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RESPIRATORY |
1 Department of Physiology, Anatomy and Genetics, University of Oxford, Sherrington Building, Parks Road, Oxford OX1 3PT, UK
2 School of Sport and Exercise Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
3 Department of Respiratory Medicine, University of Cambridge, Level 5, Box 157, Addenbrooke's Hospital, Hills Road, Cambridge CB2 2QQ, UK
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Abstract |
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Pmax) were made during euoxia and during a short period of isocapnic hypoxia. Subjects were also studied on two control days, when a saline infusion was substituted for trimetaphan. Trimetaphan had no effect on either euoxic ventilation or the sensitivity of ventilation to acute hypoxia. Trimetaphan significantly reduced Pmax in euoxia (P < 0.05), but had no significant effect on the sensitivity of Pmax to acute hypoxia once changes in cardiac output had been controlled for. The 8 h period of hypoxia elevated euoxic ventilation (P < 0.001) and Pmax (P < 0.001) and increased their sensitivities to acute hypoxia (P < 0.001 for both), indicating that significant acclimatization had occurred. Trimetaphan had no effect on the acclimatization response of any of these variables. We conclude that altered autonomic activity following 8 h of hypoxia does not underlie the acclimatization observed in ventilation or pulmonary vascular tone.
(Received 28 July 2006;
accepted after revision 28 November 2006;
first published online 30 November 2006)
Corresponding author P. A. Robbins: Department of Physiology, Anatomy and Genetics, University of Oxford, Sherrington Building, Parks Road, Oxford OX1 3PT, UK. Email: peter.robbins{at}physiol.ox.ac.uk
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Introduction |
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, pulmonary artery pressure (PAP), heart rate (HR) and cardiac output (CO) (Howard & Robbins, 1995; Dorrington et al. 1997; Clar et al. 2001). In addition, the sensitivity of some or all of these variables to brief exposures to acute hypoxia is increased following such a period of sustained hypoxia. Both of these kinds of responses are commonly subsumed under the heading ‘early acclimatization to hypoxia’, and the mechanisms underlying them remain obscure.
The autonomic nervous system plays a major role in regulating HR and CO, and a perhaps more subtle role in modulating 
and PAP. Both sympathetic and parasympathetic activities have been shown to change during exposure to hypoxia in humans (Petersen et al. 1974; Saito et al. 1988; Rowell et al. 1989). Furthermore, the elevation in sympathetic nervous system activity persists for a considerable period following relief from the hypoxic stimulus (Morgan et al. 1995). Chronic hypoxia causes a still more marked activation of both the parasympathetic and the sympathetic nervous system in healthy humans, despite a return towards normal of the arterial O2 content with acclimatization (Boushel et al. 2001; Calbet, 2003). In direct neurophysiological recording of sympathetic nerve activity, Hansen & Sander, (2003) found that acclimatization to high altitude is accompanied by marked elevations in resting levels of sympathetic neural discharge to the skeletal muscle vasculature. Furthermore, the high altitude-induced sympathetic activation persisted for days after return to sea level, demonstrating a long-lasting sympathetic neural overactivity following relief from hypoxia.
It has long been recognised that stimulation of the sympathetic nervous supply to the carotid body increases the frequency of sinus nerve chemosensory discharges (Floyd & Neil, 1952; Eyzaguirre & Lewin, 1961). More recently, two types of excitatory response have been distinguished, and it has been suggested that these relate to two distinct components of sympathetic innervation, one arising from the innervation of the vasculature and the other arising from direct sympathetic innervation of the type I cells of the carotid body (O'Regan, 1981). Using circulating and urinary noradrenaline concentrations or spillover as indirect measures of sympathetic nerve activity, a progressive increase in sympatho-excitation in humans during exposure to high altitude has been found, and this increase has been shown to correlate with the increase in 
(Asano et al. 1997).
An autonomic modulation of pulmonary vascular tone appears possible given that both sympathetic and parasympathetic branches of the autonomic nervous system innervate pulmonary vessels (Hebb, 1966; Downing & Lee, 1980). Levitzky et al. (1978) have shown in the dog that systemic arterial hypoxaemia induces a chemoreflex-mediated pulmonary vascular dilatation. A later study, employing experimental vagotomy in dogs, has suggested that the autonomic pulmonary vasodilator effect of systemic hypoxaemia has a parasympathetic origin (Wilson & Levitzky, 1989). However, other literature suggests that these effects are mediated through the pulmonary sympathetic supply (Kazemi et al. 1972; Murray et al. 1986; Brimioulle et al. 1997).
Based on the observations described above, in particular the persistence of an elevated sympathetic activity following the relief of hypoxia and the ability of the autonomic nervous system to influence 
and pulmonary vascular tone, we wished to test the hypothesis that persistently altered autonomic activity underlies part or all of early acclimatization to hypoxia. We studied the effects of a short period of exposure to isocapnic hypoxia on ventilation and the pulmonary circulation, both before and after conditioning with 8 h of hypoxia to induce acclimatization to hypoxia (first control). On a subsequent day, we repeated this sequence but we administered the short acting ganglion-blocking agent trimetaphan to abolish peripheral autonomic activity while we were testing the effects of acute exposure to isocapnic hypoxia on ventilation and the pulmonary circulation, before and after 8 h of hypoxia. On the third and final day, we undertook a second control sequence, the same as the first.
On the test day, because of its short duration of action, trimetaphan should not have been active during the period of sustained hypoxia, and therefore should not have prevented the acclimatization process from occurring. If it is the case that a persistent elevation in autonomic nervous activity is responsible for the acclimatization-induced increases in ventilatory and pulmonary vascular sensitivities to acute hypoxia, then the infusion of trimetaphan following sustained hypoxia would be expected to remove the effects of acclimatization. In this case, under the conditions of trimetaphan infusion, the ventilatory and pulmonary vascular responses to acute hypoxia should be the same before and after exposure to sustained hypoxia.
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Methods |
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Seven healthy volunteers (5 men, 2 women) with an age of 28 ± 4 years, height of 177.3 ± 8.9 cm, and weight of 71.1 ± 12.7 kg (means ± S.D.) participated in the study. Each gave informed consent before participating. The study was approved by the Oxfordshire Clinical Research Ethics Committee and performed in accordance with the Declaration of Helsinki. Each volunteer visited the laboratory twice before undertaking any of the main experimental protocols to become familiar with the laboratory and its procedures. On these visits we measured the normal end-tidal PCO2 (PET,CO2) of the participants and confirmed that they were suitable for echocardiographic measurement of tricuspid regurgitation.
Protocols
Each volunteer attended the laboratory for three main protocols. In the test protocol, trimetaphan was administered (Protocol T), which acts as a competitive inhibitor at the cholinergic binding sites in autonomic ganglia (Gilman et al. 1990). Trimetaphan also induces a degree of histamine release through an independent mechanism (McCubbin & Page, 1952), although this does not appear to play an important role in the haemodynamic responses to trimetaphan (Fahmy & Soter, 1985). The two other protocols were control protocols in which placebo was administered (Protocol C1 and Protocol C2). Each protocol lasted 1 day and the protocols were separated from each other by at least 1 week. The order of these protocols was fixed as C1, T and C2.
Participants reported to the laboratory at 08.00 h. After 30 min of supine rest, the participants performed a Valsalva manoeuvre with an expiratory strain of 40 mmHg for 20 s. The strain pressure during the Valsalva manoeuvre was monitored by a manometer. Typical changes in systemic arterial pressure (AP) and HR during the Valsalva manoeuvre were observed in all subjects before ganglion blockade (Fig. 1). In Protocol T, intravenous trimetaphan (trimetaphan camsylate, Cambridge Laboratories, Wallsend, UK) was administered at an initial rate of infusion of 2 mg min–1 after performance of the initial Valsalva manoeuvre. Three minutes after starting the infusion, the Valsalva manoeuvre was performed again to evaluate the HR and AP responses to changes in intrathoracic pressure. The infusion rate was increased incrementally until the tachycardia and AP recovery during phase II of the Valsalva manoeuvre, and overshoot in AP during phase IV of the Valsalva manoeuvre, were all abolished (Fig. 1). Once complete blockade was achieved, the infusion of trimetaphan was maintained at this peak dose throughout the time of exposure to acute hypoxia. The maximum rate of infusion used was 4 mg min–1. In Protocol C1 and C2, saline was infused instead. During the exposure to acute hypoxia, end-tidal PO2 (PET,O2) was held at 100 Torr in the first 10 min, then at 50 Torr for 20 min, and at 100 Torr again for the final 10 min. PET,CO2 was held constant at 2 Torr above each volunteer's normal baseline value throughout the exposures to acute hypoxia.
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At the end of the 8 h period of sustained hypoxia, the subject left the chamber and was left to breathe room air for 30 min. Following this, a second set of measurements was made of the responses to acute hypoxia. The procedure that was followed was identical to that used before the 8 h exposure to sustained hypoxia.
Control of end-tidal gases during the acute hypoxic exposure and respiratory measurements
During the acute hypoxic exposures, volunteers lay on their left side on a couch and breathed through a mouthpiece with the nose occluded. Ventilatory volumes were measured by a turbine volume-measuring device and flows by a pneumotachograph, both in series with the mouthpiece. Respired gases were sampled via a fine catheter close to the mouth and analysed continuously for PCO2 and PO2 by mass spectrometry. A pulse oximeter was used to monitor arterial O2 saturation. Inspired and expired volumes, end-inspiratory and end-expiratory PCO2 and PO2, and saturation were detected in real time by a computer and logged breath by breath. A dynamic end-tidal forcing system (Robbins et al. 1982) was used to control the end-tidal gases in the manner specified for the determination of the acute hypoxic ventilatory response. Before the start of each experiment, a cardiorespiratory model was used to construct a forcing function that contained the breath-by-breath values for inspiratory PCO2 and PO2 predicted to produce the desired end-tidal sequences. During the experiment, a computer-controlled gas-mixing system was used to generate this sequence in a modified manner. The modifications resulted from feedback control based on the deviations of the measured values for PET,CO2 and PET,O2 from their desired values.
Measurements of pulmonary blood pressure, cardiac output and systemic arterial pressure
The majority of people have detectable regurgitation during systole through their tricuspid valves. Doppler echocardiography can be used to detect the presence of this jet of blood and measure the velocity with which it moves back into the right atrium. On the assumption that the flow within the jet may be regarded as steady, Bernoulli's equation can be used to calculate the maximum pressure difference between the right ventricle and right atrium (Pmax) from the density of blood (
) and the peak velocity of the jet (v). This gives the relationship Pmax
=
v2/2. Assuming right atrial pressure remains constant, changes in Pmax will be equal to changes in the peak systolic PAP.
Echocardiographic measurements were performed using a Hewlett-Packard Sonos 5500 ultrasound machine with an S4 two-dimensional transducer (2–4 MHz). HR and respiratory waveform were also recorded.
A standard technique was used for the measurement of Pmax (Balanos et al. 2003). The tricuspid valve was visualized in an apical four-chamber view, and then colour Doppler format allowed the regurgitant jet to be detected. After proper alignment of the probe, continuous wave spectral analysis at a sweep speed of 50 mm s–1 was used to record the velocity profile of the jet. During analysis, the maximal velocity of the jet was measured using an electronic caliper tool. Pmax values were then computed automatically.
In order to measure CO, the flow through the aortic valve during systole was identified using an apical five-chamber view. The velocity profile was then obtained in pulsed wave spectral mode at a display screen sweep speed of 100 mm s–1. Doppler sampling of the flow was made just below the orifice of the aortic valve. An automated procedure was then used to calculate the velocity time integral (VTI) of the flow for each ventricular contraction. The diameter (D) of the aortic valve was measured from a parasternal long axis view. From these data, CO was calculated as HR x VTI x
x (D/2)2.
Systemic arterial pressure was measured continuously using finger plethysmography (Portapres, TNO-TPD Biomedical Instrumentation, the Netherlands) during Valsalva manoeuvres and the exposures to acute hypoxia.
Eight hour sustained exposure to hypoxia
The sustained hypoxic exposure was conducted using a purpose-built chamber, which was large enough to allow subjects to sit or stand comfortably. Subjects were studied individually in the chamber so that the inspiratory gas composition could be tailored to regulate their end-tidal gas composition accurately. Respired gas was sampled via a fine nasal catheter at the opening of the subject's nostril. The samples were continually analysed for PCO2 and PO2. Arterial O2 saturation was monitored using a pulse oximeter. A computer sampled the PCO2, PO2 and saturation signals at a frequency of 100 Hz. It detected end-inspiratory and end-expiratory values for PCO2 and PO2 and recorded these breath by breath along with values for saturation. The desired values for PET,CO2 and PET,O2 were entered into the computer. The computer then automatically adjusted the composition of the gas in the chamber every 5 min, or at manually overridden intervals, to minimize the error between desired and actual values for the end-tidal gases. This system has been described in detail elsewhere (Howard et al. 1995). The subject was kept under continuous observation throughout the 8 h exposure. In addition, an environmental oxygen monitor was employed within the chamber as a further safety precaution.
Modelling of hypoxic ventilatory responses
To obtain numerical values for the sensitivity of 
to the acute hypoxic exposure, a respiratory model was fitted to the data. The particular respiratory model employed was Model I of those described by Liang et al. (1997). This model separates the total ventilation, 
, into a central, hypoxia-independent component, 
, and a peripheral, hypoxia-dependant component, 
, and may be written:
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In order to allow for the autocorrelation that exists between measurements of ventilation on successive breaths, a model of the noise processes was fitted to the data in parallel with the fitting of the model of the ventilatory response to hypoxia (Liang et al. 1996). The model parameters were estimated by fitting the models to the data using a standard subroutine to minimize the sum of squares of the residuals (subroutine E04FDF, Numerical Algorithms Group, Oxford).
Modelling of pulmonary vascular responses
To quantify fully the pulmonary vascular responses to acute hypoxia observed in this study, a mathematical model was fitted to the data. In this model, it was supposed that the dynamics of the response could be represented by a simple, linear, first order process and that the changes in Pmax could be related in a linear manner to changes in saturation, at least of the range for PO2 values studied. Given these assumptions, a simple difference equation may be written of the form:
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t, S is the saturation (Severinghaus, 1979), P100 is the steady-state value for Pmax in the absence of hypoxia (i.e. when saturation is 100%), GP is the sensitivity of Pmax to a decrease in saturation and TP is the time constant for the response. The parameters of this model (P100, GP and TP) were estimated using the function LSQNONLIN in Matlab (version 7.0, The MathWorks, Inc., Natick, MA, USA) to minimize the sum of squares of the residuals. Quantification of systemic cardiovascular responses
The responses of the systemic circulation to acute hypoxia were not sufficiently well defined to quantify through a model fitting process. Accordingly simple averages of these responses were calculated under the conditions of euoxia and hypoxia.
Statistical analysis
A full factorial ANOVA was used to assess the statistical significance of the results, with fixed factors of Protocol (control protocols (Protocols C1, C2) versus Protocol T) and Time (before the sustained hypoxic exposure (AM measurements) versus after the sustained hypoxic exposure (PM measurements)) together with a random factor of Subject. The null hypothesis of the experiment was that infusion of trimetaphan would not abrogate the acclimatization observed after sustained hypoxia. The acceptance or rejection of this null hypothesis was assessed through the significance or otherwise of the interactive term between Protocol and Time.
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Results |
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Figure 2A illustrates the average time course for PET,O2 and PET,CO2 across all volunteers for both exposures to acute hypoxia in each protocol. The acute hypoxic stimulus was generated accurately in all cases. The background level of PET,CO2 was held very constant throughout each exposure. Overall, no discernable differences among the three protocols could be detected.
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Figure 2B illustrates the average responses for 
to acute hypoxia before (AM) and after (PM) exposure to the 8 h period of sustained hypoxia. Table 1 lists the parameters obtained from the model fitting. From these results, it is clear that trimetaphan had no discernable effect on the ventilatory response to acute hypoxia. Significant acclimatization to sustained hypoxia was evident in all three protocols, both through a significant increase in 
in euoxia (corresponding to parameter 
of the model) and through a significant increase in the acute ventilatory sensitivity to hypoxia (corresponding to parameter G100 of the model) in the PM measurements as compared with the AM measurements. The presence of trimetaphan in Protocol T caused no discernable reduction in the degree of acclimatization observed.
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Figure 2C illustrates the average responses for Pmax to acute hypoxia before (AM) and after (PM) exposure to 8 h period of sustained hypoxia. Table 2 lists the parameters obtained from the model fitting process. Trimetaphan significantly reduced Pmax under conditions of euoxia (corresponding to parameter P100 of the model) and significantly attenuated the sensitivity of Pmax to acute hypoxia (corresponding to parameter GP of the model). A significant acclimatization following the 8 h period of sustained hypoxia was observed through the increase in the baseline value for Pmax in the absence of hypoxia (corresponding to parameter P100 of the model) and through the increase in sensitivity of Pmax to acute hypoxia (corresponding to parameter GP of the model) in the PM values as compared with the AM values. No significant abrogation of these effects of sustained hypoxia was observed through the administration of trimetaphan in Protocol T.
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The systemic cardiovascular responses are illustrated in Fig. 3 and the average values for the various variables under both euoxic and hypoxic conditions are listed in Table 3. As expected, autonomic blockade with trimetaphan: (i) elevated HR in euoxia; (ii) abolished the normal increase in HR observed with acute hypoxia and (iii) reduced systemic vascular resistance (SVR), calculated as (mean AP)/CO. Trimetaphan did not block the reduction observed in SVR with hypoxia. In euoxia, these effects led to the expected reduction in mean AP and increase in CO. At the induction of hypoxia, the effects were associated with a loss of the normal increase in CO that occurs with acute hypoxia and also with a fall in mean AP distinct from the slight elevation in mean AP that was observed with acute hypoxia in the absence of trimetaphan.
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Discussion |
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Study design
Our study used control hypoxia protocols both before and after the trimetaphan hypoxia protocol. As there was no euoxia control protocol in the study design, in theory it is possible that there is a significant ‘time of day’ effect incorporated within the acclimatization effect. In practice we have conducted a considerable number of other studies involving 8 h exposures to hypoxia, which have included euoxia control protocols, and in none of these controls have we found a significant time of day effect (Howard & Robbins, 1995; Tansley et al. 1997; Fatemian & Robbins, 1998; Clar et al. 1999, 2000; Ren et al. 2000).
The effect of trimetaphan on the ventilatory response to hypoxia
The present study found no effect of ganglion blockade on either the ventilatory sensitivity to acute hypoxia or the acclimatization-induced changes in respiratory control. We are unaware of any other studies in humans of the effects of either complete ganglionic blockade, or indeed 
-receptor blockade, on these ventilatory responses to hypoxia. However, our results are consistent with a number of previous studies examining blockade of other specific components of the autonomic system. In particular, Clar et al. (1999) found no effect of 
-blockade with propranolol on ventilatory acclimatization to 8 h of hypoxia and, similarly, Clar et al. (2001) found no effect of parasympathetic blockade with glycopyrrolate. In neither of these studies was any effect detected on the acute ventilatory response to hypoxia – a finding consistent with the earlier results of Koller et al. (1988).
In anaesthetized cats, carotid-body sympathectomy did not affect carotid-chemoreceptor afferent activity during the first 10 min of hypoxia, but thereafter chemoreceptor discharge was augmented compared with controls (Prabhakar & Kou, 1994). However, in awake goats, ventilation and the hypoxic-ventilatory response were found to be attenuated at 1 and 4 week after carotid-body sympathectomy (Ryan et al. 1995). In that study, carotid-body sympathectomy was found to have no effect on ventilatory acclimatization to hypoxia.
Overall, whilst it is recognized that alterations in autonomic activity can undoubtedly modify respiratory control (Floyd & Neil, 1952; Eyzaguirre & Lewin, 1961), there is little evidence to suggest that either a normal level of autonomic activity, or an altered level of autonomic activity, such as can be induced by the level and duration of hypoxia in our experiments, affects respiratory control in humans.
The effect of trimetaphan on pulmonary vascular responses to hypoxia
The pulmonary circulation is innervated by both adrenergic and cholinergic nerves (Hebb, 1966; Downing & Lee, 1980). The cholinergic system appears to be vasodilatory (Nandiwada et al. 1983). Within the adrenergic system, stimulation of the receptors causes vasoconstriction and stimulation of the receptors causes vasodilatation (Silove & Grover, 1968; Howard et al. 1975; Rubin & Lazar, 1983). Numerically and functionally, the receptors tend to dominate such that sympathetic stimulation causes vasoconstriction and sympathetic blockade causes vasodilatation (Porcelli & Bergofsky, 1973). Overall, ganglionic blockade would be expected to exert both vasodilatory and vasoconstrictive influences within the lung, consistent with the variable nature of overall response observed in experimental animals (Murray et al. 1986; Lodato et al. 1988). In the present human study, the net effect of blockade was vasodilatory.
In the present study, trimetaphan caused a modest reduction in the pulmonary vascular sensitivity to acute hypoxia. One possibility is that this finding is an artifact because trimetaphan also removed the normal rise in CO that occurs with the onset of acute hypoxia. Using a value from the literature relating the increase in Pmax to the increase in CO (Balanos et al. 2005), the values obtained for the increase in Pmax with acute hypoxia may be corrected for the effect of any concomitant change in CO. Once this correction is made, the significance of the effect of trimetaphan on the sensitivity of the pulmonary vasculature to hypoxia is lost. This finding is consistent with that of Lodato et al. (1988) in conscious dogs, who concluded that, once the confounding effects of changes in CO had been controlled for, neither sympathetic blockade nor total autonomic blockade (with hexamethonium) affected the pulmonary vascular sensitivity to acute hypoxia.
In the present studies, ganglionic blockade did not abrogate the increase in basal tone and sensitivity to hypoxia induced by acclimatization with 8 h of hypoxia. We are unaware of any previous studies with which this may be reasonably compared.
The effect of trimetaphan on cardiovascular responses to hypoxia
As anticipated, autonomic blockade with trimetaphan reduced SVR and increased HR, with a consequential rise in CO and fall in mean AP. Trimetaphan blocked the increase in HR that normally occurs with the induction of acute hypoxia. This effect of trimetaphan is very similar to that observed with blockade of just the parasympathetic component using muscarinic antagonists (Petersen et al. 1974; Clar et al. 2001). Trimetaphan did not, however, block the reduction in SVR with acute hypoxia consistent with hypoxia acting as a local vasodilator (Weisbrod et al. 2001). As a result, a further rise in CO and fall in AP occurred.
Sustained hypoxia of 8 h duration induced a rise in HR under subsequent euoxic conditions and a rise in the sensitivity of HR to acute hypoxia that was similar to that reported in previous studies (Clar et al. 2000; Clar et al. 2001). Much of this was abrogated with the acute infusion of trimetaphan in Protocol T, consistent with the previous report that much of the change in HR following sustained hypoxia arises through alterations in parasympathetic function (Clar et al. 2001). Possible causes of any small elevation in HR following 8 h of hypoxia that persists during trimetaphan infusion include both alterations in humoral factors and alterations in intrinsic factors induced by hypoxia, as well as the possibility that it somehow reflects an increase in the work of breathing caused by the more vigorous ventilatory response. Longer-term studies of hypoxia report increases in both parasympathetic and sympathetic activities in regulating HR, with the result that HR in chronic hypoxia is only mildly elevated (Boushel et al. 2001; Calbet, 2003; Hansen & Sander, 2003).
Of note in this study was the absence of any conditioning effect of sustained hypoxia on SVR. This appears to be somewhat at variance with the results of Dorrington et al. (1997) where SVR appeared to remain below the control euoxic value for at least 2 h following the relief of the sustained hypoxia. In the absence of any conditioning effect of the sustained hypoxia on SVR, the hypothesis that trimetaphan might modify such conditioning immediately fails.
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Footnotes |
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References |
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) and after (
) sustained hypoxic exposure.