Arterial P CO2 and lung ventilation in man exposed to 1-5% CO2 in the inspired gas

Integrated respiratory chemoreflex-mediated regulation of cerebral blood flow in hypoxia: Implications for oxygen delivery and acute mountain sickness

NEW FINDINGS

What is the central question of this study? To what extent do hypoxia-induced changes in the peripheral and central respiratory chemoreflex modulate anterior and posterior cerebral oxygen delivery, with corresponding implications for susceptibility to acute mountain sickness? What is the main finding and its importance? We provide evidence for site-specific regulation of cerebral blood flow in hypoxia that preserves oxygen delivery in the posterior but not the anterior cerebral circulation, with minimal contribution from the central respiratory chemoreflex. External carotid artery vasodilatation might prove to be an alternative haemodynamic risk factor that predisposes to acute mountain sickness.

ABSTRACT

The aim of the present study was to determine the extent to which hypoxia-induced changes in the peripheral and central respiratory chemoreflex modulate anterior and posterior cerebral blood flow (CBF) and oxygen delivery (CDO₂ ), with corresponding implications for the pathophysiology of the neurological syndrome, acute mountain sickness (AMS). Eight healthy men were randomly assigned single blind to 7 h of passive exposure to both normoxia (21% O₂ ) and hypoxia (12% O₂ ). The peripheral and central respiratory chemoreflex, internal carotid artery, external carotid artery (ECA) and vertebral artery blood flow (duplex ultrasound) and AMS scores (questionnaires) were measured throughout. A reduction in internal carotid artery CDO₂ was observed during hypoxia despite a compensatory elevation in perfusion. In contrast, vertebral artery and ECA CDO₂ were preserved, and the former was attributable to a more marked increase in perfusion. Hypoxia was associated with progressive activation of the peripheral respiratory chemoreflex (P < 0.001), whereas the central respiratory chemoreflex remained unchanged (P > 0.05). Symptom severity in participants who developed clinical AMS was positively related to ECA blood flow (Lake Louise score, r = 0.546-0.709, P = 0.004-0.043; Environmental Symptoms Questionnaires-Cerebral symptoms score, r = 0.587-0.771, P = 0.001-0.027, n = 4). Collectively, these findings highlight the site-specific regulation of CBF in hypoxia that maintains CDO₂ selectively in the posterior but not the anterior cerebral circulation, with minimal contribution from the central respiratory chemoreflex. Furthermore, ECA vasodilatation might represent a hitherto unexplored haemodynamic risk factor implicated in the pathophysiology of AMS.

Effect of 3% CO2 inhalation on respiratory exchange ratio and cardiac output during constant work-rate exercise

BACKGROUND

The aim of this study was to examine whether the decrease in respiratory exchange ratio (RER) during constant work-rate exercise (CWE) with 3% carbon dioxide (CO<inf>2</inf>) inhalation could be caused by the combination of the decrease in CO<inf>2</inf> output (V̇CO<inf>2</inf>) and the increase in oxygen uptake (V̇O<inf>2</inf>). In addition, we investigated the effect of 3% CO<inf>2</inf> inhalation on cardiac output (Q̇) during CWE.

METHODS

Seven males (V̇O<inf>2max</inf>: 44.1±6.4 mL/min/kg) carried out transitions from low-load cycling (baseline; 40w) to light intensity exercise (45% V̇O<inf>2 max</inf>; 89.3±12.5 W) and heavy intensity exercise (80% V̇O<inf>2max</inf>; 186.5±20.2 W) while inhaling normal air (Air) or an enriched CO<inf>2</inf> gas (3% CO<inf>2</inf>, 21% O<inf>2</inf>, balance N<inf>2</inf>). Each exercise session was 6 min, and respiratory responses by Douglas bag technique and cardiac responses by thoracic bio-impedance method were measured during the experiment.

RESULTS

Ventilation for 3% CO<inf>2</inf> was higher than for air through the experiment (P<0.05). Steady and non-steady state RER and V̇CO<inf>2</inf> for 3% CO<inf>2</inf> were less than for air in both light and heavy intensities (P<0.05), but V̇O<inf>2</inf> and Q̇ did not differ between the two conditions.

CONCLUSIONS

3% CO<inf>2</inf> inhalation induced the decrease in RER during CWE at light and heavy intensities, which was due to the decrease in V̇CO<inf>2</inf>. The promoted ventilation with 3% CO<inf>2</inf> did not lead to the increase in V̇O<inf>2</inf>. Moreover, 3% CO<inf>2</inf> inhalation did not affect Q̇ during CWE at light and heavy intensities.

Effects of short-term mild hypercapnia during head-down tilt on intracranial pressure and ocular structures in healthy human subjects

Many astronauts experience ocular structural and functional changes during long-duration spaceflight, including choroidal folds, optic disc edema, globe flattening, optic nerve sheath diameter (ONSD) distension, retinal nerve fiber layer thickening, and decreased visual acuity. The leading hypothesis suggests that weightlessness-induced cephalad fluid shifts increase intracranial pressure (ICP), which contributes to the ocular structural changes, but elevated ambient CO2 levels on the International Space Station may also be a factor. We used the spaceflight analog of 6° head-down tilt (HDT) to investigate possible mechanisms for ocular changes in eight male subjects during three 1-h conditions: Seated, HDT, and HDT with 1% inspired CO2 (HDT + CO2). Noninvasive ICP, intraocular pressure (IOP), translaminar pressure difference (TLPD = IOP-ICP), cerebral and ocular ultrasound, and optical coherence tomography (OCT) scans of the macula and the optic disc were obtained. Analysis of one-carbon pathway genetics previously associated with spaceflight-induced ocular changes was conducted. Relative to Seated, IOP and ICP increased and TLPD decreased during HDT During HDT + CO2 IOP increased relative to HDT, but there was no significant difference in TLPD between the HDT conditions. ONSD and subfoveal choroidal thickness increased during HDT relative to Seated, but there was no difference between HDT and HDT + CO2 Visual acuity and ocular structures assessed with OCT imaging did not change across conditions. Genetic polymorphisms were associated with differences in IOP, ICP, and end-tidal PCO2 In conclusion, acute exposure to mild hypercapnia during HDT did not augment cardiovascular outcomes, ICP, or TLPD relative to the HDT condition.

Interaction between the ventilatory and cerebrovascular responses to hypo- and hypercapnia at rest and during exercise

Cerebrovascular reactivity to changes in the partial pressure of arterial carbon dioxide (P(a,CO(2))) via limiting changes in brain [H(+)] modulates ventilatory control. It remains unclear, however, how exercise-induced alterations in respiratory chemoreflex might influence cerebral blood flow (CBF), in particular the cerebrovascular reactivity to CO(2). The respiratory chemoreflex system controlling ventilation consists of two subsystems: the central controller (controlling element), and peripheral plant (controlled element). In order to examine the effect of exercise-induced alterations in ventilatory chemoreflex on cerebrovascular CO(2) reactivity, these two subsystems of the respiratory chemoreflex system and cerebral CO(2) reactivity were evaluated (n = 7) by the administration of CO(2) as well as by voluntary hypo- and hyperventilation at rest and during steady-state exercise. During exercise, in the central controller, the regression line for the P(a,CO(2))-minute ventilation (VE) relation shifted to higher VE and P(a,CO(2)) with no change in gain (P = 0.84). The functional curve of the peripheral plant also reset rightward and upward during exercise. However, from rest to exercise, gain of the peripheral plant decreased, especially during the hypercapnic condition (-4.1 +/- 0.8 to -2.0 +/- 0.2 mmHg l(-1) min(-1), P = 0.01). Therefore, under hypercapnia, total respiratory loop gain was markedly reduced during exercise (-8.0 +/- 2.3 to -3.5 +/- 1.0 U, P = 0.02). In contrast, cerebrovascular CO(2) reactivity at each condition, especially to hypercapnia, was increased during exercise (2.4 +/- 0.2 to 2.8 +/- 0.2% mmHg(-1), P = 0.03). These findings indicate that, despite an attenuated chemoreflex system controlling ventilation, elevations in cerebrovascular reactivity might help maintain CO(2) homeostasis in the brain during exercise.

Levels of carbon dioxide in helmet systems used during orthopaedic operations

The use of isolation helmets has gained popularity as a method of possible protection of the operating-room personnel from diseases that can be transmitted during operative procedures. However, the use of these systems has been associated with a variety of symptoms, including fatigue, diaphoresis, nausea, headache, and irritability. These symptoms have often been attributed to the mental stress of the operative procedure or the physical discomfort of the helmet. As far as we know, no manufacturers include the measured levels of carbon dioxide or the rate of air exchange of their helmet system. A possible common cause of discomfort with helmet systems is the level of carbon dioxide to which the person wearing the device is exposed. We measured the levels of carbon dioxide in four helmet systems from three different manufacturers during light exercise designed to approximate the exertion during an orthopaedic operation. All but one unit failed to meet the exposure limits recommended by the National Institute for Occupational Safety and Health and the Occupational Safety and Health Administration regarding exposure to carbon dioxide. One unit, the Stackhouse Freedom Aire self-contained system, did meet these standards, but the levels of carbon dioxide in this helmet were more than 1000 per cent greater than the ambient levels in air (440 parts per million compared with 4939 parts per million). Isolation systems must be evaluated carefully not only for comfort but also for the physiological effects caused by exposure to elevated levels of carbon dioxide. Operating-room personnel who use such systems should be aware that many of the physical symptoms that they experience may be associated with elevated levels of carbon dioxide.

Effects of inhaled CO2 and added dead space on idiopathic central sleep apnea

We hypothesized that reductions in arterial PCO2 (PaCO2) below the apnea threshold play a key role in the pathogenesis of idiopathic central sleep apnea syndrome (ICSAS). If so, we reasoned that raising PaCO2 would abolish apneas in these patients. Accordingly, patients with ICSAS were studied overnight on four occasions during which the fraction of end-tidal CO2 and transcutaneous PCO2 were measured: during room air breathing (N1), alternating room air and CO2 breathing (N2), CO2 breathing all night (N3), and addition of dead space via a face mask all night (N4). Central apneas were invariably preceded by reductions in fraction of end-tidal CO2. Both administration of a CO2-enriched gas mixture and addition of dead space induced 1- to 3-Torr increases in transcutaneous PCO2, which virtually eliminated apneas and hypopneas; they decreased from 43.7 +/- 7.3 apneas and hypopneas/h on N1 to 5.8 +/- 0.9 apneas and hypopneas/h during N3 (P < 0.005), from 43.8 +/- 6.9 apneas and hypopneas/h during room air breathing to 5.9 +/- 2.5 apneas and hypopneas/h of sleep during CO2 inhalation during N2 (P < 0.01), and to 11.6% of the room air level while the patients were breathing through added dead space during N4 (P < 0.005). Because raising PaCO2 through two different means virtually eliminated central sleep apneas, we conclude that central apneas during sleep in ICSA are due to reductions in PaCO2 below the apnea threshold.

Arterial PCO2 and pH in man during 3 days' exposure to 2.8 kPa CO2 in the inspired gas

It has not been firmly established how respiration adapts to long-term CO2 exposure in man. We have therefore exposed five healthy human subjects to 2.8 kPa CO2 in the inspired gas for about 70 h in a chamber with controlled atmospheric conditions at ambient pressure PCO2 and pH were determined in arterial or arterialized venous blood drawn before, during and after the exposure. One subject was studied twice. We found that PaCO2 increased acutely and then increased further within the 5- to 24-h period of exposure to 2.8 kPa CO2. No consistent change was observed during the following 2 days. At the end of exposure the PaCO2 was 0.5 kPa above the pre-exposure level. When the breathing gas was switched back to room air, PaCO2 promptly returned to pre-exposure values. The secondary rise in PaCO2 within the first day would correspond to a decrease in alveolar ventilation of about 10% assuming constant production and elimination of CO2. Arterial pH remained slightly below the pre-exposure level during the entire exposure period. A slight renal compensation resulting in an increase in base excess of about 1 mmol l-1 may have occurred in the middle part of the exposure period. We conclude that a significant, but moderate, respiratory adaptation takes place during the first day of exposure to an increased inspired load of CO2.

Effects of exercise and CO2 inhalation on the breathing pattern in man

Conflicting opinions exist concerning the breathing pattern in man during resting and stimulated ventilation. Some but not all investigators have reported the existence of an abrupt change, a 'breakpoint', in the relation between mean tidal volume and mean inspiratory time. Different opinions exist as to whether the slope and the intercept for the relation between mean minute ventilation and mean tidal volume are identical regardless of the mode of stimulating the ventilation. We have studied 10 subjects, at rest and during graded stimulation of ventilation by CO2 inhalation and exercise. No breakpoint was observed in the relations between (1) mean tidal volume and mean inspiratory time and (2) mean tidal volume and mean expiratory time, even if a wide range of tidal volumes was achieved in our subjects. Carbon dioxide inhalation (normoxic or hyperoxic) and exercise gave different regression lines for the relation between mean minute ventilation and mean tidal volume in 8 out of 10 subjects with a larger slope during exercise. At exercise inspiratory time decreased with any increase in tidal volume, while during CO2 breathing no consistent change in inspiratory time was seen. Mean inspiratory flow was linearly related to exercise load and apparently also to arterial carbon dioxide pressure. We conclude that CO2 breathing gives a breathing pattern which is different from that obtained with exercise in the majority of normal subjects. Furthermore, we could not confirm the existence of breakpoints in relations describing the breathing pattern of normal man.

CO2 sensitivity in humans breathing 1 or 2% CO2 in air

Ventilation increases when the concentration of CO2 in the inspired gas is increased, thereby limiting the increase in alveolar and arterial PCO2. The extent of this compensation at low levels of inspired CO2 has been debated. In five healthy humans, we have measured arterial PCO2, arterial pH and ventilation during exposure to 1 and 2% CO2 in the inspired gas. Each exposure lasted at least 7 min and arterial blood was sampled over at least 30 s during the last minute of each period. The ventilation was measured in the sixth and seventh min. The protocol included the sequences: control-test-control and test-control-test with 'test' representing CO2 loading and 'control' 0% CO2, respectively. We found that arterial PCO2 increased and pH decreased at both levels of inspired CO2. The mean increase in arterial PCO2 was 0.09 and 0.25 kPa, at CO2 1 and 2%, respectively. Three subjects were exposed to 1% CO2 in the inspired gas for 28 min flanked by similar control periods. In each period arterial blood samples were taken at 2- or 3-min intervals. Arterial PCO2 remained elevated for at least 20 min during the CO2 loading. The sensitivity to CO2 (ratio of increase in ventilation to increase in arterial PCO2) was within the range described by others at higher levels of inspired CO2. Arterial PCO2 increased by about 10% of the imposed load. We conclude that the increase in ventilation provides only incomplete compensation for exposure to CO2: arterial CO2 is increased and arterial pH decreased also at very low levels of inspired CO2.