Cerebral circulation and metabolism in chronic pulmonary emphysema, with observations on the effects of inhalation of oxygen

Comparison of cerebral blood flow in subjects with and without chronic obstructive pulmonary disease from the population-based Rotterdam Study

OBJECTIVES

Patients with chronic obstructive pulmonary disease (COPD) are at increased risk of cerebrovascular disease, which might be associated with decreases in cerebral blood flow. Since studies examining cerebral blood flow in COPD remain scarce and are limited by sample size, we aimed to study cerebral blood flow in participants with and without COPD.

DESIGN

Observational cohort study.

SETTING

Population-based Rotterdam Study.

PARTICIPANTS

4177 participants (age 68.0±8.5 years; 53% females) with and without COPD.

PREDICTOR VARIABLE

Spirometry and pulmonary diffusing capacity.

OUTCOME MEASURES

Cerebral blood flow by two-dimensional phase-contrast cerebral MRI.

RESULTS

Compared with subjects with normal spirometry (forced expiratory volume in 1 s (FEV₁)/forced vital capacity (FVC) ≥0.7 and FEV₁ ≥80%), multivariable adjusted cerebral blood flow (mL/min) was preserved in subjects with COPD Global initiative for Chronic Obstructive Lung Disease (GOLD1) (FEV₁/FVC <0.7 and FEV₁ ≥80%), but significantly lower in subjects with COPD GOLD2-3 (FEV₁/FVC <0.7 and FEV₁ <80%), even after adjustment for cardiovascular comorbidities. In sex-stratified analyses, this difference in cerebral blood flow was statistically significant in women but not in men. Cerebral blood flow was lowest in subjects with FEV₁, FVC and diffusion lung capacity for carbon monoxide % predicted values in the lowest quintile, even after adjustment for cardiovascular comorbidities and cardiac function.

CONCLUSION

We observed a lowered cerebral blood flow in subjects with COPD GOLD2-3.

Acid-base balance and cerebrovascular regulation

The regulation and defence of intracellular pH is essential for homeostasis. Indeed, alterations in cerebrovascular acid-base balance directly affect cerebral blood flow (CBF) which has implications for human health and disease. For example, changes in CBF regulation during acid-base disturbances are evident in conditions such as chronic obstructive pulmonary disease and diabetic ketoacidosis. The classic experimental studies from the past 75+ years are utilized to describe the integrative relationships between CBF, carbon dioxide tension (PCO₂ ), bicarbonate (HCO₃ ^- ) and pH. These factors interact to influence (1) the time course of acid-base compensatory changes and the respective cerebrovascular responses (due to rapid exchange kinetics between arterial blood, extracellular fluid and intracellular brain tissue). We propose that alterations in arterial [HCO₃ ^- ] during acute respiratory acidosis/alkalosis contribute to cerebrovascular acid-base regulation; and (2) the regulation of CBF by direct changes in arterial vs. extravascular/interstitial PCO₂ and pH - the latter recognized as the proximal compartment which alters vascular smooth muscle cell regulation of CBF. Taken together, these results substantiate two key ideas: first, that the regulation of CBF is affected by the severity of metabolic/respiratory disturbances, including the extent of partial/full acid-base compensation; and second, that the regulation of CBF is independent of arterial pH and that diffusion of CO₂ across the blood-brain barrier is integral to altering perivascular extracellular pH. Overall, by realizing the integrative relationships between CBF, PCO₂ , HCO₃ ^- and pH, experimental studies may provide insights to improve CBF regulation in clinical practice with treatment of systemic acid-base disorders.

Oxygen therapy improves cerebral oxygen delivery and neurovascular function in hypoxaemic chronic obstructive pulmonary disease patients

NEW FINDINGS

What is the central question of this study? How does oxygen therapy influence cerebral blood flow, cerebral oxygen delivery and neurovascular function in chronic obstructive pulmonary disease patients? What is the main finding and its importance? Oxygen therapy improves cerebral oxygen delivery and neurovascular function in chronic obstructive pulmonary disease patients. This improvement in cerebral oxygen delivery and neurovascular function might provide a physiological link between oxygen therapy and a reduced risk of cerebrovascular disease (e.g. stroke, mild cognitive impairment and dementia) in chronic obstructive pulmonary disease.

ABSTRACT

We investigated the role of hypoxaemia in cerebral blood flow (CBF), oxygen delivery (CDO₂ ) and neurovascular coupling (coupling of CBF to neural activity; NVC) in hypoxaemic chronic obstructive pulmonary disease (COPD) patients (n = 14). Resting CBF (duplex ultrasound), peripheral oxyhaemoglobin saturation (SpO2; pulse-oximetry) and NVC (transcranial Doppler) were assessed before and after a 20 min wash-in of supplemental oxygen (∼3 l min^-1 ). The peripheral oxyhaemoglobin saturation increased from 91.0 ± 3.3 to 97.4 ± 3.0% (P < 0.01), whereas CBF was unaltered (593.0 ± 162.8 versus 590.1 ± 138.5 ml min^-1 ; P = 0.91) with supplemental O₂ . In contrast, both CDO₂ (98.1 ± 25.7 versus 108.7 ± 28.4 ml dl^-1 ; P = 0.02) and NVC were improved. Specifically, the posterior cerebral artery cerebrovascular conductance was increased to a greater extent after O₂ normalization (+40%, from 20.4 ± 9.9 to 28.0 ± 10.4% increase in conductance; P = 0.04), whereas the posterior cerebral artery cerebrovascular resistance decreased to a greater extent during O₂ normalization (+22%, from -16.7 ± 7.3 to -21.4 ± 6.6% decrease in resistance; P = 0.04). The cerebral vasculature of COPD patients appears insensitive to oxygen, because CBF was unaltered in response to O₂ supplementation leading to improved CDO₂ . In patients, the improvements in CDO₂ and neurovascular function with supplemental O₂ may underlie the cognitive benefits associated with O₂ therapy.

Human cerebral blood flow control during hypoxia: focus on chronic pulmonary obstructive disease and obstructive sleep apnea

The brain is a vital organ that relies on a constant and adequate blood flow to match oxygen and glucose delivery with the local metabolic demands of active neurons. Thus exquisite regulation of cerebral blood flow (CBF) is particularly important under hypoxic conditions to prevent a detrimental decrease in the partial pressure of oxygen within the brain tissues. Cerebrovascular sensitivity to hypoxia, assessed as the change in CBF during a hypoxic challenge, represents the capacity of cerebral vessels to respond to, and compensate for, a reduced oxygen supply, and has been shown to be impaired or blunted in a number of conditions. For instance, this is observed with aging, and in clinical conditions such as untreated obstructive sleep apnea (OSA) and in healthy humans exposed to intermittent hypoxia. This review will 1) provide a brief overview of cerebral blood flow regulation and results of pharmacological intervention studies which we have performed to better elucidate the basic mechanisms of cerebrovascular regulation in humans; and 2) present data from studies in clinical and healthy populations, using a translational physiology approach, to investigate human CBF control during hypoxia. Results from studies in patients with chronic obstructive pulmonary disease and OSA will be presented to identify the effects of the disease processes on cerebrovascular sensitivity to hypoxia. Data emerging from experimental human models of intermittent hypoxia during wakefulness will also be reviewed to highlight the effects of intermittent hypoxia on the brain.

Human cerebrovascular response to combined hypoxia and hypercapnia

The N 2 O technique was used in 6 human subjects to measure cerebral blood flow and metabolism during hypoxia and hypercapnia induced by the inhalation of 10% O 2 -5% CO 2 . Ventilation increased from 7.7 to 46.3 liters/min; Pao 2 decreased from 88 to 62 mm Hg; Paco 2 increased from 38 to 45 mm Hg (for each P <.01). Mean cerebral blood flow increased from 56 to 97 ml/100 g/min ( P <.01). Because cerebral O 2 consumption was unchanged, the technique of estimating changes in cerebral flow from arterial-jugular venous O 2 differences was used to follow changes during the first 10 min of inhalation of 10% O 2 -5% CO 2 in 6 additional subjects. The rapidity of cerebral vasodilatation was increased by this combination of stimuli. The enhanced respiratory response produced by breathing 10% O 2 -5% CO 2 appeared responsible for the more rapid cerebrovascular response and may offer some protective benefits. Comparisons of the present data with previous studies lead to the conclusion that simultaneous hypoxia and hypercapnia have additive dilator effects on the cerebral vasculature. Thus, the observed increases in cerebral flow were the sum of their individual effects.

Respiratory insufficiency and chronic cor pulmonale

Because of newer technics of treatment, a small but ever increasing number of patients with chronic pulmonary disease are surviving long enough to develop chronic cor pulmonale. An attempt has been made to correlate the newer knowledge of aberrations of cardiocirculatory and respiratory physiology in chronic pulmonary disease.

Respiratory insufficiency may be due to varying proportions to the following: (1) ventilatory dysfunction or impairment of the ability to move air into or out of the lungs; this may be of two types: (a) obstructive, due primarily to airway narrowing and (b) restrictive, due to disordered function of the thoracic bellows or diminished pulmonary distensibility; (2) unequal distribution of inspired air to the alveoli (intrapulmonary mixing); (3) uneven perfusion or distribution of capillary blood flow; (4) impaired diffusion or transfer of oxygen across the alveolar capillary barrier and (5) impaired cleansing of the lung.

The same abnormal physiologic change may result from pathologic conditions which differ widely in etiology; furthermore, a particular disease may give rise to widely differing functional patterns.

The types of pulmonary disease causing chronic cor pulmonale may be divided into two main categories: (1) Type I, pulmonary diseases associated with chronic diffuse obstructive emphysema. (2) Type II, pulmonary diseases in which the pathologic process tends to be localized in or about the pulmonary vessels. In some instances, a case belongs mainly in one category but may demonstrate some features of the other.

The strain on the right ventricle is a result of: (1) increased resistance to pulmonary blood flow and (2) increased cardiac output (when present). Increased resistance to pulmonary flow may be due to: (a) reduction in caliber and distensibility of the pulmonary vascular bed which may be structural or related to hypoxia, (b) polycythemia and (c) intrapulmonary vascular shunts.

Respiratory insufficiency may be suspected from the history. Physical and roentgenologic examinations may be confirmatory, but pulmonary function tests are necessary for a precise evaluation of the functional abnormalities. The recording of a physiologic, as well as a pathologic diagnosis, is a useful practice in the individual case.

The presence of right ventricular enlargement can only be detected clinically when the chronic cor pulmonale is moderately advanced. However, clinical and roentgenologie evidence of pulmonary hypertension in chronic pulmonary disease indicates that right ventricular hypertrophy is probably present. The electrocardiogram may be helpful even in the absence of a definite right heart strain pattern since the electric position of the heart is often suggestive. Cardiac catheterization provides the best method for the early detection of pulmonary hypertension.

The treatment of chronic cor pulmonale is much more hopeful today than in the past. It is most satisfactory when cor pulmonale is due to pulmonary disease of type I. Intensive therapy of the bronchopulmonary disease is as important as specific cardiac measures. The objectives of therapy are to combat infection, produce an adequate airway and improve effective alveolar ventilation. This may partially reverse many of the pathologic changes in the lung which are responsible for hypoxia and the anatomic reduction in the pulmonary vascular bed.