Preservation of Neurovascular Coupling to Cognitive Activity in Anterior Cerebrovasculature During Incremental Ascent to High Altitude

Abstract

Background: High altitude sojourn challenges blood flow regulation in the brain, which may contribute to cognitive dysfunction. Neurovascular coupling (NVC) describes the ability to increase blood flow to working regions of the brain. Effects of high altitude on NVC in frontal regions undergoing cognitive activation are unclear but may be relevant to executive function in high-altitude hypoxia. This study sought to examine the effect of incremental ascent to very high altitude on NVC by measuring anterior cerebral artery (ACA) and middle cerebral artery (MCA) hemodynamic responses to sustained cognitive activity.

Materials and Methods: Eight adults (23 ± 7 years, four female) underwent bilateral measurement of ACA and MCA mean velocity and pulsatility index (PI) through transcranial Doppler during a 3-minute Stroop task at 1400, 3440, and 4240 m.

Results: Resting MCA and ACA PI decreased with high-altitude hypoxia (p < 0.05). Cognitive activity at all altitudes resulted in similar increases in MCA and ACA mean velocity, and decreases in ACA and MCA PI (p < 0.05 for MCA, p = 0.07 for ACA). No significant altitude-by-Stroop interactions were detected, indicating NVC was stable with increasing altitude.

Conclusions: Ascent to very high altitude (4240 m) using an incremental profile that supports partial acclimatization does not appear to disturb (1) increases in cerebral blood velocity and (2) reductions in pulsatility that characterize optimal NVC in frontal regions of the brain during cognitive activity.

Introduction

Environmental hypoxia incurred during high altitude sojourn represents a formidable perturbation to brain homeostasis and function (Ainslie et al., 2016). The brain compensates in high-altitude hypoxia by increasing blood flow to offset hypoxia-induced reductions in arterial oxygen content, ultimately maintaining cerebral oxygen delivery (Ainslie et al., 2016). Despite compensatory increases in basal cerebral blood flow, ascent to high altitude is associated with reductions in cognitive function, particularly with respect to executive function (Virues-Ortega et al., 2004; Petrassi et al., 2012; Yan, 2014).

The brain is a high-flow organ that depends on acute increases in regional blood flow and oxygen delivery to support increases in neural activity, a process known as neurovascular coupling (NVC) (Iadecola, 2004; Attwell et al., 2010; Phillips et al., 2016). NVC is sensitive to the manner in which that blood flow is delivered. Pulsatile (i.e., discontinuous) blood flow can impair oxygen extraction (Rasmussen et al., 2015), damage the microvasculature over time (Mitchell et al., 2011), and appears to disrupt optimal NVC (Heffernan et al., 2015, 2018; Lefferts et al., 2018). Indeed, changes in cerebral pulsatility have been increasingly recognized as an important hemodynamic component to characterize NVC (Squair et al., 2019). While previous studies have shown that increases in mean velocity during NVC are preserved at high altitude (Caldwell et al., 2017; Leacy et al., 2018), it is unknown if regulation of hemodynamic pulsatility during NVC is maintained in such environments. As such, assessing changes in cerebral pulsatility during neural activity may provide additional insight into the effect of high-altitude hypoxia on NVC.

NVC in the anterior cerebral circulation may be particularly important during ascent to high altitude because frontal regions of the brain appear particularly sensitive to hypoxia (Pagani et al., 2011). Executive function relies on prefrontal areas of the brain (Miller and Cohen, 2001), which are fed by the anterior cerebral artery (ACA) and middle cerebral artery (MCA). There is a paucity of data on the effects of high altitude on NVC during cognitive activity in the MCA and ACA, as previous studies have focused on NVC to light stimulation in the posterior circulation (Caldwell et al., 2017; Leacy et al., 2018). While NVC during cognitive activity may remain intact in the MCA during high altitude sojourn (Caldwell et al., 2017), it is unknown if the same holds for the ACA.

The purpose of this study was to examine the effects of a 7-day incremental ascent to very high altitude (4240 m) on NVC in the anterior cerebral circulation. To do so, we assessed changes in MCA and ACA hemodynamics during cognitive activation at 1400, 3440, and 4240 m. We hypothesized that NVC at the level of the MCA would be maintained with increasing altitude as seen in previous studies, however NVC would be impaired in the ACA and marked by unaltered mean velocities and increases in pulsatility.

Materials and Methods

Eight healthy adults (23 ± 7 years, range 18–41; height 171 ± 11 cm; weight 69.2 ± 12.5 kg; body mass index [BMI] 23.5 ± 2.4 kg/m²; four female) underwent cerebrovascular and cognitive testing during a 7-day incremental ascent to 4240 m (Fig. 1) as part of a larger trek to Everest Base Camp in the Nepal Himalaya (Lefferts et al., 2019). All participants were from North America and traveled from Syracuse, NY (116 m) to Kathmandu, Nepal (1400 m) over 2 days, with an overnight layover in Abu Dhabi, UAE (6 m). Participants were nonsmokers free from cardiovascular disease, hypertension, recent head injury (concussion), pulmonary/renal/neurological disease, and were nonobese (BMI <30 kg/m²). Oral contraceptives were used by three-quarters (n = 3) of the females. No participants used carbonic anhydrase inhibitors as an altitude prophylaxis at any point during this study. Participants were asked to refrain from caffeine (i.e., tea) consumption before testing. No restrictions were placed on nonsteroidal anti-inflammatory drug (NSAID, i.e., ibuprofen) use owing to concerns regarding participant well-being, however, no participants reported using NSAID's on test days. Menstrual phase was not controlled due to the nature of the study design and ascent profile. All testing were conducted on rest days to avoid the potential confounding effects of recent exercise on the cerebrovasculature.

FIG. 1.

Study design and ascent profile, and neurovascular coupling protocol. Gray line denotes continuous monitoring, with black line denoting epoch used for analyses. ACA, anterior cerebral artery; MCA, middle cerebral artery; NVC assessment, neurovascular coupling assessment.

This study abided by the Canadian Government Tri-Council policy on research ethics with human participants, conformed with the standards set by the Declaration of Helsinki (except for registration in a data base), and was approved by the Syracuse University and Mount Royal University Human Research Ethics Boards. All participants provided written informed consent before participation. Although these data were collected as a part of a large expedition to high altitude, the hypothesis and participant recruitment for this study were planned a priori.

Study design

The incremental ascent profile used on the trek and the study design is displayed in Figure 1. Participants underwent cognitive test familiarization and NVC testing at 1400, 3440, and 4240 m. Time of day for NVC testing during the trek was generally conducted in the morning (between 0600 and 1100) except at 1400 m (testing conducted 1000–1400) due to necessities regarding participant recruitment. Testing room illumination was kept consistent at each altitude, while room temperature varied slightly and humidity generally decreased with increasing altitude.

NVC protocol

NVC was assessed by measuring cerebrovascular hemodynamics (brachial blood pressure, peripheral arterial oxygen saturation (SPO₂), ACA, and MCA blood velocity) at rest and during a challenging cognitive task (3-minute incongruent Stroop task; Fig. 1). All testing was completed in the seated position, following ∼10 minutes of seated rest and instrumentation. Brachial blood pressure was assessed following rest/instrumentation and 30 seconds into the cognitive perturbation (Stroop test, see below). Peripheral SPO₂ and cerebral blood velocities were collected with a data acquisition system (see below) and measured continuously throughout rest and the Stroop task. The final minute of baseline data acquisition before initiating the cognitive task, and the entire 3-minute Stroop task were averaged for determining resting and Stroop test values (i.e., the NVC response).

Cognitive task

The Stroop task was chosen because it (1) elicits significant cerebrovascular responses in areas perfused by the MCA (Heffernan et al., 2018) and ACA (i.e., prefrontal cortex (Takeda et al., 2017)) and (2) examines executive function, a potentially hypoxia-sensitive cognitive domain (Asmaro et al., 2013). Participants were familiarized with the Stroop task immediately before testing at 1400 m through verbal instructions, seven example stimuli, and a full 3-minute practice bout. This cognitive perturbation has been used by our group previously and is described in detail elsewhere (Heffernan et al., 2018; Lefferts et al., 2018). In brief, participants must select one of four responses that describe the color (red, blue, green, yellow) of the target word while ignoring potentially incongruous and conflicting information from the target word itself and the color (i.e., paint) of the response words. The 3-minute task titrates response windows based on task performance (every three in a row correct, response window shortens by 300 ms, vice versa for incorrect responses). This manipulation serves to (1) standardize cognitive load between participants in attempt to produce repeatable cerebral hemodynamic responses, and (2) prevent overly high or low performing participants from disengaging. Accuracy (hits/total stimuli) and mean hit reaction times (RTs) were recorded for analysis.

Brachial blood pressure and SPO₂

Brachial blood pressure was measured using a validated, automated oscillometric sphygmomanometer (Arteriograph, TensioMed, Budapest, Hungary) at rest and ∼30 seconds into the cognitive task. Pressures were obtained on the nondominant arm so that dominant arm was free to respond to the cognitive task. Pulse pressure (PP) and mean arterial pressure (MAP) were calculated using standard equations. Brachial blood pressure was only assessed once during the task, as our previous work had documented no differences in triplicate (Heffernan et al., 2015) or duplicate (Heffernan et al., 2018; Lefferts et al., 2018) measured blood pressure during the task. Peripheral SPO₂ was measured continuously using a finger pulse oximeter secured on the nondominant hand.

Cerebral blood velocities

MCA and ACA blood velocity was measured using transcranial Doppler ultrasound using previously described and standardized procedures (Willie et al., 2011). The MCA and ACA were insonated at depths between 41–60 mm and 63–74 mm, respectively, and secured using a headset. MCA and ACA were discerned based on their direction to the probe (toward and away, respectively), and responsiveness to carotid compression/vibration. Velocities were obtained in the left MCA and right ACA for seven participants, with right MCA and left ACA insonated in one participant owing to signal quality. Mean blood velocity and pulsatility index (PI) were calculated from the raw signal in the data acquisition program (ADInstruments LabChart 8.0). PI was calculated as (V_s−V_d)/V_m, where V_s, V_d, and V_m are systolic, diastolic, and mean velocity, respectively. Heart rate was calculated in the data acquisition program from the systolic peaks contained in the cerebral blood velocity envelope. Conductance was calculated by dividing the average ACA and MCA mean velocity by brachial MAP.

Physiological variables

Additional daily physiological measures were assessed during the ascent profile to confirm the physiological effects of hypoxia during incremental ascent to high altitude. These variables were monitored in the morning (∼06:00–08:00 am), before meal consumption, on each day of testing (Fig. 1). Weight was measured using a portable digital scale (Model HBF-516B; Omron, San Ramon, CA). All physiological measures were obtained at rest in a seated position following >2 minutes rest with eyes closed and white noise played through headphones to limit distraction. Respiratory measures were obtained using a mouthpiece and nose clip. Minute ventilation was assessed using a portable analog respirometer (Haoloscale; nSpire, Longmont, CO) and respiration rate and end-tidal CO₂ (Torr; altitude adjusted) using a portable capnograph (EMMA; Masimo, Danderyd, Sweden). Expired tidal volume was calculated as V?_E/RR. Hemoglobin concentration (Hemocue Hemoglobin System, Hb201+; Angelholm, Sweden) was assessed through finger capillary blood sample using sterile lancets and universal precautions. Self-reported acute mountain sickness (AMS) scores were obtained using the standard Lake Louise Questionnaire (Roach et al., 1993).

Statistical analyses

Our previous work with NVC during normobaric hypoxia (Lefferts et al., 2016b) indicated increases in MCA velocity during the Stroop task had an effect size (Cohen's dz) of 1.17. A priori power calculations (two-tailed, paired t-test) using the effect size 1.17, between measure correlation of r = 0.93, power of 0.80, and α of 0.05 indicated eight participants would be sufficient to detect similar changes in MCA mean velocity during the Stroop task. Data normality was assessed quantitatively using the Shapiro–Wilk test. Non-normally distributed variables were log transformed to meet normality assumptions (heart rate, brachial PP, MCA mean velocity, MCA PI).

We examined the effect of incremental ascent to high altitude on NVC using a two-factor (three-altitude: 1400, 3440, 4240 m by two-time: rest, during Stroop task) repeated-measures analysis of variance (ANOVA). A main effect of Stroop indicates the effect of cognitive activation elicited by the Stroop task on a given outcome. Bonferroni corrected post hoc tests were employed to determine pairwise differences if a significant altitude-by-Stroop interaction was detected. The effect of increasing altitude on physiological variables was tested using a one-way repeated measures ANOVA across the three altitudes. The effect of increasing altitude on AMS scores was assessed through a nonparametric Friedman's test. The effect of increasing altitude on AMS was further examined through post hoc Wilcoxon signed rank tests with Bonferonni adjustment for multiple comparisons. All data were analyzed using SPSS version 24.0 and are reported as mean ± standard deviation unless otherwise noted. Statistical significance was established a priori as p < 0.05. Effect size is reported for parametric analyses using partial-η², which describes the portion of variance in the outcome accounted for by a specific main or interaction effect.

Results

Physiological effects of incremental ascent to high altitude

Significant main effects for increasing altitude exposure were detected on body weight, end-tidal CO₂, peripheral SPO₂, and AMS (Table 1; p < 0.05). By day 7 of the trek at 4240 m, body weight decreased slightly compared with baseline at 3440 m. End-tidal CO₂ and peripheral SPO₂ decreased at both 3440 and 4240 m compared with 1400 m (p < 0.05). No differences were present between pairwise comparisons of AMS after applying a Bonferroni adjustment.

Table 1.

Changes in Respiratory and Cardiovascular Measures During Ascent (Mean ± Standard Deviation)

	1400 m	3440 m (day 3)	4240 m (day 7)	p-value (partial-η2)
Weight (kg)	69.2 ± 12.5	69.1 ± 12.1	68.3 ± 11.7#	0.04 (0.4)
Minute ventilation (L/minutes)	8.24 ± 2.09	9.48 ± 2.63	9.72 ± 2.76	0.18 (0.2)
Respiration rate (minutes−1)	11.1 ± 4.0	10.6 ± 2.8	10.5 ± 2.9	0.59 (0.0)
Tidal volume (L)	0.78 ± 0.20	0.94 ± 0.34	0.96 ± 0.27	0.25 (0.2)
ET-CO2 (mmHg)	32 ± 3	25 ± 2‡	23 ± 3‡	0.01 (0.8)
SPO2 (%)	97 ± 1	91 ± 4‡	88 ± 3‡	0.01 (0.8)
Hematocrit (%)	43.9 ± 2.1	43.9 ± 3.0	45.1 ± 3.5	0.58 (0.1)
Hemoglobin (g/L)	132 ± 34	142 ± 17	143 ± 18	0.06 (0.1)
AMS scorea	0 (0–1)	1.5 (0–2)	1 (0–2)	0.02

^aNonparametric analyses presented as median (range), partial-η² not calculated.

Partial-η² quantifies effect size and portion of variance accounted for by a specific main/interaction effect ^‡altitude effect, p < 0.05 versus 1400 m; ^#altitude effect, p < 0.05 versus 3440 m.

AMS, acute mountain sickness; ET, end-tidal; SPO₂, peripheral arterial oxygen saturation.

Effects of incremental ascent to high altitude on cerebrovascular hemodynamics at rest and during cognitive activation (Stroop task)

Significant main effects for altitude was observed for peripheral SPO₂, brachial blood pressure, MCA conductance, and MCA and ACA PI (p < 0.05). Peripheral SPO₂ decreased from 1400 to 3440 m and further decreased at 4240 m (p < 0.05). Brachial SBP at 4240 m was significantly greater than 3440 and 1400 m. Brachial DBP and MAP increased from 1400 to 3440 m, and further increased at 4240 m (p < 0.05). MCA conductance was significantly lower at 4240 m compared with 1400 m. ACA PI significantly decreased from 1400 to 3440 m, however reductions in MCA PI with increasing high-altitude hypoxia were not significant following Bonferroni adjustment for multiple comparisons. Significant effects of altitude were identified for Stroop accuracy and RT (p < 0.05), which improved during the high-altitude trek. Stroop accuracy increased at 4240 m (80.9% ± 10.5%) compared with 3440 m (74.2% ± 12.1%) and 1400 m (70.0 ± 12.1), and RT decreased at both 3440 m (890 ± 78 ms) and 4240 m (974 ± 113 ms) compared with 1400 m (989 ± 105 ms; p < 0.05).

Significant main effects of the Stroop task (i.e., cognitive activation) were detected for heart rate, peripheral SPO₂ (Table 2), MCA and ACA mean velocity, and MCA PI (Fig. 2, Supplementary Table S1; p < 0.05). Heart rate, peripheral SPO₂, and MCA and ACA mean velocities all increased from rest to during the Stroop (p < 0.05). While there was a trend for a Stroop effect on ACA PI (p = 0.07), MCA PI significantly decreased from rest to during the Stroop (p < 0.05). No significant altitude-by-Stroop task interactions were detected, suggesting NVC was preserved as there was no effect of incremental ascent to high altitude on cerebrovascular responses to cognitive activity elicited by a Stroop task during the 7-day incremental ascent profile to 4240 m.

Table 2.

Cerebrovascular Hemodynamics at Rest and During Cognitive Challenge at Increasing Altitudes (Mean ± Standard Deviation)

	1400 m		3440 m (day 3)		4240 m (day 7)		p-value (partial-η2)
	Rest	Stroop	Rest	Stroop	Rest	Stroop	Alt	Stroop	AxS
Heart rate (minutes−1)^	71 ± 10	78 ± 14	75 ± 9	81 ± 12	77 ± 11	82 ± 10	0.36 (0.8)	0.01 (0.3)	0.51 (0.1)
SPO2 (%)	94 ± 2	96 ± 1	88 ± 3‡	89 ± 3‡	85 ± 2‡,#	86 ± 1‡,#	0.01 (0.9)	0.04 (0.5)	0.71 (0.0)
SBP (mmHg)	118 ± 8	119 ± 8	121 ± 7	123 ± 7	128 ± 10‡,#	134 ± 10‡,#	0.01 (0.8)	0.16 (0.3)	0.40 (0.1)
DBP (mmHg)	67 ± 10	68 ± 10	71 ± 5‡	73 ± 12‡	79 ± 6‡,#	80 ± 7‡,#	0.01 (0.8)	0.43 (0.1)	0.99 (0.0)
PP (mmHg)^	51 ± 10	51 ± 8	50 ± 10	50 ± 10	49 ± 7	54 ± 8	0.52 (0.1)	0.48 (0.1)	0.36 (0.0)
MAP (mmHg)	84 ± 8	85 ± 9	87 ± 4‡	89 ± 11‡	95 ± 7‡,#	98 ± 7‡,#	0.01 (0.8)	0.26 (0.2)	0.90 (0.0)
MCA conductance (cm/s/mmHg)	0.77 ± 0.15	0.79 ± 0.17	0.75 ± 0.12	0.77 ± 0.12	0.69 ± 0.16‡	0.71 ± 0.13‡	0.03 (0.4)	0.29 (0.2)	0.99 (0.0)
ACA conductance (cm/s/mmHg)	0.66 ± 0.12	0.69 ± 0.16	0.65 ± 0.18	0.66 ± 0.20	0.58 ± 0.18	0.58 ± 0.17	0.28 (0.2)	0.38 (0.1)	0.86 (0.0)

Partial-η² quantifies effect size and portion of variance accounted for by a specific main/interaction effect.

^{^}p-values represent transformed data to meet normality assumptions.

^‡Altitude effect, p < 0.05 versus 1400 m; ^#altitude effect, p < 0.05 versus 3440 m.

ACA, anterior cerebral artery; Alt, high-altitude hypoxia; AxS, high-altitude hypoxia-by-Stroop interaction; DBP, diastolic blood pressure; MAP, mean arterial pressure; MCA, middle cerebral artery; PP, pulse pressure; SBP, systolic blood pressure.

FIG. 2.

Effect of incremental ascent to high altitude on middle and anterior cerebral artery hemodynamics at rest and during cognitive activity (Stroop task). p-value (partial-η²); partial-η² quantifies effect size and portion of variance accounted for by a specific main/interaction effect. ^{^}p-values represent transformed data to meet normality assumptions. (A) Altitude: 0.046 (0.4), Stroop: 0.01 (0.6), altitude-by-Stroop: 0.71 (0.0).^{^} (B) Altitude: 0.01 (0.6), Stroop: 0.07 (0.4), altitude-by-Stroop: 0.14 (0.2). (C) Altitude: 0.57 (0.1), Stroop: 0.01 (0.6), altitude-by-Stroop: 0.75 (0.1).^{^} (D) Altitude: 0.93 (0.0), Stroop: 0.02 (0.6), altitude-by-Stroop: 0.95 (0.0). ^‡p < 0.05 altitude effect, p < 0.05 versus 1400 m.

Discussion

This study examined the effect of incremental ascent to very high altitude on NVC by assessing changes in cerebral blood velocity and pulsatility elicited by a cognitive task during an incremental ascent profile to a maximum of 4240 m. Cognitive activity elicited increases in both ACA and MCA mean velocity and concomitant with reductions in ACA and MCA pulsatility with incremental ascent to high altitude. The vascular hemodynamic response during cognitive activity did not change as a function of increasing altitude. Thus, NVC is preserved at the level of the ACA and MCA with incremental ascent to very high altitude.

Effects of incremental ascent to high altitude on NVC

We assessed anterior (ACA and MCA) NVC as the cerebrovascular response to neural activity, provoked by a difficult cognitive task known to activate frontal regions of the brain. We chose to examine these two vessels in the anterior cerebral circulation because this region of the brain appears particularly sensitive to hypoxia (Pagani et al., 2011), and regulation of blood flow at rest during hypoxia is vessel/region dependent (Pagani et al., 2011; Ogoh et al., 2013; Mikhail Kellawan et al., 2017). While previous data suggest NVC in the MCA appears intact at high altitude (Caldwell et al., 2017; Leacy et al., 2018), we anticipated differential responses in the ACA since (1) NVC may differ between MCA and ACA under low-altitude conditions (Kelley et al., 1992; Horton-Lambirth et al., 1998), and (2) high altitude may impair cognitive processes (executive function) associated with prefrontal brain regions (Davranche et al., 2016) perfused by the ACA. Interestingly, we noted significant increases in both MCA and ACA mean velocity during the Stroop task with ascent to high altitude. Although mean velocities were on average higher in the MCA compared with ACA, both increased with cognitive activity throughout the incremental ascent to high altitude. These data highlight that similar to resting mean velocity (Caldwell et al., 2017), the functional ability of the cerebrovasculature to accommodate acute increases in mean velocity is preserved in the ACA and MCA with incremental ascent to very high altitude. Recent data, however, underscore that NVC not only depends on changes in mean blood flow, but also requires the blood flow delivered to the active neural tissue be nonpulsatile (Heffernan et al., 2018; Squair et al., 2019).

Cerebral hemodynamic pulsatility tended to decrease under resting conditions during incremental ascent to high altitude. We noted further reductions in hemodynamic pulsatility during the Stroop task, marked by significant reductions in MCA PI, and a tendency for reductions in ACA PI during cognitive activation. While increases in pulsatility may be detrimental for NVC, we have generally observed reductions in extracranial (i.e., carotid), but not MCA PI during cognitive activity under normoxic conditions. This suggests that reductions in PI observed herein may be specific to high altitude, where even intermediate altitude (1400 m, SPO₂ ≈ 95%) elicits greater reductions in MCA PI than low-altitude conditions. Future work should further interrogate potential mechanisms (i.e., changes in mean velocity, extracranial artery stiffness, wave reflections, and cerebrovascular tone) underlying these functional decreases in PI during high altitude sojourn.

Increases in cognitive activity elicited by the Stroop task were accompanied by slight but significant increases in peripheral SPO₂. This aligns with our previous unpublished observations in young adults, and published observations in middle-aged adults, where the Stroop task causes an increase in ventilation and a modest reduction in end-tidal CO₂ (Lefferts et al., 2018). Slight increases in ventilation during the Stroop likely drove increases in peripheral SPO₂ while potentially reducing end-tidal CO₂, although this was not directly measured during the cognitive task. It is possible the small ventilation-driven reductions in end-tidal CO₂ during the Stroop task could attenuate the compensatory increases in cerebral blood velocity that support cognitive activity. Ultimately, our data suggest that slight increases in ventilation during a challenging cognitive task may increase arterial oxygen content without abolishing obligatory increases in mean velocity and reductions in pulsatility during incremental ascent to very high altitude.

The preservation of increased mean velocity with cognitive activation, despite potential vasoconstrictive effects of hypoxia-induced hypocapnia (observed herein at rest), may depend on blood pressure to ensure adequate blood flow to working areas of the brain. Indeed, we noted that increases in ACA and MCA mean velocity during the Stroop were no longer significant when expressed relative to changes in pressure (i.e., unaltered conductance). The modest pressor response that accompanies cognitive activity, while not statistically significant herein, is integral to NVC (Phillips et al., 2016) and may reflect sympathoexcitation induced by the cognitive load (i.e., effort required/task difficulty) (Hess and Ennis, 2012). This pressure-driven increase in cerebral flow is consistent with our previous work documenting cerebral responses to cognitive perturbations in normoxia (Heffernan et al., 2018; Lefferts et al., 2018) and acute normobaric hypoxia (Lefferts et al., 2016b). Ultimately, our data align with a growing number of studies revealing that high altitude and its concomitant physiological effects do not impair blood flow delivery during cognitive activity (Lefferts et al., 2016b; Caldwell et al., 2017, 2018; Rodrigues Barreto et al., 2017) and thus may not directly modulate cognitive function in this setting.

Limitations and considerations

Unlike previous field studies (Caldwell et al., 2017), we assessed NVC during cognitive activity rather than during visual stimulation alone. This design strengthened the applicability of our findings to cognitive function and we aimed to directly connect changes in cerebral hemodynamics to task performance. We unexpectedly observed improvements in Stroop task performance with no change in NVC, suggestive of a disconnect between task performance and NVC. This relation, however, is unfortunately complicated by the confounding effects of concurrent acclimatization processes on cognitive function (Subudhi et al., 2014), and learning effects that often persist during field-based trekking research (Griva et al., 2017). Jet lag may have exaggerated learning effects throughout the trek. Indeed, all individuals likely experienced some degree of jet lag for a period after arrival at 1400 m that may have falsely lowered baseline task performance, thereby amplifying learning effects with repeated testing during the ascent profile. It is possible that NVC only plays a prominent role in supporting cognitive function when redundant systems have been maximally stressed such as old age, clinical conditions, or severe hypoxia. As such, future studies may require more aggressive ascent profiles or peak altitude that challenges cognitive function to a greater extent to identify if NVC is disturbed in the presence of high altitude-mediated cognitive dysfunction (Babbar and Agarwal, 2012; Lefferts et al., 2016a; Pun et al., 2018). Ultimately, the Stroop task served as a cognitive perturbation to examine cerebrovascular responses to sustained increases of neural activity in a relevant area of the brain often relied on for complex cognitive processes. Thus, while we are unable to directly connect NVC to cognitive performance herein, our data suggest that incremental ascent to high altitude does not affect the ability to modulate ACA and MCA blood flow during cognitive activity.

There are a number of inherent limitations in the current field study. Due to the nature of the trek we were unable to control for ovarian cycle among our female participants, and this could impact their cerebrovascular responses to both incremental ascent to high altitude and cognitive activity (Peltonen et al., 2016). We were unable to safely standardize fasting windows before testing due to the complex effects of hypoxia on appetite, thus potential effects of meal timing may have contributed to variability in cerebrovascular hemodynamics/responses. There are known limitations to transcranial Doppler ultrasonography, as it cannot account for changes in vessel diameter. The degree of hypoxia encountered in our study, however, was below levels known to elicit vasodilation (Ainslie et al., 2016). This study relied on a small sample size (n = 8). Our effect size calculations indicate 9 and 30 more participants may have been required to detect a significant altitude-by-Stroop interaction for ACA or MCA PI, respectively, suggesting a small potential effect of hypoxia on this emerging component of NVC, particularly within the ACA. Conversely, altitude-by-Stroop interactions for mean velocities would require more than 60–200 additional subjects to achieve a reasonable power. However, our study was clearly sufficiently powered to detect altitude main and Stroop effects on a number of important hemodynamic response variables.

Conclusion

We sought to examine the effect of incremental ascent to very high altitude on NVC in cerebral vessels that perfuse frontal regions of the brain activated by executive function tasks. Despite classic physiological responses to hypoxia with increasing altitude, our data indicate that the cerebrovascular response to neural activity stemming from a cognitive task (NVC) is unaltered with incremental ascent to high altitude. Across all altitudes, cognitive activity elicited slight increases in SPO₂, pressure-driven increases in MCA and ACA mean velocity, and decreases in blood velocity pulsatility. Cumulatively, our data indicate that NVC during cognitive activity is preserved at the level of the MCA and ACA during an incremental ascent profile to 4240 m that supports partial acclimatization.

Authors' Contributions

W.K.L., T.D.B., T.A.D., and K.S.H.: conceived and designed the study; T.A.D.: organized data collection during the trek; W.K.L., J.P.D., J.E.S., L.M., T.S.H., and Z.R.: collected the data; T.A.D., W.K.L., S.T., and J.B.: cleaned and analyzed the data. W.K.L., T.A.D., and K.S.H.: interpreted the analyses; W.K.L.: prepared figures and drafted article; all authors edited, revised, and approved the final version of the article.

Author Disclosure Statement

No competing financial interests exist.

Funding Information

This work was partially supported by a Natural Sciences and Engineering Research Council of Canada Discovery grant RGPIN-2016-04915 (T.A.D.). W.K.L. is currently supported by the National Heart, Lung, And Blood Institute of the National Institutes of Health under Award Number T32HL134634. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.

Supplementary Material

Supplementary Table S1