Ischemic Preconditioning Improves Oxygen Saturation and Attenuates Hypoxic Pulmonary Vasoconstriction at High Altitude

Abstract

Foster, Gary P., Paresh C. Giri, Douglas M. Rogers, Sophia R. Larson, and James D. Anholm. Ischemic preconditioning improves oxygen saturation and attenuates hypoxic pulmonary vasoconstriction at high altitude. High Alt Med Biol. 15:155–161, 2014.—Exposure to hypoxic environments is associated with decreased arterial oxygen saturation and increased pulmonary artery pressures. Ischemic preconditioning of an extremity (IPC) is a procedure that stimulates vasoactive and inflammatory pathways that protect remote organ systems from ongoing or future ischemic injury. To test the effects of IPC on oxygen saturation and pulmonary artery pressures at high altitude, 12 healthy adult volunteers were evaluated in a randomized cross-over trial. IPC was administered utilizing a standardized protocol. IPC or placebo was administered daily for 5 days prior to ascent to altitude. All participants were evaluated twice at 4342 m altitude (placebo and IPC conditions separated by 4 weeks, randomized). The pulmonary artery systolic pressure (PASP) at 4342 m was significantly lower in the IPC condition than the placebo condition (36±6.0 mmHg vs. 38.1±7.6 mmHg, respectively, p=0.035). Oxygen saturation at 4342 m was significantly higher with IPC compared to placebo (80.3±8.7% vs. 75.3±9.6%, respectively, p=0.003). Prophylactic IPC treatment is associated with improved oxygen saturation and attenuation of the normal hypoxic increase in pulmonary artery pressures following ascent to high altitude.

Introduction

Exposure to hypoxic environments is typically associated with decreased arterial oxygen saturation and increased pulmonary artery pressures, both of which may contribute to the impaired exercise performance at high altitude. Although controversial, some investigators attribute a significant portion of this impairment to hypoxic pulmonary vasoconstriction (HPV) (Ghofrani et al., 2004; Faoro et al., 2009; Anholm and Foster, 2011; Naeije et al., 2010, 2011). This response leads to increased pulmonary arterial pressure resulting in increased right ventricular afterload and decreased cardiac output. The complex underlying mechanisms responsible for HPV have recently been reviewed elsewhere, but are largely mediated through vasoactive and inflammatory pathways (Aaronson et al., 2006; Sylvester et al., 2012; Swenson, 2013). Pharmacologic interventions to attenuate HPV at simulated and high altitude environments improve arterial oxygen saturation (S_po₂) but have mixed results on exercise performance in (Ghofrani et al., 2004; Hsu et al., 2006; Faoro et al., 2009; Olfert et al., 2011). These findings in healthy study subjects have important implications since common high morbidity clinical scenarios such as sleep apnea syndromes, chronic obstructive pulmonary disease, critically ill patients requiring respiratory support, and heart failure are often complicated by HPV.

Ischemic preconditioning (IPC) is a procedure that has the effect of markedly attenuating ischemia reperfusion injury. The IPC procedure is performed by repetitive occlusion of arterial blood flow to an organ or extremity (e.g., 5 minutes occlusion, followed by 5 minutes of restored blood flow, repeated several times). The protective effects of IPC on local and remote tissues are largely attributed to effects on vasoactive and inflammatory pathways (Gho et al., 1996; Birnbaum et al., 1997; Kharbanda et al., 2002; Auchampach et al., 2004; Ali et al., 2007; Botker et al., 2010). The protective effects of IPC occur in two phases, early (lasting 2–3 hours) and late (returning at 12–24 hours and lasting 48–72 hours) (Murry et al., 1986; Marber et al., 1993).

The systemic or “remote” effects of IPC can be profound. As an example, IPC to one arm can dramatically reduce the size of a myocardial infarction, mitigate the incidence of arrhythmias, and improve recovery of myocardial contractility and vascular function in both animal models and human trials (Murry et al., 1986; Birnbaum et al., 1997; Auchampach et al., 2004; Botker et al., 2010). These effects also extend to the pulmonary vasculature, where our group demonstrated that the hypoxic increase in pulmonary artery systolic pressure during acute simulated altitude conditions is significantly attenuated IPC in humans (Foster et al., 2011).

The intent of the present study was to test the effects of the late phase of IPC on pulmonary artery pressures and S_po₂, following exercise at high altitude through a randomized cross-over interventional trial.

Materials and Methods

Subjects

Fourteen healthy, nonacclimatized adults (42±14 years, twelve male and two female) habituated to strenuous exercise were enrolled. These subjects provided written informed consent for study participation, which was approved by the VA Loma Linda Healthcare System Institutional Review Board (IRB). The subjects had no medical conditions limiting exercise or known heart or lung disease. None of the subjects had a history of abnormal HPV response to high altitude exposure (such as high altitude pulmonary edema). Two of the subjects developed illnesses (one sprained ankle, one bronchitis) precluding completion of the protocol and were excluded from the study. The study protocol was completed in 12 of the initial study subjects (eleven male, one female). The subjects had a mean height 179.9±8.7 cm, a mean weight of 74.3±12.0 kg, and a mean BMI of 23.3±2.6 kg/m².

Study design

Initial testing was performed at the VA Loma Linda Healthcare System facility in Loma Linda, CA (altitude 370 meters, ambient Pb ∼727 mmHg, P_Io₂ ∼142 mmHg). After initial screening, baseline echocardiography was performed on all study subjects. Subjects were then randomized to either IPC first or placebo first groups in a crossover study design. Echocardiography was repeated at low altitude (LA) after 4 days of IPC or placebo treatment. Subjects were then taken to Barcroft Laboratory, White Mountain Research Station, CA (3800 m). This trip was completed in each instance over the course of 6 hours. Following a brief rest, each subject completed a 12.8 km run from a point at 3560 m to the summit of White Mountain, at 4342 m. At an average of 27±7 min after reaching the summit, each subject underwent pulse oximetry and echocardiography before and then after breathing 100% oxygen with a non-rebreather mask (Hudson RCI 1060, Teleflex Medical, Research Triangle Park, NC) for 15 min.

Ischemic preconditioning

Ischemic preconditioning (IPC) was completed by placing a blood pressure thigh cuff (Tycos, hand held aneroid sphygmomanometer 5098-03 and Welch-Allyn durable two piece cuff 5098-02 cb, Skaneateles Falls, NY) around one thigh, inflating it to a pressure of ∼200 mmHg for 5 min, then deflating for 5 min of restored blood flow. This sequence was repeated for a total of four cycles over an elapsed time of 40 min [4×(5 min of arterial occlusion +5 min nonocclusion)=40 min]. IPC was administered daily for 5 days with the fifth IPC en-route to Barcroft laboratory, prior to ascent. The placebo condition was timed identical to the IPC condition with the blood pressure cuff inflated to only 40 mmHg.

Echocardiography

Echocardiography was performed with a portable ultrasound machine (Siemens, P50 ultrasound system, Siemens Medical Solutions USA Inc., Ultrasound Division, Mountain View, CA) using standard techniques. Echocardiographic data was recorded digitally and anonymized for analysis. The sonographers (and all study staff ) were blinded to the subject study condition (placebo or IPC) at the time of echocardiography and data analysis for all phases of the study. Two sonographers, blinded to subject condition, independently analyzed the echocardiographic data off-line. Spectral Doppler tracings were independently graded for quality and excluded if deemed not interpretable by either sonographer. Pulmonary artery systolic pressure (PASP) was calculated using the modified Bernoulli equation: (4V²)+5 mmHg, where V equals the peak end-expiratory tricuspid regurgitation velocity (TRV) by continuous wave spectral Doppler and 5 mmHg is the assumed right atrial pressure (Yock and Popp, 1984; Rudski et al., 2010). A minimum of three TRVs were averaged for each subject and each condition. As a means of providing external validation of the PASP findings, the mean pulmonary artery pressure (mPAP) was calculated using the empirically derived formula and methods described by Kitabatake et al. (1983), where the mPAP=10^ [−2.8 (AT/ET) +2.4]. AT equals systolic acceleration time across the main pulmonary artery measured by pulsed wave spectral Doppler and calculated from initiation to peak (sec) and ET is the right ventricular systolic ejection time measured utilizing the same Doppler wave, from initiation to end (sec). Cardiac output was calculated by assessment of the left ventricular outflow tract (LVOT), using the formula: cardiac output=πr²×VTI×heart rate, where VTI is the velocity-time integral of blood flow across the LVOT (Christie et al., 1987; Weyman, 1994).

Pulse oximetry

Pulse oximetry was measured and recorded continuously at Barcroft Lab (3800 meters) and the summit (4342 meters) throughout the course of echocardiography using a pulse oximeter (Masimo Corporation, RDS-1 Signal Extraction Pulse Oximeter, Irvine, CA) that was placed on the index finger or earlobe. S_po₂ values were recorded every 2 sec with the patient in a supine position over the course of a minimum of 10 min by automated recordings. Low quality sensor recordings were excluded from the data set.

Lake Louise Acute Mountain Sickness Score

The Lake Louise Scoring System (LLS) for Acute Mountain Sickness was used to assess symptoms of mountain sickness (Sutton and Houston, 1992). All subjects completed the questionnaire at the summit of White Mountain following exercise for both IPC and placebo conditions.

Exercise task

The study subjects performed a 12.8 km run from an altitude of 3560 m to 4342 m on two occasions (once for each testing condition), separated by at least 4 weeks. Heart rate was measured throughout the course using a chest mounted radio transmitted heart rate monitor (Polar Vantage NV, Polar Electro Inc., Lake Success, NY). Aid stations were provided at four locations along the course to provide water, nutrition, or other support to the study subjects.

Statistical analysis

Based on a randomized crossover study design, it was estimated that 8–10 subjects would be required for an 80% power to detect a 5 mmHg change in PASP with an α value of 0.05. Normally distributed continuous variables were evaluated using two-way repeated measures analysis of variance (ANOVA). The ANOVA included terms for high altitude and IPC and compared low altitude, high altitude, IPC, and no IPC conditions. Post-hoc testing for continuous variables was performed using a paired 2-tailed Student's t-test. Continuous variables are reported as mean values±one standard deviation (SD). A p value of<0.05 was considered significant.

Results

Effect of IPC on oxygen saturation

At the summit, oxygen saturation was significantly higher in the IPC condition compared to the placebo.

Effect of IPC on pulmonary pressures

Eight of the twelve subjects had spectral Doppler tricuspid regurgitation data that met quality standards for paired matching of testing conditions determined independently by two sonographers. Analysis of variance (ANOVA) demonstrated a significant increase in PASP at the summit vs. low altitude (p=0.001) for all conditions and a significant lowering of PASP after IPC vs. placebo at the summit. The altitude-related increase in PASP was attenuated by 21.6% following IPC compared to placebo (p<0.05). To further evaluate these pressure changes, we administered 100% oxygen for 15 min at the summit and then repeated echocardiography. Following 100% oxygen administration for 15 min at the summit, the PASP normalized to low altitude values for both the placebo and IPC conditions.

The response of the mPAP for each condition was similar to the PASP findings described above. Paired post oxygen data were limited to 8 of 12 study subjects due to equipment failures. In these 8 individuals, supplemental oxygen decreased the mPAP for both the placebo and IPC conditions (Fig. 1).

FIG. 1.

(A) Mean pulmonary artery pressure (mPAP) and (B) pulmonary vascular resistance (PVR) in study subjects in the placebo (dots, solid lines) and IPC (triangles, dotted lines) conditions at low and high altitude and at high altitude following supplemental oxygen. 4342 m=summit values at rest, 4342 m with O₂=summit values in 8 study subjects after 15 min of 100% supplemental oxygen, *=significant difference between IPC and placebo conditions, p<0.01 for both mPAP and PVR.

Effect of IPC on exercise performance

The average time to complete the 12.8 km run was significantly faster when pretreated with IPC compared with placebo pretreatment. See Table 1 and Figure 2 for study results and percentage change graphs.

FIG. 2.

Graphs A through D demonstrate the percentage change from the placebo condition for selected variables displayed in a paired fashion where each study subject serves at their own control. Graph A demonstrates a significant decrease in pulmonary artery systolic pressure in the IPC condition in the 8 individuals with satisfactory tricuspid regurgitation signal quality. Graph B demonstrates a significant decrease in mean pulmonary artery pressure in the IPC condition. Graph C demonstrates a significant increase in O₂ saturation in the IPC condition. Graph D demonstrates significantly faster course completion times in the IPC condition. Bars represent mean and 95% confidence intervals.

Table 1.

Effects of IPC on Physiologic Variables at Sea Level (370 m) and Altitude (4342 m), n=12

Parameter	LA Placebo	LA IPC	Summit Placebo^*	Summit IPC^*	P Value¹	P Value²
Heart rate (bpm)	56±10	54±12	79±20	82±20	0.27	0.71
O₂ saturation (%)	-	-	75.3±9.6	80.3±8.7	-	0.003
Cardiac output (L)	2.8±0.8	3.2±0.8	4.7±2.1	4.7±2.0	0.29	0.87
^§PASP (mmHg)	24.7±3.3	25.5±2.6	38.1±7.6	36±6.0	0.38	0.035
mPAP (mmHg)	14.1±4.5	14.0±3.9	26.1±9.0	19.6±6.2	0.91	0.005
PVR (wood units)	5.4±2.4	4.8±2.3	6.9±4.2	5.0±2.7	0.30	0.008
Course time (min)	-	-	137.8±33.8	128.1±27.4	-	0.037
AMS score	-	-	5.1±2.1	4.9±2.2	-	0.35

bpm, beats per minute; CO, cardiac output; IPC, ischemic preconditioning of the lower extremity; L, liters; LA, low altitude; min, minutes; mPAP, mean pulmonary artery pressure; PASP, estimated pulmonary artery systolic pressure; PVR, pulmonary vascular resistance.

Summit data represent resting values obtained at the time of echocardiography, 20–30 min following completion of the course run.

1, LA Placebo vs. LA IPC; 2, Summit Placebo vs. Summit IPC.

8 subjects with matched data for the 4 study conditions.

Discussion

This study demonstrated significant attenuation of hypoxic pulmonary vasoconstriction and improved oxygen saturation at altitude following IPC treatment. To our knowledge, this is the first study to test and demonstrate the late IPC effects on HPV in humans at high altitude.

Historical perspective

Ischemic preconditioning has a number of profound physiologic effects highlighted by three important studies. Murry et al. (1986) demonstrated that when IPC was applied to canine myocardial tissue, there was a 2–3 hour protective effect against subsequent ischemic insult. This initial protective effect has been termed the “early” or “classic” phase. Marber et al. (1993) demonstrated a second less intense protective period after IPC that returned at hour 12–24 and lasted 48–72 hours. Finally, Przyklenk et al. (1993) discovered that IPC applied to a single canine coronary artery distribution induced protective effects to surrounding coronary distributions. Hausenloy, Ali, and Dave individually extended the findings of Przyklenk, and demonstrated the protective effects of IPC on distant organs including the heart, kidneys, and brain, when IPC is applied to an extremity (Dave et al., 2006; Ali et al., 2007; Hausenloy et al., 2007). In addition, Birnbaum et al. (1997) also reported a significant protective effect of remote IPC (lower limb ischemia-preconditioning) on the size of myocardial infarcts following acute coronary occlusion in rabbits. Interestingly, these researchers also included the paradigm of skeletal muscle electrical stimulation and found that IPC at a distant site can afford protection to the myocardium independent of lower limb muscle stimulation (Birnbaum et al., 1997). The protective effects observed in distant vascular beds has been termed “remote IPC” ( Przyklenk et al., 1993; Birnbaum et al., 1997; Kharbanda et al., 2002).

Effect of IPC on pulmonary pressures

Although the mechanisms responsible for HPV continue to be debated, evidence suggests that endothelin (Berger et al., 2009), increased sympathetic nervous system activity (Duplain et al., 1999), the loss of bioactive metabolites, and increased free radical production (Bailey et al., 2010) all play important modulating roles. Our hypothesis that IPC would attenuate HPV was postulated from the fact that IPC activates vasodilatory and anti-inflammatory pathways that may counteract the mechanisms responsible for HPV. The systemic effects of IPC are largely attributed to the release of vasoactive molecules, such as nitric oxide, bradykinin, oxygen free radicals, and adenosine (Vanden Hoek et al., 1998; Jaberansari et al., 2001; Yellon and Downey, 2003; Auchampach et al., 2004; Eckle et al., 2008), which are also known to affect the pulmonary vasculature (Mentzer et al., 1975; Russell et al., 1990; Balanos et al., 2002; Connolly and Aaronson, 2010; Ketabchi et al., 2012). Additionally, hypoxia-inducible factor 1-α is central in both the physiologic response to altitude and cardioprotection from IPC (Eckle et al., 2008).

Compelling evidence demonstrates that IPC has beneficial effects in ischemic reperfusion injury of the heart (Gho et al., 1996; Ali et al., 2007; Botker et al., 2010), kidneys (Ali et al., 2007), and brain (Dave et al., 2006). Our findings suggest that IPC also has important vasoactive roles in the pulmonary circulation and might be considered in future randomized controlled trials to determine if this simple treatment could have clinical implications for disease states associated with exaggerated HPV such as high altitude pulmonary edema and pulmonary hypertension. If so, IPC could be easily applied in remote high altitude emergency situations where other treatments modalities (decent from high altitude, supplemental oxygen, dexamethasone, phosphodiesterase inhibitors, etc) are not readily available.

Effect of IPC on oxygen saturation

In our study subjects, S_po₂ was significantly higher in the IPC group compared to the control group at high altitude. The specific mechanisms by which IPC may have influenced oxygen saturation are unknown but likely operate through changes in \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes {10} {9} {7} {6} \begin{document} $$\dot{\rm V}_{\rm A}/ \dot{\rm Q}$$ \end{document} matching, O₂ diffusion, and alveolar ventilation. Our observations are consistent with other studies demonstrating that vasodilators such as sildenafil and bosentan may improve arterial oxygenation while simultaneously decreasing pulmonary pressure in hypoxic conditions (Ghofrani et al., 2004; Hsu et al., 2006; Olfert et al., 2011). The effect of IPC on hypoxic ventilatory responsiveness is unknown but it is possible that changes in alveolar ventilation made a significant contribution to our observed changes in S_po₂. Recent work suggests an inverse correlation between hypoxic pulmonary vasoconstriction and hypoxic ventilatory responsiveness (Albert and Swenson, 2014). Measures of ventilation were not performed as a part of our study, leaving unresolved the possible contribution from this mechanism.

The protective effects of IPC occur in two phases. The first effect peaks at about 90 minutes after treatment, and the second appears 24 hours after treatment. Our study was designed to test the effects of the second phase of IPC, which has been attributed to the upregulation of nitric oxide (NO). While the second phase is known to be less potent than the early phase, it is more robust and ubiquitous (Bolli et al., 1997; Hausenloy and Yellon, 2010). Local NO has been shown to induce changes in mitochondrial respiration, becoming more potent as O₂ concentration decreases (Shiva et al., 2007). It follows that NO-mediated effects on mitochondrial respiration may decrease O₂ utilization at the cellular level, thereby maintaining arterial O₂ saturation. Although speculative, IPC could induce changes in mitochondrial respiration via NO thereby contributing to improved oxygen saturation.

Effect of IPC on exercise performance

Hypoxia-induced impairment of exercise performance is largely influenced by cardiac output, arterial oxygenation, vasodilation, and oxygen uptake capabilities, including \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes {10} {9} {7} {6} \begin{document} $$\dot{\rm V}_{\rm A}/ \dot{\rm Q}$$ \end{document} mismatch and diffusion limitations ( Reeves et al., 1987; Wagner et al., 1987; Ghofrani et al., 2004; Naeije, 2010; Wagner, 1996; 2010). While exercise performance improvements coincided with other physiologic parameters, the prolonged nature of our exercise task did not control for environmental variables (temperature, humidity, wind, cloud cover, and trail conditions). Studies with higher fidelity measurements in conjunction with appropriate controls should be considered in future studies where exercise performance is used as a primary outcome variable.

Limitations

Due to the nature of the intervention, our study subjects were not completely blinded to IPC and placebo conditions. While knowledge of study conditions may have affected exercise performance, it is unlikely that this knowledge could have affected pulmonary pressures or O₂ saturation. Our study was also limited to a small number of healthy subjects who were conditioned to intense exercise regimens. Therefore, our findings are not extendable to the general population or to individuals who suffer from pathological conditions of the cardiopulmonary system in which these parameters play a critical role. Specifically, whether IPC affects pulmonary artery pressures, pulse oximetry, or exercise capacity in nonathletic or even unhealthy individuals, with or without confounding co-morbidities, remains unresolved and provides a compelling avenue for future research.

Conclusions

This study demonstrates significant attenuation of the normal hypoxic increase of pulmonary artery pressures and improvement of oxygen saturation at altitude more than 3 hours following IPC treatment. Future studies are warranted to identify the molecular mechanisms responsible for these findings and to determine the extent of the effect on less healthy individuals.

Footnotes

Acknowledgments

Numerous individuals donated their financial, technical, and time resources to this project. We wish to particularly acknowledge the invaluable efforts of Laura Weaver-Carnahan (study nurse), Brent Carnahan (logistical support), Bertha Jadovitz (registered diagnostic cardiac sonographer), Brendan Matus, Moussa Saleh, Claire-Alyce Andrews, Jeremy Van't Hof, and each of the study subjects who donated their time and effort to make this project possible. Thanks to Siemens for loaning the portable P50 ultrasound machines without which the study data could not have been collected. Additionally, we wish to thank the staff at the Barcroft Laboratory, White Mountain Research Station for the use of their excellent facilities.

Author Disclosure Statement

No competing financial interests exist.

References

Aaronson

, Robertson

, Knock

, et al. (2006). Hypoxic pulmonary vasoconstriction: Mechanisms and controversies. J Physiol, 570:53–58.

Albert

, and Swenson

. (2014). Peripheral chemoreceptor responsiveness and hypoxic pulmonary vasoconstriction in humans. High Alt Med Biol, 15:15–20.

Ali

, Callaghan

, Lim

, et al. (2007). Remote ischemic preconditioning reduces myocardial and renal injury after elective abdominal aortic aneurysm repair: A randomized controlled trial. Circulation, 116:198–105.

Anholm

, and Foster

. (2011). Con: Hypoxic pulmonary vasoconstriction is not a limiting factor of exercise at high altitude. High Alt Med Biol, 12:313–317.

Auchampach

, Jin

, Moore

, et al. (2004). Comparison of three different A1 adenosine receptor antagonists on infarct size and multiple cycle ischemic preconditioning in anesthetized dogs. J Pharmacol Exp Ther, 308:846–856.

Bailey

, Dehnert

, Luks

, et al. (2010). High-altitude pulmonary hypertension is associated with a free radical-mediated reduction in pulmonary nitric oxide bioavailability. J Physiol, 588:4837–4847.

Balanos

, Dorrington

, and Robbins

. (2002). Desferrioxamine elevates pulmonary vascular resistance in humans: Potential for involvement of HIF-1. J Appl Physiol, 92:2501–2507.

Berger

, Dehnert

, Bailey

, et al. (2009). Transpulmonary plasma ET-1 and nitrite differences in high altitude pulmonary hypertension. High Alt Med Biol, 10:17–24.

Birnbaum

, Hale

, and Kloner

. (1997). Ischemic preconditioning at a distance: Reduction of myocardial infarct size by partial reduction of blood supply combined with rapid stimulation of the gastrocnemius muscle in the rabbit. Circulation, 96:1641–1646.

10.

Bolli

, Manchikalapudi

, Tang

, Takano

, Qiu

, Guo

, Zhang

, and Jadoon

. (1997). The protective effect of late preconditioning against myocardial stunning in conscious rabbits is mediated by nitric oxide synthase. Evidence that nitric oxide acts both as a trigger and as a mediator of the late phase of ischemic preconditioning. Circ Res, 81:1094–1107.

11.

Botker

, Kharbanda

, Schmidt

, et al. (2010). Remote ischaemic conditioning before hospital admission, as a complement to angioplasty, and effect on myocardial salvage in patients with acute myocardial infarction: A randomised trial. Lancet, 375:727–734.

12.

Christie

, Sheldahl

, Tristani

, Sagar

, Ptacin

, and Wann

. (1987). Determination of stroke volume and cardiac output during exercise: Comparison of two-dimensional and Doppler echocardiography, Fick oximetry, and thermodilution. Circulation, 76:539–547.

13.

Connolly

, and Aaronson

. (2010). Cell redox state and hypoxic pulmonary vasoconstriction: Recent evidence and possible mechanisms. Respir Physiol Neurobiol, 174:165–174.

14.

Dave

, Saul

, Prado

, Busto

, and Perez-Pinzon

. (2006). Remote organ ischemic preconditioning protect brain from ischemic damage following asphyxial cardiac arrest. Neurosci Lett, 404:170–175.

15.

Duplain

, Vollenweider

, Delabays

, Nicod

, Bartsch

, and Scherrer

. (1999). Augmented sympathetic activation during short-term hypoxia and high-altitude exposure in subjects susceptible to high-altitude pulmonary edema. Circulation, 99:1713–1718.

16.

Eckle

, Kohler

, Lehmann

, El Kasmi

, and Eltzschig

. (2008). Hypoxia-inducible factor-1 is central to cardioprotection: A new paradigm for ischemic preconditioning. Circulation, 118:166–175.

17.

Faoro

, Boldingh

, Moreels

, et al. (2009). Bosentan decreases pulmonary vascular resistance and improves exercise capacity in acute hypoxia. Chest, 135:1215–1222.

18.

Foster

, Westerdahl

, Foster

, Hsu

, and Anholm

. (2011). Ischemic preconditioning of the lower extremity attenuates the normal hypoxic increase in pulmonary artery systolic pressure. Respir Physiol Neurobiol, 179:248–253.

19.

Gho

, Schoemaker

, van den Doel

, Duncker

, and Verdouw

. (1996). Myocardial protection by brief ischemia in noncardiac tissue. Circulation, 94:2193–2200.

20.

Ghofrani

, Reichenberger

, Kohstall

, et al. (2004). Sildenafil increased exercise capacity during hypoxia at low altitudes and at Mount Everest base camp: A randomized, double-blind, placebo-controlled crossover trial. Ann Intern Med, 141:169–177.

21.

Hausenloy

, Mwamure

, Venugopal

, et al. (2007). Effect of remote ischaemic preconditioning on myocardial injury in patients undergoing coronary artery bypass graft surgery: A randomised controlled trial. Lancet, 370:575–579.

22.

Hausenloy

, and Yellon

. (2010). The second window of preconditioning (SWOP) where are we now?. Cardiovasc Drugs Ther, 24:235–254.

23.

Hsu

, Barnholt

, Grundmann

, Lin

, McCallum

, and Friedlander

. (2006). Sildenafil improves cardiac output and exercise performance during acute hypoxia, but not normoxia. J Appl Physiol, 100:2031–2040.

24.

Jaberansari

, Baxter

, Muller

, Latouf

, Roth

, Opie

, and Yellon

. (2001). Angiotensin-converting enzyme inhibition enhances a subthreshold stimulus to elicit delayed preconditioning in pig myocardium. J Am Coll Cardiol, 37:1996–2001.

25.

Ketabchi

, Ghofrani

, Schermuly

, et al. (2012). Effects of hypercapnia and NO synthase inhibition in sustained hypoxic pulmonary vasoconstriction. Respir Res, 13:7

26.

Kharbanda

, Mortensen

, White

, et al. (2002). Transient limb ischemia induces remote ischemic preconditioning in vivo. Circulation, 106:2881–2883.

27.

Kitabatake

, Inoue

, Asao

, et al. (1983). Noninvasive evaluation of pulmonary hypertension by a pulsed Doppler technique. Circulation, 68:302–309.

28.

Marber

, Latchman

, Walker

, and Yellon

. (1993). Cardiac stress protein elevation 24 hours after brief ischemia or heat stress is associated with resistance to myocardial infarction. Circulation, 88:1264–1272.

29.

Mentzer

Jr , Rubio

, and Berne

. (1975). Release of adenosine by hypoxic canine lung tissue and its possible role in pulmonary circulation. Am J Physiol, 229:1625–1631.

30.

Murry

, Jennings

, and Reimer

. (1986). Preconditioning with ischemia: A delay of lethal cell injury in ischemic myocardium. Circulation, 74:1124–1136.

31.

Naeije

. (2010). Physiological adaptation of the cardiovascular system to high altitude. Prog Cardiovasc Dis, 52:456–466.

32.

Naeije

. (2011). Pro: Hypoxic pulmonary vasoconstriction is a limiting factor of exercise at high altitude. High Alt Med Biol, 12:309–312.

33.

Naeije

, Huez

, Lamotte

, Retailleau

, Neupane

, Abramowicz

, and Faoro

. (2010). Pulmonary artery pressure limits exercise capacity at high altitude. Eur Respir J, 36:1049–1055.

34.

Olfert

, Loeckinger

, Treml

, et al. (2011). Sildenafil and bosentan improve arterial oxygenation during acute hypoxic exercise: A controlled laboratory trial. Wilderness Environ Med, 22:211–221.

35.

Przyklenk

, Bauer

, Ovize

, Kloner

, and Whittaker

. (1993). Regional ischemic ‘preconditioning’ protects remote virgin myocardium from subsequent sustained coronary occlusion. Circulation, 87:893–899.

36.

Reeves

, Groves

, Sutton

, et al. (1987). Operation Everest II: Preservation of cardiac function at extreme altitude. J Appl Physiol, 63:531–539.

37.

Rudski

, Lai

, Afilalo

, et al. (2010). Guidelines for the echocardiographic assessment of the right heart in adults: A report from the American Society of Echocardiography endorsed by the European Association of Echocardiography, a registered branch of the European Society of Cardiology, and the Canadian Society of Echocardiography. J Am Soc Echocardiogr, 23:685–713.

38.

Russell

, Emery

, Cai

, Barer

, and Howard

. (1990). Enhanced reactivity to bradykinin, angiotensin I and the effect of captopril in the pulmonary vasculature of chronically hypoxic rats. Eur Respir J, 3:779–785.

39.

Shiva

, Huang

, Grubina

, et al. (2007). Deoxymyoglobin is a nitrite reductase that generates nitric oxide and regulates mitochondrial respiration. Circ Res, 100:654–661.

40.

Sutton

, and Houston

. (1992). The Lake Louise Consensus on the Definition and Quantification of Altitude Illness. In: Hypoxia and Mountain Medicine. Sutton

JR CG

, Houston

, ed. Queen City Printers, Burlington, Vermont.

41.

Swenson

. (2013). Hypoxic pulmonary vasoconstriction. High Alt Med Biol, 14:101–110.

42.

Sylvester

, Shimoda

, Aaronson

, and Ward

. (2012). Hypoxic pulmonary vasoconstriction. Physiol Rev, 92:367–520.

43.

Vanden Hoek

, Becker

, Shao

, Li

, and Schumacker

. (1998). Reactive oxygen species released from mitochondria during brief hypoxia induce preconditioning in cardiomyocytes. J Biol Chem, 273:18092–18098.

44.

Wagner

. (1996). Determinants of maximal oxygen transport and utilization. Annu Rev Physiol, 58:21–50.

45.

Wagner

. (2010). The physiological basis of reduced VO2max in Operation Everest II. High Alt Med Biol, 11:209–215.

46.

Wagner

, Sutton

, Reeves

, Cymerman

, Groves

, and Malconian

. (1987). Operation Everest II: Pulmonary gas exchange during a simulated ascent of Mt. Everest. J Appl Physiol, 63:2348–2359.

47.

Weyman

. (1994). Doppler Estimation of Volumetric Flow. In: Principles and Practice of Echocardiography. Weyman

, ed. Lea & Febiger, Malvern, Pennsylvania. pp 963.

48.

Yellon

, and Downey

. (2003). Preconditioning the myocardium: From cellular physiology to clinical cardiology. Physiol Rev, 83:1113–1151.

49.

Yock

, and Popp

. (1984). Noninvasive estimation of right ventricular systolic pressure by Doppler ultrasound in patients with tricuspid regurgitation. Circulation, 70:657–662.