Specific carotid body chemostimulation is sufficient to elicit phrenic poststimulus frequency decline in a novel in situ dual-perfused rat preparation

Reduced Stroke Volume and Brain Perfusion Drive Postural Hyperventilation in Postural Orthostatic Tachycardia Syndrome

Postural hyperventilation has been implicated as a cause of postural orthostatic tachycardia syndrome (POTS), yet the precise mechanisms underlying the heightened breathing response remain unclear. This study challenges current hypotheses by revealing that exaggerated peripheral chemoreceptor activity is not the primary driver of postural hyperventilation. Instead, significant contributions from reduced stroke volume and compromised brain perfusion during orthostatic stress were identified. These findings shed light on our understanding of POTS pathophysiology, emphasizing the critical roles of systemic hemodynamic status. Further research should explore interventions targeting stroke volume and brain perfusion for more effective clinical management of POTS.

Neuroanatomic and neurophysiologic evidence of pulmonary nociceptor and carotid chemoreceptor convergence in the nucleus tractus solitarius and nucleus ambiguus

Pulmonary vagal nociceptors defend the airways. Cardiopulmonary vagal nociceptors synapse in the nucleus tractus solitarius (NTS). Evidence has demonstrated the convergence of cardiopulmonary nociceptors with afferents from carotid chemoreceptors. Whether sensory convergence occurs in motor nuclei and how sensory convergence affects reflexive efferent motor output directed toward the airways are critical knowledge gaps. Here, we show that distinct tracer injection into the pulmonary nociceptors and carotid chemoreceptors leads to co-labeled neurons in the nucleus tractus solitarius and nucleus ambiguus. Precise simultaneous stimulation delivered to pulmonary nociceptors and carotid chemoreceptors doubled efferent vagal output, enhanced phrenic pause, and subsequently augmented phrenic motor activity. These results suggest that multiple afferents are involved in protecting the airways and concurrent stimulation enhances airway defensive reflex output.

NEW & NOTEWORTHY Sensory afferents have been shown to converge onto nucleus tractus solitarius primary neurons. Here, we show sensory convergence of two distinct sets of sensory afferents in motor nuclei of the nucleus ambiguus, which results in augmentation of airway defense motor output.

Advancing respiratory-cardiovascular physiology with the working heart-brainstem preparation over 25 years

Twenty‐five years ago, a new physiological preparation called the working heart–brainstem preparation (WHBP) was introduced with the claim it would provide a new platform allowing studies not possible before in cardiovascular, neuroendocrine, autonomic and respiratory research. Herein, we review some of the progress made with the WHBP, some advantages and disadvantages along with potential future applications, and provide photographs and technical drawings of all the customised equipment used for the preparation. Using mice or rats, the WHBP is an in situ experimental model that is perfused via an extracorporeal circuit benefitting from unprecedented surgical access, mechanical stability of the brain for whole cell recording and an uncompromised use of pharmacological agents akin to in vitro approaches. The preparation has revealed novel mechanistic insights into, for example, the generation of distinct respiratory rhythms, the neurogenesis of sympathetic activity, coupling between respiration and the heart and circulation, hypothalamic and spinal control mechanisms, and peripheral and central chemoreceptor mechanisms. Insights have been gleaned into diseases such as hypertension, heart failure and sleep apnoea. Findings from the in situ preparation have been ratified in conscious in vivo animals and when tested have translated to humans. We conclude by discussing potential future applications of the WHBP including two‐photon imaging of peripheral and central nervous systems and adoption of pharmacogenetic tools that will improve our understanding of physiological mechanisms and reveal novel mechanisms that may guide new treatment strategies for cardiorespiratory diseases.

Cardiorespiratory plasticity in humans following two patterns of acute intermittent hypoxia

NEW FINDINGS

What is the central question of this study? Do cardiorespiratory experience-dependent effects (EDEs) differ between two different stimulus durations of acute isocapnic intermittent hypoxia (IHx; 5-min vs. 90-s cycles between hypoxia and normoxia)? What is the main finding and its importance? There was long-term facilitation in ventilation and blood pressure in both IHx protocols, but there was no evidence of progressive augmentation or post-hypoxia frequency decline. Not all EDEs described in animal models translate to acute isocapnic IHx responses in humans, and cardiorespiratory responses to 5-min versus 90-s on/off IHx protocols are largely similar.

ABSTRACT

Peripheral respiratory chemoreceptors monitor breath-by-breath changes in arterial CO₂ and O₂ , and mediate ventilatory changes to maintain homeostasis. Intermittent hypoxia (IHx) elicits hypoxic ventilatory responses, with well-described experience-dependent effects (EDEs), derived mostly from animal work involving intermittent 5-min cycles of hypoxia and normoxia. These EDEs include post-hypoxia frequency decline (PHxFD), progressive augmentation (PA) and long-term facilitation (LTF). Comparisons of these EDEs between animal models and humans using similar IHx protocols are lacking. In addition, it is unknown whether shorter bouts of hypoxia, which may be more relevant to clinical conditions, elicit EDEs of similar magnitudes in humans. Respiratory (frequency, tidal volume and minute ventilation ( V ̇ I ) and cardiovascular (heart rate and mean arterial pressure (MAP)) variables were measured during and following two patterns of acute isocapnic IHx in 14 healthy human participants (four female): (1) 5 × 5 min and (2) 5 × 90 s on/off hypoxia. Participants' end-tidal P O 2 was clamped at 45 Torr during hypoxia and 100 Torr during normoxia. We found that (1) PHxFD and PA were not present in either IHx pattern (P > 0.14), (2) LTF was present in V ̇ I following both 5-min (P < 0.001) and 90-s isocapnic IHx trials (P < 0.001), and (3) LTF was present in MAP following 5-min isocapnic IHx (P < 0.001), and trended towards significance following 90-s IHx (P = 0.058). We demonstrate that acute isocapnic IHx alone may not elicit all of the EDEs that have been described in animal models. Additionally, ventilatory LTF occurred regardless of the length of hypoxia-normoxia cycles.

Preventing acute asthmatic symptoms by targeting a neuronal mechanism involving carotid body lysophosphatidic acid receptors

Asthma accounts for 380,000 deaths a year. Carotid body denervation has been shown to have a profound effect on airway hyper-responsiveness in animal models but a mechanistic explanation is lacking. Here we demonstrate, using a rat model of asthma (OVA-sensitized), that carotid body activation during airborne allergic provocation is caused by systemic release of lysophosphatidic acid (LPA). Carotid body activation by LPA involves TRPV1 and LPA-specific receptors, and induces parasympathetic (vagal) activity. We demonstrate that this activation is sufficient to cause acute bronchoconstriction. Moreover, we show that prophylactic administration of TRPV1 (AMG9810) and LPA (BrP-LPA) receptor antagonists prevents bradykinin-induced asthmatic bronchoconstriction and, if administered following allergen exposure, reduces the associated respiratory distress. Our discovery provides mechanistic insight into the critical roles of carotid body LPA receptors in allergen-induced respiratory distress and suggests alternate treatment options for asthma.

Acute bronchoconstriction is the leading cause of asthmatic sudden death following allergen exposure. The authors show that the systemic increase of LPA following inhaled allergen or bradykinin challenge activates the carotid bodies through TRPV1 and LPA-specific receptors and that systemic TRPV1 and LPA-specific receptor antagonists ameliorate acute bronchoconstriction.

Integration of Central and Peripheral Respiratory Chemoreflexes

A debate has raged since the discovery of central and peripheral respiratory chemoreceptors as to whether the reflexes they mediate combine in an additive (i.e., no interaction), hypoadditive or hyperadditive manner. Here we critically review pertinent literature related to O2 and CO2 sensing from the perspective of system integration and summarize many of the studies on which these seemingly opposing views are based. Despite the intensity and quality of this debate, we have yet to reach consensus, either within or between species. In reviewing this literature, we are struck by the merits of the approaches and preparations that have been brought to bear on this question. This suggests that either the nature of combination is not important to system responses, contrary to what has long been supposed, or that the nature of the combination is more malleable than previously assumed, changing depending on physiological state and/or respiratory requirement.

Stress peptide PACAP stimulates and stabilizes neonatal breathing through distinct mechanisms

Pituitary adenylate cyclase-activating peptide (PACAP) is an important mediator of the stress response and is crucial in maintaining breathing in neonates. Here we investigate the role of exogenously applied PACAP in neonatal breathing using the neonatal rat in situ working heart-brainstem preparation. A 1-min bolus of 250 nM PACAP-38 caused an increased in respiratory frequency that was rapid and transient, but had no effect on neural tidal volume or neural minute ventilation. Denervation of the carotid body abolished this effect. PACAP had a persistent effect on breathing stability in both carotid body-intact and -denervated preparations, as shown by decreases in respiratory variability 5 min following application. These data suggest that PACAP released during stress acts via carotid body dependent and independent mechanisms to stimulate and stabilize breathing. These mechanisms may account for PACAP's critical role in defending neonatal breathing against environmental stress.

Serotonergic mechanisms are necessary for central respiratory chemoresponsiveness in situ

Evidence from in vivo and in vitro experiments conclude that serotonin (5-HT) neurons are involved in and play an important role in central respiratory CO2/H(+) chemosensitivity. This study was designed to assess the importance of 5-HT neurons and 5-HT receptor activation in the frequency and amplitude components of the hypercapnic response of the respiratory network in the unanesthetized perfused in situ juvenile rat brainstem preparation that exhibits patterns of phrenic nerve discharge similar to breathing in vivo. Exposure to a hypercapnic perfusate increased phrenic burst frequency and/or amplitude, the neural correlates of breathing frequency and tidal volume in vivo. Hypercapnic responses were also assessed during exposure to ketanserin (5-HT2 receptor antagonist), and 8-OH-DPAT (inhibiting 5-HT neurons via 5-HT1A autoreceptors). Neither of these drugs substantially altered baseline activity, however, both abolished hypercapnic responses of the respiratory network. These data illustrate that 5-HT neurons and 5-HT receptor activation are not required for respiratory rhythm generation per se, but are critical for CO2 responses in situ, supporting the hypothesis that 5-HT neurons play an important role in central ventilatory chemosensitivity in vivo.

The essential role of peripheral respiratory chemoreceptor inputs in maintaining breathing revealed when CO2 stimulation of central chemoreceptors is diminished

Central sleep apnoea is a condition characterized by oscillations between apnoea and hyperpnoea during sleep. Studies in sleeping dogs suggest that withdrawal of peripheral chemoreceptor (carotid body) activation following transient ventilatory overshoots plays an essential role in causing apnoea, raising the possibility that sustaining carotid body activity during ventilatory overshoots may prevent apnoea. To test whether sustained peripheral chemoreceptor activation is sufficient to drive breathing, even in the absence of central chemoreceptor stimulation and vagal feedback, we used a vagotomized, decerebrate dual-perfused in situ rat preparation in which the central and peripheral chemoreceptors are independently and artificially perfused with gas-equilibrated medium. At varying levels of carotid body stimulation (CB PO2/PCO2: 40/60, 100/40, 200/15, 500/15 Torr), we decreased the brainstem perfusate PCO2 in 5 Torr steps while recording phrenic nerve activity to determine the central apnoeic thresholds. The central apnoeic thresholds decreased with increased carotid body stimulation. When the carotid bodies were strongly stimulated (CB 40/60), the apnoeic threshold was 3.6 ± 1.4 Torr PCO2 (mean ± SEM, n = 7). Stimulating carotid body afferent activity with either hypercapnia (60 Torr PCO2) or the neuropeptide pituitary adenylate cyclase-activating peptide restored phrenic activity during central apnoea. We conclude that peripheral stimulation shifts the central apnoeic threshold to very hypocapnic levels that would likely increase the CO2 reserve and have a protective effect on breathing. These data demonstrate that peripheral respiratory chemoreceptors are sufficient to stave off central apnoeas when the brainstem is perfused with low to no CO2.

Acute and chronic effects of carotid body denervation on ventilation and chemoreflexes in three rat strains

Brown Norway (BN) rats have a relatively specific deficit in CO2 sensitivity. This deficit could be due to an abnormally weak carotid body contribution to CO2 sensitivity. Accordingly, we tested the hypothesis that CBD would have less of an effect on eupnoeic breathing and CO2 sensitivity in the BN rats compared to other rat strains.We measured ventilation and blood gases at rest (eupnoea) and during hypoxia (FIO2 =0.12) or hypercapnia (FICO2 =0.07) before and up to 23 days after bilateral or Sham CBD in BN, Sprague–Dawley (SD) and Dahl Salt-Sensitive (SS) rats. In all three rat strains, CBD elicited eupnoeic hypoventilation (PaCO2 +8.7–11.0 mmHg) 1–2 days post-CBD (P <0.05), and attenuated ventilatory responses to hypoxia (P <0.05) and venous sodium cyanide (NaCN; P<0.05), while sham CBD had no effect on resting breathing, blood gases or chemoreflexes (P >0.05). In contrast, CBD had no effect on CO2 sensitivity (˙VE/PaCO2) in all strains (P>0.05). Eupnoeic PaCO2 returned to pre-CBD values within 15–23 days post-CBD. Thus, the effects of CBD in rats (1) further support an important role for the carotid bodies in eupnoeic blood gas regulation, (2) suggest that the carotid bodies are not a major determinant of CO2 sensitivity in rats, and (3) may not support the concept of an interaction among the peripheral and central chemoreceptors in rats.

Hypercapnia attenuates inspiratory amplitude and expiratory time responsiveness to hypoxia in vagotomized and vagal-intact rats

A negative influence of central chemosensitivity on peripheral chemoreflex response has been demonstrated recently in a decerebrate-vagotomized rat preparation in situ with separate carotid body and brainstem perfusions. Here, we report similar negative influences of hypercapnia on the hypoxic respiratory response in anesthetized, spontaneously breathing rats before and after vagotomy and anesthetized, artificially ventilated rats after vagotomy. Baseline breathing patterns and responsiveness to hypercapnia and hypoxia varied widely between the three respiratory modes. Despite this, the responses in inspiratory amplitude and expiratory duration (and hence respiratory frequency and neural ventilation) to hypoxia varied inversely with the background CO2 level in all three groups. Results demonstrate a hypoadditive hypercapnic-hypoxic interaction in vivo that resembles the hypoadditive central-peripheral chemoreceptor interaction in situ for these respiratory variables in the rat, regardless of differences in vagal feedback, body temperature and ventilation method. These observations stand in contrast to previous reports of hyperadditive peripheral-central chemoreceptor interaction.

Species differences in respiratory rhythm generation in rodents

We examined the role of riluzole (RIL)- and flufenamic acid (FFA)-sensitive mechanisms in respiratory rhythmogenesis in rats and hamsters using the in situ arterially perfused preparation. Based on the hypothesis that respiratory networks in animals capable of autoresuscitation would have a greater prevalence of membrane mechanisms that promote endogenous bursting, we predicted that older (weaned) hamsters (a hibernating species) would be more sensitive to the blockade of RIL- and FFA-sensitive mechanisms than age-matched rats and that younger (preweaned) rats would behave more like hamsters. Consistent with this, we found that respiratory motor output in weaned hamsters [>21 days postnatal (P21)] was highly sensitive to RIL (0.2–20 μM), while in young rats (P12–14) it was less so (only affected at higher concentrations of RIL), and weaned rats were not affected at all. On the other hand, respiratory motor output was equally reduced by FFA (0.25–25 μM) in both young and weaned rats but was unaffected in weaned hamsters. Coapplication of RIL and FFA (RIL + FFA) produced greater inhibition of respiration in both young and weaned rats compared with either drug alone. In contrast, in weaned hamsters, FFA coapplication offset the inhibitory effect of RIL alone. Increasing respiratory drive with hypercapnia/acidosis ameliorated the respiratory inhibition produced by RIL + FFA in weaned rats but had no effect in young rats. Data from the present study indicate that respiratory rhythmogenesis in young rats is more dependent on excitatory RIL-sensitive and FFA-sensitive mechanisms than older rats and that fundamental differences exist in the respiratory rhythmogenic mechanisms between rats and hamsters.

A negative interaction between brainstem and peripheral respiratory chemoreceptors modulates peripheral chemoreflex magnitude

Interaction between central (brainstem) and peripheral (carotid body) respiratory chemosensitivity is vital to protect blood gases against potentially deleterious fluctuations, especially during sleep. Previously, using an in situ arterially perfused, vagotomized, decerebrate preparation in which brainstem and peripheral chemoreceptors are perfused separately (i.e. dual perfused preparation; DPP), we observed that the phrenic response to specific carotid body hypoxia was larger when the brainstem was held at 25 Torr P(CO(2)) compared to 50 Torr P(CO(2)). This suggests a negative (i.e. hypo-additive) interaction between chemoreceptors. The current study was designed to (a) determine whether this observation could be generalized to all carotid body stimuli, and (b) exclude the possibility that the hypo-additive response was the simple consequence of ventilatory saturation at high brainstem P(CO(2)). Specifically, we tested how steady-state brainstem P(CO(2)) modulates peripheral chemoreflex magnitude in response to carotid body P(CO(2)) and P(O(2)) perturbations, both above and below eupnoeic levels. We found that the peripheral chemoreflex was more responsive the lower the brainstem P(CO(2)) regardless of whether the peripheral chemoreceptors received stimuli which increased or decreased activation. These findings demonstrate a negative interaction between brainstem and peripheral chemosensitivity in the rat in the absence of ventilatory saturation. We suggest that a negative interaction in humans may contribute to increased controller gain associated with sleep-related breathing disorders and propose that the assumption of simple addition between chemoreceptor inputs used in current models of the respiratory control system be reconsidered.

Chronic intermittent hypoxia reduces ventilatory long-term facilitation and enhances apnea frequency in newborn rats

Ventilatory long-term facilitation (LTF; defined as gradual increase of minute ventilation following repeated hypoxic exposures) is well described in adult mammals and is hypothesized to be a protective mechanism against apnea. In newborns, LTF is absent during the first postnatal days, but its precise developmental pattern is unknown. Accordingly, this study describes this pattern of postnatal development. Additionally, we tested the hypothesis that chronic intermittent hypoxia (CIH) from birth alters this development. LTF was estimated in vivo using whole body plethysmography by exposing rat pups at postnatal days 1, 4, and 10 (P1, P4, and P10) to 10 brief hypoxic cycles (nadir 5% O2) and respiratory recordings during the following 2 h (recovery, 21% O2). Under these conditions, ventilatory LTF (gradual increase of minute ventilation during recovery) was clearly expressed in P10 rats but not in P1 and P4. In a second series of experiments, rat pups were exposed to CIH during the first 10 postnatal days (6 brief cyclic exposures at 5% O2 every 6 min followed by 1 h under normoxia, 24 h a day). Compared with P10 control rats, CIH enhanced hypoxic ventilatory response (estimated during the hypoxic cycles) specifically in male rat pups. Ventilatory LTF was drastically reduced in P10 rats exposed to CIH, which was associated with higher apnea frequency during recovery. We conclude that CIH from birth enhances hypoxic chemoreflex and disrupts LTF development, thus likely contributing to increase apnea frequency.

Inspiratory activation is not required for episodic hypoxia-induced respiratory long-term facilitation in postnatal rats

Episodic hypoxia causes repetitive inspiratory activation that induces a form of respiratory plasticity termed long-term facilitation (LTF). While LTF is a function of the hypoxic exposures and inspiratory activation, their relative importance in evoking LTF is unknown. The aims of this study were to: (1) dissociate the relative roles played by episodic hypoxia and respiratory activation in LTF; and (2) determine whether the magnitude of LTF varies as a function of hypoxic intensity. We did this by examining the effects of episodic hypoxia in postnatal rats (15-25 days old), which unlike adult rats exhibit a prominent hypoxia-induced respiratory depression. We quantified inspiratory phrenic nerve activity generated by the in situ working-heart brainstem before, during and for 60 min after episodic hypoxia. We demonstrate that episodic hypoxia evokes LTF despite the fact that it potently suppresses inspiratory activity during individual hypoxic exposures (P < 0.05). Specifically, we show that after episodic hypoxia (three 5 min periods of 10% O2) respiratory frequency increased to 40 +/- 3.3% above baseline values over the next 60 min (P < 0.001). Continuous hypoxia (15 min of 10% O2) had no lasting effects on respiratory frequency (P > 0.05). To determine if LTF magnitude was affected by hypoxic intensity, the episodic hypoxia protocol was repeated under three different O2 tensions. We demonstrate that the magnitude and time course of LTF depend on hypoxic severity, with more intense hypoxia inducing a more potent degree of LTF. We conclude that inspiratory activation is not required for LTF induction, and that hypoxia per se is the physiological stimulus for eliciting hypoxia-induced respiratory LTF.

Ventilatory roll off during sustained hypercapnia is gender specific in pekin ducks

The objective of the present study was to examine the relative roles of peripheral versus central mechanisms in producing ventilatory adjustments in pekin ducks during prolonged (5 h) hypercapnia (5% inspired CO2), and to determine whether these adjustments differed between male and female ducks. After 20 min of CO2 exposure, intact ducks increased total ventilation (VE) 2.5-3-fold above control values, due to large increases (approximately 200%) in tidal volume (VT) and slightly smaller increases (approximately 140%) in breathing frequency (fR). This response was accompanied by respiratory acidosis (pHa fell from approximately 7.46 to approximately 7.41) and hypercapnia (PaCO2 increased from approximately 35 to approximately 40 Torr). In males, VE fell progressively thereafter due exclusively to a fall in fR, in parallel with a rapid partial recovery of pH (to 7.44) while PaCO2 continued to climb (to approximately 42 Torr). In females, VE remained elevated during hypercapnia, and no pH recovery occurred. This suggests that a respiratory decline resulting from acid-base compensation (probably due to HCO3- mobilization) occurred in males but not in females. Bicarbonate mobilization, and thus pH compensation, may have been reduced in females due to the CaCO3 requirements of eggshell formation. In males, the acute ventilatory response was reduced slightly by denervation of the carotid bodies or intrapulmonary chemoreceptors, but there was no effect of denervation of either receptor group on the responses to prolonged CO2. We conclude that pH compensation triggered by constant or increasing PaCO2, acting at central chemoreceptors, likely mediates the respiratory adjustments seen in male pekin ducks during hypercapnia. Furthermore, we suggest that this ventilatory response be considered a gender-specific hypercapnic ventilatory roll off, in the context of the various time domains of the hypercapnic ventilatory response.

Brainstem PCO2 modulates phrenic responses to specific carotid body hypoxia in an in situ dual perfused rat preparation

Inputs from central (brainstem) and peripheral (carotid body) respiratory chemoreceptors are coordinated to protect blood gases against potentially deleterious fluctuations. However, the mathematics of the steady-state interaction between chemoreceptors has been difficult to ascertain. Further, how this interaction affects time-dependent phenomena (in which chemoresponses depend upon previous experience) is largely unknown. To determine how central P(CO2) modulates the response to peripheral chemostimulation in the rat, we utilized an in situ arterially perfused, vagotomized, decerebrate preparation, in which central and peripheral chemoreceptors were perfused separately (i.e. dual perfused preparation (DPP)). We carried out two sets of experiments: in Experiment 1, we alternated steady-state brainstem P(CO2) between 25 and 50 Torr in each preparation, and applied specific carotid body hypoxia (60 Torr P(O2) and 40 Torr P(CO2)) under both conditions; in Experiment 2, we applied four 5 min bouts (separated by 5 min) of specific carotid body hypoxia (60 Torr P(O2) and 40 Torr P(CO2)) while holding the brainstem at either 30 Torr or 50 Torr P(CO2). We demonstrate that the level of brainstem P(CO2) modulates (a) the magnitude of the phrenic responses to a single step of specific carotid body hypoxia and (b) the magnitude of time-dependent phenomena. We report that the interaction between chemoreceptors is negative (i.e. hypo-additive), whereby a lower brainstem P(CO2) augments phrenic responses resulting from specific carotid body hypoxia. A negative interaction may underlie the pathophysiology of central sleep apnoea in populations that are chronically hypocapnic.

Ventilatory responses to isocapnic and poikilocapnic hypoxia in humans

We examined the hypoxic ventilatory response (HVR) including breathing frequency (f(R)) and tidal volume (V(T)) responses during 20 min of step isocapnic (IH) and poikilocapnic (PH) hypoxia (45 Torr). We hypothesized an index related to [Formula: see text] (pHPR) may be more robust during PH. Peak HVR was suppressed during PH (P<0.001), and mediated by V(T) during PH and both V(T) and f(R) during IH. The relative magnitude of HVD remained similar between conditions indicating a suppressive role of hypocapnia in development of the HVR unrelated to the degree of subsequent HVD, implying a primarily O(2) dependant mechanism. Post-hypoxic frequency decline was observed following both IH (3.4+/-3.7 bpm, P<0.05) and PH (3.6+/-3.1 bpm, P<0.01), despite no f(R) response during exposure to PH. Use of pHPR improved the signal to noise ratio during PH, though failed to detect the peak ventilatory response, and therefore may not be appropriate when describing peak responses.