Abstract
BACKGROUND: Pulmonary microcirculation abnormalities are the main determinants of pulmonary arterial hypertension (PAH) pathophysiology. We hypothesized that PAH patients have peripheral tissue microcirculation alterations that might benefit from hyperoxic breathing. We evaluated peripheral muscle microcirculation with near-infrared spectroscopy, before and after hyperoxic breathing.
METHODS: Eight PAH subjects, 8 healthy subjects (controls) matched for age, sex, and body mass index, and 16 subjects with chronic heart failure and matched for functional capacity with the PAH subjects underwent near-infrared spectroscopy. Tissue O2 saturation, defined as the hemoglobin saturation (%) in the microvasculature compartments, was measured on the thenar muscle. Then the 3-min brachial artery occlusion technique was applied before, during, and after 15 min of breathing 100% O2. We calculated the oxygen consumption rate (%/min), the reactive hyperemia time, and the time needed for tissue O2 saturation to reach its baseline value after the release of the occlusion.
RESULTS: Compared to the controls, the PAH subjects had a significantly lower resting tissue O2 saturation (65.8 ± 14.9% vs 82.1 ± 4.0%, P = .005), a trend toward a lower oxygen consumption rate (35.3 ± 9.1%/min vs 43.4 ± 19.7%/min, P = .60), and a significantly higher reactive hyperemia time (3.0 ± 0.6 min vs 2.0 ± 0.3 min, P < .001). The PAH subjects also had lower tissue O2 saturation (P = .08), lower peripheral arterial oxygen saturation (P = .01), and higher reactive hyperemia time (P = .02) than the chronic heart failure subjects. After hyperoxic breathing, the PAH subjects had increased tissue O2 saturation (65.8 ± 14.9% to 71.4 ± 14.5%, P = .01), decreased oxygen consumption rate (35.3 ± 9.1%/min to 25.1 ± 6.6%/min, P = .01), and further increased reactive hyperemia time (3.0 ± 0.6 min to 4.2 ± 0.7 min, P = .007).
CONCLUSIONS: The PAH subjects had substantial impairments of peripheral muscle microcirculation, decreased tissue O2 saturation (possibly due to hypoxemia), slower reactive hyperemia time, (possibly due to endothelium dysfunction), and peripheral systemic vasoconstriction. Acute hyperoxic breathing improved resting tissue O2 saturation (an expression of higher oxygen delivery) and decreased the oxygen consumption rate and reactive hyperemia time during reperfusion, possibly due to increased oxidative stress and evoked vasoconstriction.
- endothelium
- microcirculation
- near-infrared spectroscopy
- oxygen breathing
- pulmonary arterial hypertension
- hyperoxia
Introduction
Pulmonary arterial hypertension is (PAH) a life-threatening disease that it is associated with poor prognosis.1 PAH patients are characterized by exercise intolerance, with marked fatigue and dyspnea during exercise, limiting their daily life activity. The profound cardiopulmonary abnormalities in PAH lead to chronic hypoxemia and right ventricular failure.2
Endothelial cell dysfunction plays an important role in the pulmonary vascular remodeling of PAH.3 Increased levels of endothelin-1 and thromboxane, and decreased levels of nitric oxide synthase and prostacyclin increase vessel tone and cause cell proliferation and vasoconstriction in the pulmonary microcirculation, with progressive increase in pulmonary vascular resistance.4–6
Remodeling of small pulmonary arteries and pulmonary microcirculation abnormalities are the key elements of PAH pathophysiology; the role of the peripheral systemic microcirculation has been less investigated. Recent interesting studies found that PAH is associated with peripheral endothelial dysfunction.7–9
Current treatment for PAH is based mainly on restoring the imbalance between vasoactive mediators and correcting the hypoxemia.10 Oxygen therapy relieves the tissue hypoxia and hypoxic pulmonary vasoconstriction, but its effect on the peripheral microcirculation is not yet clearly defined.
Near-infrared spectroscopy with vascular occlusion is a valid, noninvasive technique to evaluate peripheral tissue microcirculation.11–18 We hypothesized that PAH patients have systemic peripheral tissue microcirculation alterations that might benefit from oxygen therapy, and we measured peripheral muscle microcirculation with near-infrared spectroscopy in PAH patients before, during, and after hyperoxic breathing. Figure 1 illustrates the scheme for near-infrared spectroscopy with arterial occlusion in a PAH subject and in a healthy control subject.
QUICK LOOK
Current knowledge
Pulmonary arterial hypertension (PAH) is a life-threatening disease associated with a poor prognosis. Remodeling of small pulmonary arteries and pulmonary microcirculation abnormalities are the key elements of PAH pathophysiology.
What this paper contributes to our knowledge
PAH patients exhibited impaired peripheral muscle microcirculation and decreased tissue oxygen saturation, possibly due to hypoxemia and slower reactive hyperemia time. Breathing 100% oxygen improved tissue oxygen saturation, due to higher oxygen delivery, but reactive hyperemia time decreased during reperfusion after a 3-min brachial artery occlusion, due to increased oxidative stress and evoked vasoconstriction.
Methods
This study was approved by our institution's human study committee; the study protocol conforms with the ethics guidelines of the 1975 Declaration of Helsinki, and all stages and aspects of the study conformed with the principles of ethical publishing.19
Subjects
We recruited 8 subjects (2 men and 6 women) with stable PAH, 16 subjects with stable systolic chronic heart failure (8 ischemia, 8 dilated cardiomyopathy), and 8 healthy subjects. The diagnosis of PAH was confirmed via right-heart catheterization that showed moderate to severe PAH and normal pulmonary capillary wedge pressure, and an echocardiogram that showed normal left-ventricular ejection fraction (Table 1). Right-heart catheterization in the PAH subjects was performed within 1 month of the near-infrared spectroscopy evaluation, as part of their PAH follow-up in our institution. None of the PAH subjects had history/evidence of thromboembolic pulmonary disease on isotope perfusion scanning of the lungs, or other secondary causes of PAH. All the chronic heart failure subjects were stable and on optimal medical treatment, including beta-blockers, angiotensin-converting-enzyme inhibitors, aldosterone antagonists, and furosemide for at least 3 months. The chronic heart failure group was matched with the PAH group for age, sex, body mass index, and functional exercise capacity (New York Heart Association class).
In the PAH group: 4 subjects had PAH associated with congenital cardiac disease (2 had an atrial septal defect surgically corrected, 1 had a common ventricle with transposition of the great vessels, and 1 had an atrial septal defect not surgically corrected); 2 subjects had PAH associated with connective tissue disease (1 systemic lupus erythematosus, and 1 systemic scleroderma); 1 subject had PAH associated with hereditary spherocytosis, without hemolytic anemia (hemoglobin 16.6 g/dL), without thrombocytosis, and with history of splenectomy; and 1 subject had PAH associated with chronic drug abuse. One of the subjects with atrial septal defect had a permanent pacemaker VVI (ventricle paced, ventricle sensed, inhibited pace if beat sensed), implanted 5 years before enrollment in the study, due to complete atrioventricular block. Seven of the PAH subjects had clinical functional capacity of New York Heart Association class II, and one subject was classified as New York Heart Association class III. Seven of the PAH subjects were under treatment with diuretics, 6 with sildenafil, 5 subjects with bosentan, 3 with digoxin, 2 with nifedipine, and 6 with anticoagulants. We excluded people who had a history of smoking, alcohol, diabetes mellitus, systemic arterial hypertension, dyslipidemias, or obesity.
The control group consisted of 8 healthy subjects individually matched with the PAH subjects for age, sex, and body mass index. None of the healthy subjects was taking medications.
Study Design
All the subjects were evaluated under the same conditions: in the morning, in the seated position, at rest for at least 20 min, and without a meal for at least 8 h. Peripheral microcirculation was measured with near-infrared spectroscopy and the 3-min vascular occlusion technique, as previously described.12–14,17,18 The healthy subjects and the chronic heart failure subjects underwent near-infrared spectroscopy while breathing room air. The PAH subjects underwent near-infrared spectroscopy in 3 conditions: while breathing room air; after 15 min of breathing 100% oxygen via non-rebreathing mask; and 15 min after stopping the oxygen inhalation. SpO2 and heart rate were continuously monitored during near-infrared spectroscopy.
Near-Infrared Spectroscopy
The use of near-infrared spectroscopy in humans has been previously validated.20 With a probe placed on the surface of the skin, near-infrared spectroscopy uses the principles of light spectroscopy to measure tissue O2 saturation, the percentage of hemoglobin saturation in all 3 tissue vascular compartments (arterial, capillary, venous). The light transmitted from the probe has a penetration depth of approximately 12.5 mm, thus enabling measurement within the muscle.
We conducted near-infrared spectroscopy (InSpectra, Hutchinson Technology, Hutchinson, Minnesota) on the thenar muscle. Tissue O2 saturation (%) was monitored continuously before, during, and after brachial artery occlusion with a pneumatic cuff inflated to 50 mm Hg above the subject's systolic blood pressure, for 3 min. Movement of the upper limb was not allowed during the measurements. The tissue O2 saturation curves were analyzed with the spectrascope's software (Inspectra Analysis 2.0, Hutchinson Technology, Hutchinson, Minnesota) running in statistics software (MatLab 7.0, MathWorks, Natick, Massachusetts). The first degree slope of the saturation curve during arterial occlusion was used to calculate the oxygen consumption rate (%/min) (see Fig. 1). The first degree slope of the saturation curve after release of the brachial artery occlusion was used to calculate the reperfusion rate (%/min). Endothelial function was estimated by calculating the time needed for tissue O2 saturation to reach the highest value at the end of the occlusion period. Tissue oxygen restoration rate, another index of endothelial function, was calculated as the reactive hyperemia time, which is the time needed for tissue O2 saturation to return to baseline after the release of the occlusion.
Statistical Analysis
Continuous variables are presented as mean ± SD. Prior to analysis, the continuous variables were tested with the Kolmogorov-Smirnov test. Group means of continuous variables that showed a normal distribution were compared with the unpaired Student t test. Continuous variables with non-normal distribution were compared with the non-parametric Mann-Whitney U test. Analysis of variance with post hoc Bonferroni multiple comparisons was performed for continuous variables for between-group differences (PAH vs chronic heart failure vs control). Repeated-measures analysis of variance was used to compare the within-group before, during, and after data. The level of significance was set at P < .05. The statistical analysis was performed with statistics software (SPSS 14.0, SPSS, Chicago, Illinois).
Results
There were no statistical differences in age, sex, or body mass index between the subjects and controls. The chronic heart failure subjects had lower left-ventricular ejection fraction and larger left-ventricular end-diastolic diameter than the PAH subjects (P < .001).
PAH Subjects Versus Healthy Subjects
The subjects with PAH had significantly lower tissue O2 saturation and SpO2 than the control subjects (P = .005 and P = .01, respectively). The reactive hyperemia time was significantly longer in the PAH subjects than in the controls (P < .001). There was no significant difference in oxygen consumption rate between the PAH subjects and the control subjects (P = .60). There were trends toward a shorter time from lowest to highest tissue O2 saturation after occlusion (P = .11) and reperfusion rate (P = .06) in the PAH subjects, compared to the control subjects (Table 2).
PAH Subjects Versus Chronic Heart Failure Subjects
The PAH subjects had lower tissue O2 saturation (P = .08) and SpO2 (P = .01), and higher reactive hyperemia time (P = .02) than the chronic heart failure subjects (see Table 2). There was no significant difference in oxygen consumption rate between the PAH and chronic heart failure subjects.
Tissue O2 Saturation in the PAH Subjects
In the PAH subjects, after 15 min of breathing 100% O2 there was a significant increase in tissue O2 saturation (P = .01) and reactive hyperemia time (Fig. 2) (P = .007), and a significant decrease in oxygen consumption rate (P = .01), compared to the baseline values. All the measurements had a trend to return to baseline after discontinuation of oxygen (Table 3).
Discussion
This study shows that PAH patients are characterized by several skeletal muscle tissue microcirculation alterations, as assessed with near-infrared spectroscopy during vascular occlusion. More specifically, the PAH subjects had a lower resting skeletal muscle tissue oxygenation, a trend toward a decreased oxygen consumption rate, and a slower tissue oxygen restoration rate during reperfusion than did the control subjects. The PAH subjects also had a slower tissue oxygen restoration rate and a trend toward a lower resting skeletal muscle tissue oxygenation than the chronic heart failure subjects, who were matched with the PAH subjects for exercise capacity.
By applying the near-infrared spectroscopy with occlusion technique we have assessed several novel aspects of peripheral tissue microcirculation in PAH subjects, monitoring in this way tissue microcirculation changes during hyperoxic breathing. To our knowledge, this study is the first to evaluate skeletal muscle microcirculation with near-infrared spectroscopy and vascular occlusion in PAH subjects, and to monitor systemic microcirculation changes after chemo-reflex deactivation.
Recent studies have found impaired peripheral endothelial function in subjects with PAH, in relation to disease severity.7–9 Endothelium function was measured with ultrasound during brachial artery flow-mediated vasodilatation during reactive hyperemia7 and with noninvasive plethysmography assessing post-occlusion brachial artery flow-mediated vasodilatation.8,9 Our study confirms those findings related to endothelium dysfunction in PAH subjects.
In our study the resting tissue oxygen saturation differences between PAH subjects and healthy subjects can be explained mainly by the hypoxemia in the PAH subjects, which decreases oxygen delivery and tissue oxygen saturation. Low cardiac output in PAH might also be a contributing factor. Low tissue oxygen saturation is related to disease severity in patients with chronic left-ventricular dysfunction, and can be improved with intravenous inotropic agents13 and by exercise training.14 In our study the PAH subjects had lower tissue oxygen saturation than the chronic heart failure subjects, but PAH subjects did not have low cardiac output, making it less likely that low cardiac output was the cause of their tissue oxygen saturation. Another possible explanation might be increased sympathetic tone in PAH, which decreases tissue perfusion. Interestingly, recent studies have shown increased sympathetic nerve activity21 and impaired autonomic nervous activity, which is related to disease severity,22,23 in PAH patients.
During vascular occlusion we found a trend toward a decreased oxygen consumption rate in the PAH subjects, compared to the control subjects, indicating a relatively low tissue oxygen diffusion rate, possibly due to skeletal muscle abnormalities (reduction of oxidative muscle fibers, depletion of mitochondria). This might be explained by the presence of skeletal muscle atrophy due to chronic low tissue perfusion and deconditioning in PAH patients. Recent studies have shown substantial skeletal muscle dysfunction in PAH patients.24,25 Imbalance between vasoconstrictor and vasodilator microvascular tone regulated mainly by local (endothelial function) and systemic neuro-humoral factors seems to be the main reason for the slower tissue oxygen restoration rate during reperfusion in PAH patients. In PAH there is an imbalance between the excretion of thromboxane and prostacyclin metabolites,6 decreased nitric oxide synthase,5 and high plasma endothelin-1.4 Endothelium dysfunction plays a central role in the initiation and progression of PAH.3 Gabrielli et al26 recently found that adults with PAH have increased oxidative stress and endothelial dysfunction markers. Friedman et al27 found that children with idiopathic PAH have substantial systemic endothelial dysfunction, in association with disease severity. However, whether those abnormalities in systemic endothelium function affect the PAH disease process or are a consequence of PAH is not yet known. Further longitudinal studies are needed to investigate the exact mechanism. Another important finding in our study is that supplemental oxygen improves resting skeletal muscle tissue oxygenation, while decreasing the oxygen consumption rate and the tissue oxygen restoration rate in PAH subjects.
The acute changes of peripheral tissue microcirculation with oxygen supplementation in PAH subjects consisted of improving resting tissue oxygen saturation, mainly as an expression of higher oxygen delivery due to improved hypoxemia, and less possibly to higher cardiac output. The acute correction of hypoxemia induces a slight reduction in sympathetic activity in PAH patients,21,28 which indirectly reduces the stiffness of the main pulmonary artery,28 lowering right-ventricular after-load. However, studies in healthy subjects found that high FIO2 induces a vagus-dependent decrease in heart rate, a rate-dependent decrease in cardiac index, and an increase in peripheral systemic resistance.29–32
Interestingly, the oxygen consumption rate was slowed, and there was a more pronounced delay in the tissue oxygen restoration rate after hyperoxic breathing, suggesting deterioration in peripheral tissue microcirculation in PAH subjects. The exact mechanism of these changes is not known, but a possible explanation is increased oxidative stress, the formation of reactive oxygen species, and the peripheral vasoconstriction induced by high FIO2.29 The latter mechanism has also been proposed by recent investigators who used flow-mediated vasodilatation assessment in conductance arteries and in the microvasculature in healthy adults30 and in patients with high cardiovascular risk.31 In a previous important study it was shown that pulmonary vascular resistance decreased and systemic vascular resistance increased after hyperoxic breathing in patients with severe pulmonary vascular obstructive disease, the effect being more pronounced on the systemic than on the pulmonary vascular bed.33 Furthermore, as recent systemic reviews have shown, high FIO2 markedly reduces coronary blood flow and myocardial oxygen consumption,34 while there is limited evidence of routine use of oxygen therapy in uncomplicated myocardial infarction.35 Interestingly, in a recent study detrimental effects of oxygen therapy were noted in patients with heart failure.36 The risk of detrimental effects from high FIO2 on peripheral tissue microcirculation must be the aim of future studies to establish the benefits and safety limits of oxygen supplementation in PAH patients, and near-infrared spectroscopy during arterial occlusion might be a useful tool in that research.
Clinical Implications
PAH patients present with several abnormalities in peripheral tissue microcirculation that can be detected with near-infrared spectroscopy during arterial occlusion. Oxygen supplementation induces changes in peripheral tissue microcirculation of PAH patients that can be evaluated and monitored with near-infrared spectroscopy. This simple, noninvasive technique provides valuable information for monitoring and prognosis, and may detect PAH patients with worse peripheral tissue microcirculation abnormalities, who might benefit from endothelin antagonists, prostanoid analogs, phosphodiesterase inhibitors, angiotensin-converting enzyme inhibitors, oxygen therapy, and/or exercise training.
Limitations
Our small number of PAH subjects did not allow us to study a possible correlation with disease severity. While we did not directly evaluate endothelium function, we used the near-infrared spectroscopy during arterial occlusion as an indirect evaluation of peripheral tissue microcirculation. A non-significantly lower oxygen consumption rate was found in the PAH subjects than in the controls. This was possibly due to the small number of PAH subjects. We did not provide cardiac output in the chronic heart failure subjects, because only a few of these subjects had had a recent right-heart catheterization. However, the chronic heart failure group might have had a rather low cardiac output, compared to the PAH group, which had a normal cardiac output (see Table 2). Also, the PAH subjects were receiving endothelin antagonists and sildenafil, which might have affected our results. However, endothelin antagonists and sildenafil are potent vasodilators that improve endothelial function, so this treatment enhances rather than favors the negative findings in our PAH subjects. We identified systemic microcirculation changes in the PAH subjects by comparing their results to those of healthy subjects, but the exact cause of these findings cannot be confirmed by our study, so future studies are required.
Conclusions
PAH patients have alterations in peripheral tissue microcirculation, as assessed with near-infrared spectroscopy during arterial occlusion. Specifically, PAH patients exhibit decreased tissue oxygen saturation, possibly due to hypoxemia, and a slower tissue oxygen restoration rate, possibly due to endothelium dysfunction and peripheral vasoconstriction in the systemic microcirculation. High FIO2 improved the resting tissue oxygen saturation, as an expression of higher oxygen delivery, and decreased the tissue oxygen consumption rate and tissue oxygen restoration rate during reperfusion, possibly due to increased oxidative stress and evoked vasoconstriction.
Footnotes
- Correspondence: Stavros Dimopoulos MD, Cardiopulmonary Exercise Testing and Rehabilitation Laboratory, First Critical Care Medicine Department, Evgenidio Hospital, National and Kapodistrian University of Athens, Papadiamantopoulou Street 20, Athens, 11528, Greece. E-mail: a-icu{at}med.uoa.gr.
Dr Dimopoulos presented a version of this paper at the International Conference of the American Thoracic Society, held May 13–18, 2011, in Denver, Colorado.
This research was supported by a grant from the National and Kapodistrian University of Athens, Athens, Greece. The authors have disclosed no conflicts of interest.
- Copyright © 2013 by Daedalus Enterprises
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