Hypoxemia increases blood-brain barrier permeability during extreme apnea in humans

Voluntary asphyxia imposed by static apnea challenges blood-brain barrier (BBB) integrity in humans through transient extremes of hypertension, hypoxemia and hypercapnia. In the present study, ten ultra-elite breath-hold divers performed two maximal dry apneas preceded by normoxic normoventilation (NX: severe hypoxemia and hypercapnia) and hyperoxic hyperventilation (HX: absence of hypoxemia with exacerbating hypercapnia) with measurements obtained before and immediately after apnea. Transcerebral exchange of NVU proteins (ELISA, Single Molecule Array) were calculated as the product of global cerebral blood flow (gCBF, duplex ultrasound) and radial arterial to internal jugular venous concentration gradients. Apnea duration increased from 5 m 6 s in NX to 15 m 59 s in HX (P = <0.001) resulting in marked elevations in gCBF and venous S100B, glial fibrillary acidic protein, ubiquitin carboxy-terminal hydrolase-L1 and total tau (all P < 0.05 vs. baseline). This culminated in net cerebral output reflecting mildly increased BBB permeability and increased neuronal-gliovascular reactivity that was more pronounced in NX due to more severe systemic and intracranial hypertension (P < 0.05 vs. HX). These findings identify the hemodynamic stress to which the apneic brain is exposed, highlighting the critical contribution of hypoxemia and not just hypercapnia to BBB disruption.

Vascular dysfunction following breath-hold diving

The pathogenesis of predominantly neurological decompression sickness (DCS) is multifactorial. In SCUBA diving, besides gas bubbles, DCS has been linked to microparticle release, impaired endothelial function, and platelet activation. This study focused on vascular damage and its potential role in the genesis of DCS in breath-hold diving. Eleven breath-hold divers participated in a field study comprising eight deep breath-hold dives with short surface periods and repetitive breath-hold dives lasting for 6 h. Endothelium-dependent vasodilation of the brachial artery, via flow-mediated dilation (FMD), and the number of microparticles (MPs) were assessed before and after each protocol. All measures were analyzed by two-way within-subject ANOVA (2 × 2 ANOVA; factors: time and protocol). Absolute FMD was reduced following both diving protocols (p < 0.001), with no interaction (p = 0.288) or main effect of protocol (p = 0.151). There was a significant difference in the total number of circulating MPs between protocols (p = 0.007), where both increased post-dive (p = 0.012). The number of CD31+/CD41- and CD66b+ MP subtypes, although different between protocols (p < 0.001), also increased by 41.0% ± 56.6% (p = 0.050) and 60.0% ± 53.2% (p = 0.045) following deep and repetitive breath-hold dives, respectively. Both deep and repetitive breath-hold diving lead to endothelial dysfunction that may play an important role in the genesis of neurological DCS.

Physiology of static breath holding in elite apneists

NEW FINDINGS

What is the topic of this review? This review provides an up-to-date assessment of the physiology involved with extreme static dry-land breath holding in trained apneists. What advances does it highlight? We specifically highlight the recent findings involved with the cardiovascular, cerebrovascular and metabolic function during a maximal breath hold in elite apneists.

ABSTRACT

Breath-hold-related activities have been performed for centuries, but only recently, within the last ∼30 years, has it emerged as an increasingly popular competitive sport. In apnoea sport, competition relates to underwater distances or simply maximal breath-hold duration, with the current (oxygen-unsupplemented) static breath-hold record at 11 min 35 s. Remarkably, many ultra-elite apneists are able to suppress respiratory urges to the point where consciousness fundamentally limits a breath-hold duration. Here, arterial oxygen saturations as low as ∼50% have been reported. In such cases, oxygen conservation to maintain cerebral functioning is critical, where responses ascribed to the mammalian dive reflex, e.g. sympathetically mediated peripheral vasoconstriction and vagally mediated bradycardia, are central. In defence of maintaining global cerebral oxygen delivery during prolonged breath holds, the cerebral blood flow may increase by ∼100% from resting values. Interestingly, near the termination of prolonged dry static breath holds, recent studies also indicate that reductions in the cerebral oxidative metabolism can occur, probably attributable to the extreme hypercapnia and irrespective of the hypoxaemia. In this review, we highlight and discuss the recent data on the cardiovascular, metabolic and, particularly, cerebrovascular function in competitive apneists performing maximal static breath holds. The physiological adaptation and maladaptation with regular breath-hold training are also summarized, and future research areas in this unique physiological field are highlighted; particularly, the need to determine the potential long-term health impacts of extreme breath holding.

Influence of lung volume on the interaction between cardiac output and cerebrovascular regulation during extreme apnoea

NEW FINDINGS

What is the central question of this study? Does the reduction in cardiac output observed during extreme voluntary apnoea, secondary to high lung volume, result in a reduction in cerebral blood flow, perfusion pressure and oxygen delivery in a group of elite free divers? What is the main finding and its importance? High lung volumes reduce cardiac output and ventricular filling during extreme apnoea, but changes in cerebral blood flow are observed only transiently during the early stages of apnoea. This reveals that whilst cardiac output is important in regulating cerebral haemodynamics, the role of mean arterial pressure in restoring cerebral perfusion pressure is of greater significance to the regulation of cerebral blood flow. We investigated the role of lung volume-induced changes in cardiac output (Q̇) on cerebrovascular regulation during prolonged apnoea. Fifteen elite apnoea divers (one female; 185 ± 7 cm, 82 ± 12 kg, 29 ± 7 years old) attended the laboratory on two separate occasions and completed maximal breath-holds at total lung capacity (TLC) and functional residual capacity (FRC) to elicit disparate cardiovascular responses. Mean arterial pressure (MAP), internal jugular venous pressure and arterial blood gases were measured via cannulation. Global cerebral blood flow was quantified by ultrasound and cardiac output was quantified by via photoplethysmography. At FRC, stroke volume and Q̇ did not change from baseline (P > 0.05). In contrast, during the TLC trial stroke volume and Q̇ were decreased until 80 and 40% of apnoea, respectively (P < 0.05). During the TLC trial, global cerebral blood flow was significantly lower at 20%, but subsequently increased so that cerebral oxygen delivery was comparable to that during the FRC trial. Internal jugular venous pressure was significantly higher throughout the TLC trial in comparison to FRC. The MAP increased progressively in both trials but to a greater extent at TLC, resulting in a comparable cerebral perfusion pressure between trials by the end of apnoea. In summary, although lung volume has a profound effect on Q̇ during prolonged breath-holding, these changes do not translate to the cerebrovasculature owing to the greater sensitivity of cerebral blood flow to arterial blood gases and MAP; regulatory mechanisms that facilitate the maintenance of cerebral oxygen delivery.

Competitive apnea and its effect on the human brain: focus on the redox regulation of blood-brain barrier permeability and neuronal-parenchymal integrity

Static apnea provides a unique model that combines transient hypertension, hypercapnia, and severe hypoxemia. With apnea durations exceeding 5 min, the purpose of the present study was to determine how that affects cerebral free-radical formation and the corresponding implications for brain structure and function. Measurements were obtained before and following a maximal apnea in 14 divers with transcerebral exchange kinetics, measured as the product of global cerebral blood flow (duplex ultrasound) and radial arterial to internal jugular venous concentration differences ( a-v_D). Apnea increased the systemic (arterial) and, to a greater extent, the regional (jugular venous) concentration of the ascorbate free radical, resulting in a shift from net cerebral uptake to output ( P < 0.05). Peroxidation (lipid hydroperoxides, LDL oxidation), NO bioactivity, and S100β were correspondingly enhanced ( P < 0.05), the latter interpreted as minor and not a pathologic disruption of the blood-brain barrier. However, those changes were insufficient to cause neuronal-parenchymal damage confirmed by the lack of change in the a-v_D of neuron-specific enolase and human myelin basic protein ( P > 0.05). Collectively, these observations suggest that increased cerebral oxidative stress following prolonged apnea in trained divers may reflect a functional physiologic response, rather than a purely maladaptive phenomenon.-Bain, A. R., Ainslie, P. N., Hoiland, R. L., Barak, O. F., Drvis, I., Stembridge, M., MacLeod, D. M., McEneny, J., Stacey, B. S., Tuaillon, E., Marchi, N., De Maudave, A. F., Dujic, Z., MacLeod, D. B., Bailey, D. M. Competitive apnea and its effect on the human brain: focus on the redox regulation of blood-brain barrier permeability and neuronal-parenchymal integrity.

Hypercapnia is essential to reduce the cerebral oxidative metabolism during extreme apnea in humans

The cerebral metabolic rate of oxygen (CMRO₂) is reduced during apnea that yields profound hypoxia and hypercapnia. In this study, to dissociate the impact of hypoxia and hypercapnia on the reduction in CMRO₂, 11 breath-hold competitors completed three apneas under: (a) normal conditions (NM), yielding severe hypercapnia and hypoxemia, (b) with prior hyperventilation (HV), yielding severe hypoxemia only, and (c) with prior 100% oxygen breathing (HX), yielding the greatest level of hypercapnia, but in the absence of hypoxemia. The CMRO₂ was calculated from the product of cerebral blood flow (ultrasound) and the radial artery-jugular venous oxygen content difference (cannulation). Secondary measures included net-cerebral glucose/lactate exchange and nonoxidative metabolism. Reductions in CMRO₂ were largest in the HX condition (-44 ± 15%, p < 0.05), with the most severe hypercapnia (PaCO₂ = 58 ± 5 mmHg) but maintained oxygen saturation. The CMRO₂ was reduced by 24 ± 27% in NM ( p = 0.05), but unchanged in the HV apnea where hypercapnia was absent. A net-cerebral lactate release was observed at the end of apnea in the HV and NM condition, but not in the HX apnea (main effect p < 0.05). These novel data support hypercapnia/pH as a key mechanism mediating reductions in CMRO₂ during apnea, and show that severe hypoxemia stimulates lactate release from the brain.

Ventilation inhibits sympathetic action potential recruitment even during severe chemoreflex stress

This study investigated the influence of ventilation on sympathetic action potential (AP) discharge patterns during varying levels of high chemoreflex stress. In seven trained breath-hold divers (age 33 ± 12 yr), we measured muscle sympathetic nerve activity (MSNA) at baseline, during preparatory rebreathing (RBR), and during 1) functional residual capacity apnea (FRC_Apnea) and 2) continued RBR. Data from RBR were analyzed at matched (i.e., to FRC_Apnea) hemoglobin saturation (HbSat) levels (RBR_Matched) or more severe levels (RBR_End). A third protocol compared alternating periods (30 s) of FRC and RBR (FRC-RBR_ALT). Subjects continued each protocol until 85% volitional tolerance. AP patterns in MSNA (i.e., providing the true neural content of each sympathetic burst) were studied using wavelet-based methodology. First, for similar levels of chemoreflex stress (both HbSat: 71 ± 6%; P = NS), RBR_Matched was associated with reduced AP frequency and APs per burst compared with FRC_Apnea (both P < 0.001). When APs were binned according to peak-to-peak amplitude (i.e., into clusters), total AP clusters increased during FRC_Apnea (+10 ± 2; P < 0.001) but not during RBR_Matched (+1 ± 2; P = NS). Second, despite more severe chemoreflex stress during RBR_End (HbSat: 56 ± 13 vs. 71 ± 6%; P < 0.001), RBR_End was associated with a restrained increase in the APs per burst (FRC_Apnea: +18 ± 7; RBR_End: +11 ± 5) and total AP clusters (FRC_Apnea: +10 ± 2; RBR_End: +6 ± 4) (both P < 0.01). During FRC-RBR_ALT, all periods of FRC elicited sympathetic AP recruitment (all P < 0.001), whereas all periods of RBR were associated with complete withdrawal of AP recruitment (all P = NS). Presently, we demonstrate that ventilation per se restrains and/or inhibits sympathetic axonal recruitment during high, and even extreme, chemoreflex stress.NEW & NOTEWORTHY The current study demonstrates that the sympathetic neural recruitment patterns observed during chemoreflex activation induced by rebreathing or apnea are restrained and/or inhibited by the act of ventilation per se, despite similar, or even greater, levels of severe chemoreflex stress. Therefore, ventilation modulates not only the timing of sympathetic bursts but also the within-burst axonal recruitment normally observed during progressive chemoreflex stress.

Effect of pulmonary hyperinflation on central blood volume: An MRI study

Pulmonary hyperinflation attained by glossopharyngeal insufflation (GPI) challenges the circulation by compressing the heart and pulmonary vasculature. Our aim was to determine the amount of blood translocated from the central blood volume during GPI. Cardiac output and cardiac chamber volumes were assessed by magnetic resonance imaging in twelve breath-hold divers at rest and during apnea with GPI. Pulmonary blood volume was determined from pulmonary blood flow and transit times for gadolinium during first-pass perfusion after intravenous injection. During GPI, the lung volume increased by 0.8±0.6L (11±7%) above the total lung capacity. All cardiac chambers decreased in volume and despite a heart rate increase of 24±29 bpm (39±50%), pulmonary blood flow decreased by 2783±1820mL (43±20%). The pulmonary transit time remained unchanged at 7.5±2.2s and pulmonary blood volume decreased by 354±176mL (47±15%). In total, central blood volume decreased by 532±248mL (46±14%). Voluntary pulmonary hyperinflation leads to ∼50% decrease in pulmonary and central blood volume.

Forced vital capacity and not central chemoreflex predicts maximal hyperoxic breath-hold duration in elite apneists

The determining mechanisms of a maximal hyperoxic apnea duration in elite apneists have remained unexplored. We tested the hypothesis that maximal hyperoxic apnea duration in elite apneists is related to forced vital capacity (FVC) but not the central chemoreflex (for CO₂). Eleven elite apneists performed a maximal dry static-apnea with prior hyperoxic (100% oxygen) pre-breathing, and a central chemoreflex test via a hyperoxic re-breathing technique (hyperoxic-hypercapnic ventilatory response: HCVR); expressed as the increase in ventilation (pneumotachometry) per increase in arterial CO₂ tension (PaCO₂; radial artery). FVC was assessed using standard spirometry. Maximal apnea duration ranged from 807 to 1262s (mean=1034s). Average HCVR was 2.0±1.2Lmin^-1mmHg^-1 PaCO₂. The hyperoxic apnea duration was related to the FVC (r²=0.45, p<0.05), but not the HCVR (r²<0.01, p>0.05). These findings were interpreted to suggest that during a hyperoxic apnea, a larger initial lung volume prolongs the time before reaching intolerable discomfort associated with pending lung squeeze, while CO₂ sensitivity has little impact.

Blood pooling in extrathoracic veins after glossopharyngeal insufflation

PURPOSE

Trained breath-hold divers hyperinflate their lungs by glossopharyngeal insufflation (GPI) to prolong submersion time and withstand lung collapse at depths. Pulmonary hyperinflation leads to profound hemodynamic changes.

METHODS

Thirteen divers performed preparatory breath-holds followed by apnea with GPI. Filling of extrathoracic veins was determined by ultrasound and magnetic resonance imaging and peripheral extravasation of fluid was assessed by electrical impedance. Femoral vein diameter was measured by ultrasound throughout the easy-going and struggle phase of apnea with GPI in eight divers in a sub-study.

RESULTS

After GPI, pulmonary volume increased by 0.8 ± 0.6 L above total lung capacity. The diameter of the superior caval (by 36 ± 17%) and intrathoracic part of the inferior caval vein decreased (by 21 ± 16%), while the diameters of the internal jugular (by 53 ± 34%), hepatic (by 28 ± 40%), abdominal part of the inferior caval (by 28 ± 28%), and femoral veins (by 65 ± 50%) all increased (P < 0.05). Blood volume of the internal jugular, the hepatic, the abdominal part of the inferior caval vein, and the combined common iliac and femoral veins increased by 145 ± 115, 80 ± 88, 61 ± 60, and 183 ± 197%, respectively. In the sub-study, femoral vein diameter increased by 44 ± 33% in the easy-going phase of apnea with GPI, subsequently decreasing by 20 ± 16% during the struggle phase. Electrical impedance remained unchanged over the thigh and forearm, thus excluding peripheral fluid extravasation.

CONCLUSIONS

GPI leads to heart and pulmonary vessel compression, resulting in redistribution of blood to extrathoracic capacitance veins proximal to venous valves. This is partially reversed by the onset of involuntary breathing movements.

Immune and inflammatory responses to freediving calculated from leukocyte gene expression profiles

Freedivers hold their breath while diving, causing blood oxygen levels to decrease (hypoxia) while carbon dioxide increases (hypercapnia). Whereas blood gas changes are presumably involved in the progression of respiratory diseases, less is known about their effect on healthy individuals. Here we have used gene expression profiling to analyze elite athletes' immune and inflammatory responses to freediving. Blood was collected before and 1 and 3 h after a series of maximal dynamic and static freediving apneas in a pool, and peripheral blood gene expression was mapped on genome-wide microarrays. Fractions of phenotypically distinct immune cells were computed by deconvolution of the gene expression data using Cibersort software. Changes in gene activity and associated biological pathways were determined using R and GeneGo software. The results indicated a temporary increase of neutrophil granulocytes, and a decrease of cytotoxic lymphocytes; i.e., CD8+ T cells and resting NK cells. Biological pathway associations indicated possible protective reactions: genes involved in anti-inflammatory responses to proresolving lipid mediators were upregulated, whereas central factors involved in granule-mediated lymphocyte cytotoxicity were downregulated. While it remains unresolved whether freediving alters the immune system's defensive function, these results provide new insight into leukocyte responses and the protection of homeostasis in healthy athletes.

Cerebral oxidative metabolism is decreased with extreme apnoea in humans; impact of hypercapnia

KEY POINTS

The present study describes the cerebral oxidative and non-oxidative metabolism in man during a prolonged apnoea (ranging from 3 min 36 s to 7 min 26 s) that generates extremely low levels of blood oxygen and high levels of carbon dioxide. The cerebral oxidative metabolism, measured from the product of cerebral blood flow and the radial artery-jugular venous oxygen content difference, was reduced by ∼29% at the termination of apnoea, although there was no change in the non-oxidative metabolism. A subset study with mild and severe hypercapnic breathing at the same level of hypoxia suggests that hypercapnia can partly explain the cerebral metabolic reduction near the apnoea breakpoint. A hypercapnia-induced oxygen-conserving response may protect the brain against severe oxygen deprivation associated with prolonged apnoea.

ABSTRACT

Prolonged apnoea in humans is reflected in progressive hypoxaemia and hypercapnia. In the present study, we explore the cerebral metabolic responses under extreme hypoxia and hypercapnia associated with prolonged apnoea. We hypothesized that the cerebral metabolic rate for oxygen (CMRO2 ) will be reduced near the termination of apnoea, attributed in part to the hypercapnia. Fourteen elite apnoea-divers performed a maximal apnoea (range 3 min 36 s to 7 min 26 s) under dry laboratory conditions. In a subset study with the same divers, the impact of hypercapnia on cerebral metabolism was determined using varying levels of hypercapnic breathing, against the background of similar hypoxia. In both studies, the CMRO2 was calculated from the product of cerebral blood flow (ultrasound) and the radial artery-internal jugular venous oxygen content difference. Non-oxidative cerebral metabolism was calculated from the ratio of oxygen and carbohydrate (lactate and glucose) metabolism. The CMRO2  was reduced by ∼29% (P < 0.01, Cohen's d = 1.18) near the termination of apnoea compared to baseline, although non-oxidative metabolism remained unaltered. In the subset study, in similar backgrounds of hypoxia (arterial O2 tension: ∼38.4 mmHg), severe hypercapnia (arterial CO2 tension: ∼58.7 mmHg), but not mild-hypercapnia (arterial CO2 tension: ∼46.3 mmHg), depressed the CMRO2 (∼17%, P = 0.04, Cohen's d = 0.87). Similarly to the apnoea, there was no change in the non-oxidative metabolism. These data indicate that hypercapnia can partly explain the reduction in CMRO2 near the apnoea breakpoint. This hypercapnic-induced oxygen conservation may protect the brain against severe hypoxaemia associated with prolonged apnoea.

β1-Blockade increases maximal apnea duration in elite breath-hold divers

We hypothesized that the cardioselective β₁-adrenoreceptor antagonist esmolol would improve maximal apnea duration in elite breath-hold divers. In elite national-level divers (n = 9), maximal apneas were performed in a randomized and counterbalanced order while receiving either iv esmolol (150 μg·kg^-1·min^-1) or volume-matched saline (placebo). During apnea, heart rate (ECG), beat-by-beat blood pressure, stroke volume (SV), cardiac output (CO), and total peripheral resistance (TPR) were measured (finger photoplethysmography). Myocardial oxygen consumption (MV̇o₂) was estimated from rate pressure product. Cerebral blood flow through the internal carotid (ICA) and vertebral arteries (VA) was assessed using Duplex ultrasound. Apnea duration improved in the esmolol trial when compared with placebo (356 ± 57 vs. 323 ± 61 s, P < 0.01) despite similar end-apnea peripheral oxyhemoglobin saturation (71.8 ± 10.3 vs. 74.9 ± 9.5%, P = 0.10). The HR response to apnea was reduced by esmolol at 10-30% of apnea duration, whereas MAP was unaffected. Esmolol reduced SV (main effect, P < 0.05) and CO (main effect; P < 0.05) and increased TPR (main effect, P < 0.05) throughout apnea. Esmolol also reduced MV̇o₂ throughout apnea (main effect, P < 0.05). Cerebral blood flow through the ICA and VA was unchanged by esmolol at baseline and the last 30 s of apnea; however, global cerebral blood flow was reduced in the esmolol trial at end-apnea (P < 0.05). Our findings demonstrate that, in elite breath-hold divers, apnea breakpoint is improved by β₁-blockade, likely owing to an improved total body oxygen sparring through increased centralization of blood volume (↑TPR) and reduced MV̇o₂NEW & NOTEWORTHY The governing bodies for international apnea competition, the Association Internationale pour le Développment de l'Apnée and La Confédération Mondaile des Activités Subaquatiques, have banned the use of β-blockers based on anecdotal reports that they improve apnea duration. Using a randomized placebo-controlled trial, we are the first to empirically confirm that β-blockade improves apnea duration. This improvement in apnea duration coincided with a reduced myocardial oxygen consumption.

Organ perfusion during voluntary pulmonary hyperinflation; a magnetic resonance imaging study

Pulmonary hyperinflation is used by competitive breath-hold divers and is accomplished by glossopharyngeal insufflation (GPI), which is known to compress the heart and pulmonary vessels, increasing sympathetic activity and lowering cardiac output (CO) without known consequence for organ perfusion. Myocardial, pulmonary, skeletal muscle, kidney, and liver perfusion were evaluated by magnetic resonance imaging in 10 elite breath-hold divers at rest and during moderate GPI. Cardiac chamber volumes, stroke volume, and thus CO were determined from cardiac short-axis cine images. Organ volumes were assessed from gradient echo sequences, and organ perfusion was evaluated from first-pass images after gadolinium injection. During GPI, lung volume increased by 5.2 ± 1.5 liters (mean ± SD; P < 0.001), while spleen and liver volume decreased by 46 ± 39 and 210 ± 160 ml, respectively (P < 0.05), and inferior caval vein diameter by 4 ± 3 mm (P < 0.05). Heart rate tended to increase (67 ± 10 to 86 ± 20 beats/min; P = 0.052) as right and left ventricular volumes were reduced (P < 0.05). Stroke volume (107 ± 21 to 53 ± 15 ml) and CO (7.2 ± 1.6 to 4.2 ± 0.8 l/min) decreased as assessed after 1 min of GPI (P < 0.01). Left ventricular myocardial perfusion maximum upslope and its perfusion index decreased by 1.52 ± 0.15 s(-1) (P < 0.001) and 0.02 ± 0.01 s(-1) (P < 0.05), respectively, without transmural differences. Pulmonary tissue, spleen, kidney, and pectoral-muscle perfusion also decreased (P < 0.05), and yet liver perfusion was maintained. Thus, during pulmonary hyperinflation by GPI, CO and organ perfusion, including the myocardium, as well as perfusion of skeletal muscles, are reduced, and yet perfusion of the liver is maintained. Liver perfusion seems to be prioritized when CO decreases during GPI.

Peripheral chemoreflex inhibition with low-dose dopamine: new insight into mechanisms of extreme apnea

The purpose of this study was to determine the impact of peripheral chemoreflex inhibition with low-dose dopamine on maximal apnea time, and the related hemodynamic and cerebrovascular responses in elite apnea divers. In a randomized order, participants performed a maximal apnea while receiving either intravenous 2 μg·kg(-1)·min(-1) dopamine or volume-matched saline (placebo). The chemoreflex and hemodynamic response to dopamine was also assessed during hypoxia [arterial O2 tension, (PaO2 ) ∼35 mmHg] and mild hypercapnia [arterial CO2 tension (PaCO2 ) ∼46 mmHg] that mimicked the latter parts of apnea. Outcome measures included apnea duration, arterial blood gases (radial), heart rate (HR, ECG), mean arterial pressure (MAP, intra-arterial), middle (MCAv) and posterior (PCAv) cerebral artery blood velocity (transcranial ultrasound), internal carotid (ICA) and vertebral (VA) artery blood flow (ultrasound), and the chemoreflex responses. Although dopamine depressed the ventilatory response by 27 ± 41% (vs. placebo; P = 0.01), the maximal apnea duration was increased by only 5 ± 8% (P = 0.02). The PaCO2 and PaO2 at apnea breakpoint were similar (P > 0.05). When compared with placebo, dopamine increased HR and decreased MAP during both apnea and chemoreflex test (P all <0.05). At rest, dopamine compared with placebo dilated the ICA (3.0 ± 4.1%, P = 0.05) and VA (6.6 ± 5.0%, P < 0.01). During apnea and chemoreflex test, conductance of the cerebral vessels (ICA, VA, MCAv, PCAv) was increased with dopamine; however, flow (ICA and VA) was similar. At least in elite apnea divers, the small increase in apnea time and similar PaO2 at breakpoint (∼31 mmHg) suggest the apnea breakpoint is more related to PaO2 , rather than peripheral chemoreflex drive to breathe.

Effect of Maximal Apnoea Easy-Going and Struggle Phases on Subarachnoid Width and Pial Artery Pulsation in Elite Breath-Hold Divers

Purpose

The aim of the study was to assess changes in subarachnoid space width (sas-TQ), the marker of intracranial pressure (ICP), pial artery pulsation (cc-TQ) and cardiac contribution to blood pressure (BP), cerebral blood flow velocity (CBFV) and cc-TQ oscillations throughout the maximal breath hold in elite apnoea divers. Non-invasive assessment of sas-TQ and cc-TQ became possible due to recently developed method based on infrared radiation, called near-infrared transillumination/backscattering sounding (NIR-T/BSS).

Methods

The experimental group consisted of seven breath-hold divers (six men). During testing, each participant performed a single maximal end-inspiratory breath hold. Apnoea consisted of the easy-going and struggle phases (characterised by involuntary breathing movements (IBMs)). Heart rate (HR) was determined using a standard ECG. BP was assessed using the photoplethysmography method. SaO₂ was monitored continuously with pulse oximetry. A pneumatic chest belt was used to register thoracic and abdominal movements. Cerebral blood flow velocity (CBFV) was estimated by a 2-MHz transcranial Doppler ultrasonic probe. sas-TQ and cc-TQ were measured using NIR-T/BSS. Wavelet transform analysis was performed to assess cardiac contribution to BP, CBFV and cc-TQ oscillations.

Results

Mean BP and CBFV increased compared to baseline at the end of the easy phase and were further augmented by IBMs. cc-TQ increased compared to baseline at the end of the easy phase and remained stable during the IBMs. HR did not change significantly throughout the apnoea, although a trend toward a decrease during the easy phase and recovery during the IBMs was visible. Amplitudes of BP, CBFV and cc-TQ were augmented. sas-TQ and SaO₂ decreased at the easy phase of apnoea and further decreased during the IBMs.

Conclusions

Apnoea increases intracranial pressure and pial artery pulsation. Pial artery pulsation seems to be stabilised by the IBMs. Cardiac contribution to BP, CBFV and cc-TQ oscillations does not change throughout the apnoea.