiac variability in bottlenose dolphins associated with respiration occurs at their typical breathing frequencies between 0.025 and 0.1 Hz. The results also highlight a second frequency of cardiac variability during breathing in the range of 0.1–0.2 Hz, possibly associated with blood pressure regulation. The data presented in the current study imply that both exhalation and inhalation alter if _H, and that breathing frequency alters cardiac function and heart rate.

Figure 1 shows the if _H following Full breaths, with panel A) showing the response during repeated Full breaths, and panel B) following a single Full breath followed by an exhalation. The most parsimonious model showed that if _H increased by 19.9% following a Full breath (Eq. 1; Table 1; Figure 1, df = 2, χ² = 84.2). When breaths were separated into exhalation and inhalation, Ex resulted in a 33.5% decrease in if _H. In resulted in an additional 20.1% increase in if _H (Eq. 1; Table 1; Figures 2, 3), similar to what we recorded during a normal breathing cycle. Δ f H   % = 19.9   2.8   –   33.5   3.9   •   E x + 20.1   5.5   •   I n (1)

For repeated Ex, the decrease in if _H was not affected by the number of repeated exhalations (df = 1, Loglikelihood = 447.4, χ² = 0.03).

In the current study, we show a respiratory tachycardia associated with normal breaths in the bottlenose dolphin, similar to what has been previously reported in this species and also other small to medium sized cetaceans (Irving, 1939; Ridgway, 1972; Fahlman et al., 2020b; Blawas et al., 2021b). We also report that when exhalation and inhalation are separated, the changes in if _H associated with exhalation and inhalation appear to occur in opposite directions. In addition, we confirm that the sinusoidal variation in if _H of between 0.1 and 0.2 Hz that is present following Full breaths (Figure 1B) (Cauture et al., 2019; Fahlman et al., 2020a; Blawas et al., 2021a; Blawas et al., 2021b), also is present after exhalations and inhalations (Figures 2, 3). We propose that this respiratory tachycardia may be important to enhance gas exchange during inspiration and help reduce cardiac work during the extended inter-breath intervals in intermittent breathers. The respiratory tachycardia we report in the bottlenose dolphin has implications for how to define resting heart rate, which may help improve comparative studies.

The data provided in the current study show that following a Full breath, there is a temporary 19.9% increase in if _H (Figure 1; Eq. 1; Table 1). We restricted the analysis of if _H to a 2-s interval between 1 and 3 s before or after the breathing phase for two reasons; first, we wanted to show that the response develops soon following a breath phase, and second, to be able to assess the effect of repeated exhalation. Under conditions of held phases of respiration, our data further suggest a decrease in if _H by 33.5% following exhalation (Figures 1, 2), with a 20.1% increase during inhalation (Figure 3; Eq. 1; Table 1). Notably, the magnitude of increase in if _H during the short period following a Full breath (19.9%, range: 14.9%–59.8%) was less than has been reported in previous studies in cetaceans where the increase can be as high as 100% (Ridgway, 1972; Fahlman et al., 2019a; Cauture et al., 2019; Blawas et al., 2021a; Blawas et al., 2021b). One reason for this could be the variation in the temporal response during Full breaths, where the maximal if _H following the breath can take several seconds to develop (Figure 1A). Thus, the increase may not have fully developed during our restricted period of analysis following a Full breath. When the In was separated from the Ex phase, it resulted in an additional 20.1% increase in if _H (Figure 3, Table 1, Eq. 1). One possible explanation for this result could be that during a normal breath the Ex phase partially suppresses the effects of the In phase, which gives rise to the temporary increase in if _H that we report. However, when breathing phases are separated the responses represent the distinct effect of the In and Ex phases. Thus, separating the breathing phases allows the effect of lung inflation and deflation on if _H to be observed on an extended time scale. (Eq. 1).

Following a Full breath in the bottlenose dolphin, the transient increase followed by an exponential decrease in if _H between breaths commonly does not reach a stable value until after 7–12 s, but can be as long as 15 s (Figure 1A) (Irving, 1939; Ridgway, 1972; Fahlman et al., 2019b; Cauture et al., 2019; Blawas et al., 2021a; Blawas et al., 2021b). In the present study, when the exhalation was separated from the inhalation, the if _H decreased by 33.5% within the 1–3 s after the exhalation. One alternative explanation to the decrease in if _H that we observed is that this merely represents the progressive sinusoidal decline during the inter-breath period (Figure 1B). Thus, it is possible that the decrease in if _H following exhalation alone is the result of this continuous decline. If so, Ex may not result in a further decrease in heart rate, however, we do not believe this is the case as studies in other mammals show a decrease in heart rate during a reduction in lung volume (Hayano et al., 1996; Looga, 1997; Mortola et al., 2015). Finally, in a limited number of samples (n = 64), we analyzed the change in if _H between 1 and 5 s following the inhalation or exhalation (i.e., a 4-s interval), which showed no further increase in if _H compared to if _H between 1 to 3 s following inhalation, but that following exhalation if _H decreased by an average of 42.6%. Thus, if Ex did not further reduce if _H there would not have been a further reduction.

Although these results are preliminary, we show that the Ex and In phases of respiration result in rapid and opposite f _H responses. Thus, after breathing, the tachycardia followed by a slowly declining if _H, may be a compound effect of the rapid inhalation and/or lung inflation resulting in transient vagal withdrawal or sympathetic activation, followed by the gradual extinction of this chronotropic stimulation. In addition, it has been shown that both f _R and VT alter the magnitude of the change in if _H (Fahlman et al., 2019b; Cauture et al., 2019; Blawas et al., 2021a; Blawas et al., 2021b). For example, when varying the f _R from 2 breaths • min⁻¹ to 6 breaths • min⁻¹ the average resting if _H increased by 40% (Cauture et al., 2019; Fahlman et al., 2020b; Blawas et al., 2021a; Blawas et al., 2021b). Consequently, this makes it difficult to assign a resting f _H that can be used to compare within and between species (Kooyman, 1985). In comparative physiology, resting f _H is an important measure as it provides a link between how cardiac function relates to energy use, e.g., allometric scaling (He et al., 2023). In this context, the resting f _H should be the value that represents no limitation to blood flow to support organs with O₂ for aerobic metabolism (Kooyman, 1985). However, variation in respiratory effort (f _R and VT) likely causes large variation between species and studies. Thus, without accounting for the cardiorespiratory coupling, studies, such as allometric scaling and diving physiology, that use an estimated “resting” f _H rate from intermittent breathers may have inflated variance.

Cardiac variability is a well-known phenomenon in continuous breathing mammals, referred to as respiratory sinus arrhythmia (RSA), where the if _H varies throughout the respiratory phase, with increase and decrease of if _H related to the inspiration and exhalation phases (Ben-Tal et al., 2012). The cyclical variation is thought to improve gas exchange or to maintain blood gases while minimizing the work of breathing (Hirsch and Bishop, 1981; Ben-Tal et al., 2012; Mortola et al., 2015). We propose that the respiratory tachycardia that we report in intermittent breathing bottlenose dolphin serves a similar purpose. Following inhalation, the pulmonary PO₂ and PCO₂ are, respectively higher and lower, which enhances the diffusion rate of both gases with the blood in the pulmonary capillary. The increase in instantaneous cardiac output, through increase in if _H and an increase in the instantaneous stroke volume, helps to enhance O₂ uptake and CO₂ removal (Fahlman et al., 2020b). Inspiration likely also results in a transient change in arterial blood pressure (Figure 1B), which then is seen as the sinusoidal variation in if _H during the inter-breath interval (Larsen et al., 2010). The initial increase followed by a decrease in both f _H and stroke volume helps reduce cardiac work, while also enhancing gas exchange. This cardiorespiratory strategy may be especially important as dolphins return to the surface following a dive, when reducing the recovery time at the surface allows the dolphin to return to the foraging patch. Past data in marine mammals have shown that the duration of the surface interval is mainly driven by CO₂ removal from the tissues and blood (Reed et al., 1994; Reed et al., 2000; Boutilier et al., 2001; Fahlman et al., 2008) and elevated cardiac output helps transport CO₂ to the lungs and enhances gas exchange. Thus, this respiratory tachycardia has a dual effect to reduce the surface interval.

In summary, in the current study we show that cardiorespiratory coupling in the bottlenose dolphin results in a respiratory tachycardia associated with the inhalation. When we separated the two phases of breathing, we observed that inhalation results in tachycardia similar to what we observed during a normal breathing cycle, while exhalation reduces if _H. The sinusoidal variation in if _H seen throughout the inter-breath interval is likely the effect variation in arterial blood pressure (Figure 1B), which in turn alters if _H. We propose that the cardiorespiratory coupling reported in the bottlenose dolphin, which results in a temporary increase in cardiac output, improves gas exchange and reduces recovery time during a surface interval following a dive.