Can intravenous endothelin-1 be used to enhance hypoxic pulmonary vasoconstriction in healthy humans?

BACKGROUND: Hypoxic pulmonary vasoconstriction (HPV) helps match pulmonary perfusion to ventilation. The peptide endothelin-1 (ET-1) may be involved in the cellular mechanisms of this response. We hypothesized that increasing plasma ET-1 concentration during hypoxia would enhance HPV in humans and might represent a strategy for improving gas exchange during single-lung anaesthesia or respiratory disease. METHODS: Nine healthy volunteers were each exposed twice to a 7-h protocol consisting of 1 h breathing air, 4 h of eucapnic hypoxia (end-tidal Po(2), 50 mm Hg), and 2 h of eucapnic euoxia (end-tidal Po(2), 100 mm Hg). Volunteers received a 7-h i.v. infusion of ET-1 during one protocol (1.0-2.5 ng kg(-1) min(-1)) and normal saline during the other. At intervals of 30-60 min, cardiac output and the maximum tricuspid pressure gradient during systole (DeltaP(max), an index of HPV) were measured using Doppler echocardiography, systemic arterial pressure was measured using sphygmomanometry, and plasma samples were obtained to determine ET-1 concentration. RESULTS: During hypoxia, DeltaP(max) increased for around 2 h before reaching a plateau. Compared with saline, ET-1 had no effect on DeltaP(max), either at baseline or during hypoxia. ET-1 infusion slightly increased diastolic arterial pressure and reduced cardiac output, but had no specific effect on the change in these variables during hypoxia. During the final 1 h of hypoxia, plasma ET-1 concentration was 1.7 (0.4) pg ml(-1) [mean (sd)] in the saline protocol and 21.9 (12.2) pg ml(-1) in the ET-1 protocol. CONCLUSIONS: ET-1 infusion seems unlikely to represent a therapeutic strategy for enhancing HPV during acute (<4 h) hypoxia.

Mutation of the von Hippel-Lindau gene alters human cardiopulmonary physiology.

Intracellular responses to hypoxia are coordinated by the von Hippel-Lindau--hypoxia-inducible factor (VHL-HIF) transcriptional system. This study investigated the potential role of the VHL-HIF pathway in human systems-level physiology. Patients diagnosed with Chuvash polycythaemia, a rare disorder in which VHL signalling is specifically impaired, were studied during acute hypoxia and hypercapnia. Subjects breathed through a mouthpiece and ventilation was measured while pulmonary vascular tone was assessed echocardiographically. The patients were found to have elevated basal ventilation and pulmonary vascular tone, and ventilatory, pulmonary vasoconstrictive and heart rate responses to acute hypoxia were greatly increased, as were heart rate responses to hypercapnia. The patients also had abnormal pulmonary function on spirometry. This study's findings demonstrate that the VHL-HIF signalling pathway, which is so central to intracellular oxygen sensing, also regulates the organ systems upon which cellular oxygen delivery ultimately depends.

Non-dimensional quantification of the interactions between hypoxia, hypercapnia and exercise on ventilation in humans.

The purpose of this study was to develop a non-dimensional approach towards the description of interaction between the three respiratory stimuli of hypoxia, hypercapnia and exercise and to use this approach to quantify the relative strengths of their interactions. Only a part of the study related to the overall interaction of the three stimuli is presented here. Nine volunteers took part in the study and their ventilatory responses to hypoxia were measured under four different conditions of rest-eucapnia, rest-hypercapnia, exercise-eucapnia and exercise-hypercapnia. Non-dimensional linear functions of hypercapnia (x), hypoxia (y) and exercise (z) were defined such that a value of one would double the resting ventilation. Non-dimensional ventilation v was derived as: v(x,y,z) = 1+ x + y + z + g1xy + g2xz + g3yz + g4xyz, where g1, g2, g3 and g4 provide non-dimensional measures of the strength of stimulus interaction. These interactions were calculated from the parameters obtained by fitting simple respiratory models to the data. The values for g1, g3 and g4 were significantly different from zero (p < 0.05, t-test). An intriguing result of this study is the overall negative interaction of the three stimuli, which may suggest that the linear, stimulus-response models commonly used to describe respiratory data may not be adequate for describing these complex interactions.

Release by hypoxia of a soluble vasoconstrictor from rabbit small pulmonary arteries.

BACKGROUND: Soluble pulmonary vasoconstrictors released in response to hypoxia have been reported in pig and rat preparations, but not in rabbit preparations. METHODS: We used myography to evaluate the contribution of a soluble factor to constriction in rabbit small pulmonary arteries (external diameter 300-475 microm) exposed to 45 min hypoxia (PO(2)=9 mm Hg). RESULTS: Hypoxia produced gradually intensifying constriction. Return to euoxia (PO(2)=145 mm Hg) for 30 min relaxed only approximately 30% of the constriction, whereas elution of the myograph bath yielded full relaxation. Reapplication of the eluent gradually restored the constriction to its pre-elution level over a 30-min period. CONCLUSIONS: In this closed system, a soluble factor contributes substantially to hypoxic pulmonary vasoconstriction.

Effects of 8 h of isocapnic hypoxia with and without muscarinic blockade on ventilation and heart rate in humans.

This study examined the role of muscarinic parasympathetic mechanisms in generating the progressive increases in ventilation (V(E)) and heart rate previously reported with 8 h exposures to hypoxia. The sensitivities of V(E) (G(p)) and heart rate (G(HR)) to acute variations in hypoxia, and V(E) and heart rate during acute hyperoxia were assessed in 10 subjects before and after two 8 h exposures to isocapnic hypoxia (end-tidal P(O2) = 50 mmHg). The responses were measured during muscarinic blockade with glycopyrrolate (0.015 mg kg(-1)) and without glycopyrrolate, as a control. There were significant increases in G(p) (P < 0.01) and V(E) during hyperoxia (P < 0.01) following hypoxic exposure, but these were unaffected by glycopyrrolate. G(HR) increased significantly by 0.29 +/- 0.08 beats min(-1) %(-1) (mean +/- S.E.M.) following exposure to hypoxia under control conditions, but only non-significantly by 0.10 +/- 0.08 beats min(-1) %(-1) with glycopyrrolate. This difference was significant. Changes in heart rate during hyperoxia were slight and inconclusive. We conclude that muscarinic mechanisms play little role in the progressive ventilatory changes that occur over 8 h of hypoxia, but that they do mediate much of the progressive increase in heart rate. Experimental Physiology (2001) 86.4, 529-538.

Very mild exposure to hypoxia for 8 h can induce ventilatory acclimatization in humans.

Ventilatory acclimatization to altitude is associated with a progressive increase in ventilation, a progressive decrease in end-tidal PCO2 and a progressive increase in the acute ventilatory sensitivity to hypoxia. Ventilatory acclimatization has been observed with mild exposure to hypoxia when the duration of exposure has been of some length (e.g. days), and with shorter duration exposures (e.g. 8 h) when the degree of hypoxia has been more severe. The purpose of this study was to determine whether short-duration exposures to very mild hypoxia, such as are commonly associated with the reduction in cabin pressure during commercial airline flight, can also induce some degree of ventilatory acclimatization. Twelve subjects were exposed in a chamber to both 8 h mild hypoxia (inspired PO2 127 mmHg) and 8 h air-breathing as a control. Exposures were on different days in random order. Following the hypoxic exposure, there was a significant reduction in end-tidal PCO2 during air breathing (from 39.2+/-1.8 to 38.11+/-1.5 mmHg, mean +/- SD, P<0.05), and a significant increase in ventilatory sensitivity to hypoxia (from 0.84+/-0.54 l/min/% to 1.13+/-0.66 l/min/%, P<0.05). We conclude that shortterm exposures to very mild hypoxia do induce significant acclimatization within the respiratory control system.

Respiratory control in humans after 8 h of lowered arterial PO2, hemodilution, or carboxyhemoglobinemia.

In humans exposed to 8 h of isocapnic hypoxia, there is a progressive increase in ventilation that is associated with an increase in the ventilatory sensitivity to acute hypoxia. To determine the relative roles of lowered arterial PO2 and oxygen content in generating these changes, the acute hypoxic ventilatory response was determined in 11 subjects after four 8-h exposures: 1) protocol IH (isocapnic hypoxia), in which end-tidal PO2 was held at 55 Torr and end-tidal PCO2 was maintained at the preexposure value; 2) protocol PB (phlebotomy), in which 500 ml of venous blood were withdrawn; 3) protocol CO, in which carboxyhemoglobin was maintained at 10% by controlled carbon monoxide inhalation; and 4) protocol C as a control. Both hypoxic sensitivity and ventilation in the absence of hypoxia increased significantly after protocol IH (P < 0.001 and P < 0.005, respectively, ANOVA) but not after the other three protocols. This indicates that it is the reduction in arterial PO2 that is primarily important in generating the increase in the acute hypoxic ventilatory response in prolonged hypoxia. The associated reduction in arterial oxygen content is unlikely to play an important role.

Selected contribution: chemoreflex responses to CO2 before and after an 8-h exposure to hypoxia in humans.

The ventilatory sensitivity to CO2, in hyperoxia, is increased after an 8-h exposure to hypoxia. The purpose of the present study was to determine whether this increase arises through an increase in peripheral or central chemosensitivity. Ten healthy volunteers each underwent 8-h exposures to 1) isocapnic hypoxia, with end-tidal PO2 (PET(O2)) = 55 Torr and end-tidal PCO2 (PET(CO2)) = eucapnia; 2) poikilocapnic hypoxia, with PET(O2) = 55 Torr and PET(CO2) = uncontrolled; and 3) air-breathing control. The ventilatory response to CO2 was measured before and after each exposure with the use of a multifrequency binary sequence with two levels of PET(CO2): 1.5 and 10 Torr above the normal resting value. PET(O2) was held at 250 Torr. The peripheral (Gp) and the central (Gc) sensitivities were calculated by fitting the ventilatory data to a two-compartment model. There were increases in combined Gp + Gc (26%, P < 0.05), Gp (33%, P < 0.01), and Gc (23%, P = not significant) after exposure to hypoxia. There were no significant differences between isocapnic and poikilocapnic hypoxia. We conclude that sustained hypoxia induces a significant increase in chemosensitivity to CO2 within the peripheral chemoreflex.

Methods for averaging irregular respiratory flow profiles in awake humans.

Respiratory flow profiles have been of interest as an output of the respiratory controller. In determining average flow profiles, however, previous methods that align individual breaths in the time domain are susceptible to distortions caused by the great variability, both between breaths and within breaths. We aimed to develop a method for determining typical flow profiles that circumvents such distortions. Our method aligns different breaths by phase of respiratory cycle, which is defined as the angle associated with the point on the normalized flow-volume diagram (a phase-plane plot). Over a number of breaths, median values for flow, volume, and elapsed time from the start of the breath at each phase angle are determined. Because these estimates are mutually semi-independent and in general violate the laws of mass balance, an adjustment was performed such that the volume was precisely the time integral of the flow. The method produced typical flow profiles with characteristics that were significantly closer to the mean values obtained from the individual cycles than those obtained by the technique of Benchetrit and co-workers (Benchetrit G, Shea SA, Dinh TP, Bodocco S, Baconnier P, and Guz A, Respir Physiol 75: 199-210, 1989), which reconstructs the typical flow profile from Fourier coefficients.

Is ventilatory acclimatization to hypoxia a phenomenon that arises through mechanisms that have an intrinsic role in the regulation of ventilation at sea level?

The purpose of this article is to set out the hypothesis that arterial PO2 may play a significant role in the regulation of breathing at sea level. The following points are made: 1) Although CO2 is clearly the dominant feedback signal in the acute setting, there is evidence, particularly clinical observation, that the ventilatory response to CO2 may adapt. 2) Although the ventilatory response to an acute variation in alveolar PO2 around sea-level values is feeble, studies at altitude have shown that over longer-time periods alveolar PO2 is a more powerful regulator of ventilation. 3) Recent evidence suggests that mechanisms associated with ventilatory acclimatization to hypoxia are active at sea-level values for PO2, and indeed affect the acute ventilatory response to hypoxia. 4) While most evidence suggests that the peripheral and central chemoreflexes are independent and additive in their contributions to ventilation, experiments over longer durations suggest that peripheral chemoreceptor afferents may play an important role in regulating central chemoreflex sensitivity to CO2. This is potentially an important mechanism by which oxygen can alter the acute chemoreflex responses to CO2. In conclusion, the mechanisms underlying ventilatory acclimatization to hypoxia may have an important role in regulating the respiratory system at sea level.

Cardiovascular effects of 8 h of isocapnic hypoxia with and without beta-blockade in humans.

This study seeks to confirm the progressive changes in cardiac output and heart rate previously reported with 8 h exposures to constant hypoxia, and to examine the role of sympathetic mechanisms in generating these changes. Responses of ten subjects to four 8 h protocols were compared: (1) air breathing with placebo; (2) isocapnic hypoxia (end-tidal PO2 = 50 mm Hg) with placebo; (3) isocapnic hypoxia with beta-blockade; and (4) air breathing with beta -blockade. Regular measurements of heart rate and cardiac output (using ultrasonography and N2O rebreathing techniques) were made with subjects seated in the upright position. The sensitivity of heart rate to rapid variations in hypoxia (GHR) and heart rate in the absence of hypoxia were measured at times 0, 4 and 8 h. No significant progressive effect of hypoxia on cardiac output was detected. There was a gradual rise in heart rate with hypoxia of 11+/-2 beats min(-1) in the placebo protocol and of 10+/-2 beats min(-1) in the beta-blockade protocol over 8 h, compared to the air breathing protocols. The rise in heart rate was progressive (P<0.001) and accompanied by progressive increases in both GHR (P<0.001) and heart rate measured in the absence of hypoxia (P<0.05). No significant effect of beta-blockade was detected on any of these progressive changes. We conclude that sympathetic mechanisms that act via beta -receptors play little role in the progressive changes in heart rate observed over 8 h of moderate hypoxia.

Changes in respiratory control in humans induced by 8 h of hyperoxia.

In humans, 8 h of isocapnic hypoxia causes a progressive rise in ventilation associated with increases in the acute ventilatory responses to hypoxia (AHVR) and hypercapnia (AHCVR). To determine whether 8 h of hyperoxia causes the converse of these effects, three 8-h protocols were compared in 14 subjects: 1) poikilocapnic hyperoxia, with end-tidal PO(2) (PET(O(2))) = 300 Torr and end-tidal PCO(2) (PET(CO(2))) uncontrolled; 2) isocapnic hyperoxia, with PET(O(2)) = 300 Torr and PET(CO(2)) maintained at the subject's normal air-breathing level; and 3) control. Ventilation was measured hourly. AHVR and AHCVR were determined before and 0.5 h after each exposure. During isocapnic hyperoxia, after an initial increase, ventilation progressively declined (P < 0.01, ANOVA). After exposure to hyperoxia, 1) AHVR declined (P < 0.05); 2) ventilation at fixed PET(CO(2)) decreased (P < 0.05); and 3) air-breathing PET(CO(2)) increased (P < 0.05); but 4) no significant changes in AHCVR or intercept were demonstrated. In conclusion, 8 h of hyperoxia have some effects opposite to those found with 8 h of hypoxia, indicating that there may be some "acclimatization to hypoxia" at normal sea-level values of PO(2).

Effects of desferrioxamine on serum erythropoietin and ventilatory sensitivity to hypoxia in humans.

In cell culture, hypoxia stabilizes a transcriptional complex called hypoxia-inducible factor-1 (HIF-1) that increases erythropoietin (Epo) formation. One hallmark of HIF-1 responses is that they can be induced by iron chelation. The first aim of this study was to examine whether an infusion of desferrioxamine (DFO) increased serum Epo in humans. If so, this might provide a paradigm for identifying other HIF-1 responses in humans. Consequently a second aim was to determine whether an infusion of DFO would mimic prolonged hypoxia and increase the acute hypoxic ventilatory response (AHVR). Sixteen volunteers undertook two protocols: 1) continuous infusion of DFO over 8 h and 2) control. Epo and AHVR were measured at fixed times during and after the protocols. The results show that 1) compared with control, Epo increased in most subjects at 8 h [52.8 +/- 57.7 vs. 6.9 +/- 2.5 (SD) mIU/ml, for DFO = 4 g/70 kg body wt, P < 0.05] and 12 h (63.7 +/- 76.3 vs. 7.3 +/- 2.5 mIU/ml, P < 0.001) after the start of DFO administration and 2) DFO had no significant effect on AHVR. We conclude that, whereas infusions of DFO mimic hypoxia by increasing Epo, they do not mimic prolonged hypoxia by augmenting AHVR.

Peripheral chemoreflex function in hyperoxia following ventilatory acclimatization to altitude.

After a period of ventilatory acclimatization to high altitude (VAH), a degree of hyperventilation persists after relief of the hypoxic stimulus. This is likely, in part, to reflect the altered acid-base status, but it may also arise, in part, from the development during VAH of a component of carotid body (CB) activity that cannot be entirely suppressed by hyperoxia. To test this hypothesis, eight volunteers undergoing a simulated ascent of Mount Everest in a hypobaric chamber were acutely exposed to 30 min of hyperoxia at various stages of acclimatization. For the second 10 min of this exposure, the subjects were given an infusion of the CB inhibitor, dopamine (3 microg. kg(-1). min(-1)). Although there was both a significant rise in ventilation (P < 0.001) and a fall in end-tidal PCO(2) (P < 0.001) with VAH, there was no progressive effect of dopamine infusion on these variables with VAH. These results do not support a role for CB in generating the persistent hyperventilation that remains in hyperoxia after VAH.

Methodological and physiological variability within the ventilatory response to hypoxia in humans.

Measurement of the acute hypoxic ventilatory response (AHVR) requires careful choice of the hypoxic stimulus. If the stimulus is too brief, the response may be incomplete; if the stimulus is too long, hypoxic ventilatory depression may ensue. The purpose of this study was to compare three different techniques for assessing AHVR, using different hypoxic stimuli, and also to examine the between-day variability in AHVR. Ten subjects were studied, each on six different occasions, which were >/=1 wk apart. On each occasion, AHVR was assessed using three different protocols: 1) protocol SW, which uses square waves of hypoxia; 2) protocol IS, which uses incremental steps of hypoxia; and 3) protocol RB, which simulates an isocapnic rebreathing test. Mean values for hypoxic sensitivity were 1.02 +/- 0.48, 1.15 +/- 0.55, and 0.93 +/- 0.60 (SD) l. min(-1). %(-1) for protocols SW, IS, and RB, respectively. These differed significantly (P < 0.01). The coefficients of variation for measurement of AHVR were 20, 23, and 36% for the three protocols, respectively. These were not significantly different. There was a significant physiological variation in AHVR (F (50,100) = 3.9, P < 0. 001), with a coefficient of variation of 26%. We conclude that there was relatively little systematic variation between the three protocols but that AHVR varies physiologically over time.