Sodium sensitivity in young adults with high resting end-tidal CO2

Capnometric feedback training decreases 24-h blood pressure in hypertensive postmenopausal women

BACKGROUND

High normal resting pCO₂ is a risk factor for salt sensitivity of blood pressure (BP) in normotensive humans and has been associated with higher resting systolic BP in postmenopausal women. To date, however, no known studies have investigated the effects of regular practice of voluntary mild hypocapnic breathing on BP in hypertensive patients. The objective of the present research was to test the hypothesis that capnometric feedback training can decrease both resting pCO₂ and 24-h BP in a series of mildly hypertensive postmenopausal women.

METHODS

A small portable end tidal CO₂ (etCO₂) monitor was constructed and equipped with software that determined the difference between the momentary etCO₂ and a pre-programmed criterion range. The monitor enabled auditory feedback for variations in CO₂ outside the criterion range. 16 mildly hypertensive postmenopausal women were individually trained to sustain small decreases in etCO₂ during six weekly sessions in the clinic and daily sessions at home. 24-h BP monitoring was conducted before and after the intervention, and in 16 prehypertensive postmenopausal women in a control group who did not engage in the capnometric training.

RESULTS

Following the intervention, all 16 capnometric training participants showed decreases in resting etCO₂ (- 4.3 ± 0.4 mmHg; p < .01) while 15 showed decreases in 24-h systolic BP (- 7.6 ± 2.0 mmHg; p < .01). No significant changes in either measure was observed in the control group. In addition, nighttime (- 9.5 ± 2.6; p < .01) and daytime (- 6.7 ± 0.2 mmHg) systolic BP were both decreased following capnometric training, while no significant changes in nighttime (- 2.8 ± 2.2 mmHg; p = .11) or daytime (- 0.7 ± 1.0 mmHg; p ≤ .247) systolic BP were observed in the control group.

CONCLUSIONS

These findings support the hypothesis that regular practice of mild hypocapnic breathing that decreases resting etCO₂ reliably decreases 24-h blood pressure in hypertensive postmenopausal women. The extent to which these effects persist beyond the training period or can be observed in other hypertensive subgroups remains to be investigated.

Dietary sodium effects on heart rate variability in salt sensitivity of blood pressure

High dietary sodium intake is a risk factor for hypertension, and heart rate variability (HRV) is decreased in hypertension. Effects of dietary sodium intake on HRV of normotensive persons have not, however, been investigated to date. The present study examined effects of low and high sodium diets on blood pressure, heart rate, and HRV in 36 healthy, normotensive women, ages 40-70. Each was placed on a low sodium diet for 6 days followed by a high sodium diet for 6 days. The high salt diet increased mean systolic blood pressure, decreased heart rate, and increased high frequency HRV (HF). Cardiac vagal tone, estimated at baseline from heart period and a time domain estimate of respiratory sinus arrhythmia, was higher in salt-sensitive than salt-insensitive subjects. The finding of increased vagal tone in normotensive persons on high salt intake indicates that dietary sodium status should be considered in behavioral studies of HRV.

Determining the relationship of heart rate and blood pressure using voluntary cardio-respiratory synchronization (VCRS)

Voluntary cardio-respiratory synchronization (VCRS) was used to investigate heart rate and blood pressure changes in the supine position in 21 subjects. VCRS involves a breathing pattern that is synchronized with the cardiac cycle. The signals to inhale and exhale are derived from the ECG. In this study, the subjects inspired for four heart beats and expired for four heart beats for 35 cycles. This technique is designed to have the heart beat occur at exactly the same phase in the respiratory cycle and lends itself to the study of the influence of the respiration cycle on heart rate and blood pressure changes. The heart rate and blood pressure changed simultaneously in the same direction, with the largest significant positive change occurring on the second heart beat during inspiration. The authors discuss the potential of VCRS for research, and clinical applications as a respiration modulator for hypertension therapy or increased heart rate variability.

Nitric oxide inhibition as a mechanism for blood pressure increase during salt loading in normotensive postmenopausal women

OBJECTIVES

Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide (NO), which plays an important role in natriuresis. We determined whether changes in endothelium-dependent vasodilation (EDD) and plasma ADMA predict changes in blood pressure (BP) after salt loading in normotensive postmenopausal women (PMW).

METHODS

In 15 normotensive PMW (age 50-60 years), not receiving estrogen, ambulatory 24-h BP, plasma lipids, and ADMA were measured after 4 days of a low-salt diet (70 mEq/day) and following 7 days of high-salt intake (260 mEq/day). Brachial artery diameter at rest, during reactive hyperemia, i.e. EDD, and after sublingual nitroglycerin, i.e. non-EDD, were measured by ultrasound. The 24-h urinary NO metabolite (NOx) was measured by Griess reaction. Plasma ADMA was measured by high-pressure liquid chromatography.

RESULTS

During low-salt, 24-h BP levels averaged 121 +/- 11 and 69 +/- 7 mmHg for systolic BP (SBP) and diastolic BP (DBP), respectively. After salt loading, average 24-h BP increases were: 7.6 mmHg for SBP, 2.2 mmHg for DBP, and 5.5 mmHg for pulse pressure (PP). Increases of 24-h SBP and 24-h PP after salt loading correlated directly with changes in ADMA (partial R2 = 0.16 for 24-h SBP and 0.17 for 24-h PP, P < 0.05 for both) and inversely with changes in EDD (partial R2 = 0.13, P = 0.09 for 24 h SBP and partial R2 = 0.15, P = 0.07 for 24-h PP), after adjustment for age and cholesterol.

CONCLUSIONS

Inhibition of NO bioavailability by ADMA and a subsequent reduction in EDD contribute to the increase in BP during high-salt intake in normotensive PMW not receiving estrogen.

Respiratory psychophysiology in hypertension research

The pathogenesis of hypertension results from an interaction of genetic and environmental factors. Behavioral factors might participate in sodium sensitive forms of hypertension via a cascade of physiological responses triggered by conditioned inhibition of breathing. When an individual decreases ventilation sufficient to increase pCO2 but not sufficient to activate chemoreceptor reflexes, plasma pH decreases transiently to stimulate a renal mechanism that can expand plasma volume via sodium retention. The combination of high resting pCO2 and high sodium intake elevates resting blood pressure in laboratory animals and healthy human participants. In the natural environment, this mechanism seems to be more important for the development of hypertension in women than in men, perhaps due to differential expression of anger and aggression. Studies are needed to clarify the role of breathing pattern in individual differences in resting pCO2 and the effects of breathing interventions on salt sensitivity and sodium sensitive forms of hypertension.

Racial differences in resting end-tidal CO2 and circulating sodium pump inhibitor

Previous studies have shown that high end-tidal CO2 (PetCO2) is a marker for sodium sensitivity of blood pressure (BP) in White Americans, and that the BP of African Americans is more sensitive to high sodium intake than that of whites. The present study tested the hypothesis that resting PetCO2 is higher in normotensive African Americans than in whites. Resting end-tidal CO2 of 395 white and 125 African American participants in the Baltimore Longitudinal Study on Aging was monitored for 20 min with a respiratory gas monitor, and BP and heart rate were recorded every 5 min by oscillometric methodology. Twenty-four-hour urinary excretion of a circulating sodium pump inhibitor marinobufagenin-like compound (MBG), which increases when plasma volume is expanded, was also analyzed by fluoroimmunoassay in racial groups. Mean resting PetCO2 of African American men was higher than that of white men (38.1+/-0.5 v 36.4+/-0.3 mm Hg), and resting PetCO2 of African American women was higher than that of white women (37.7+/-0.3 v 36.2+/-0.3 mm Hg). The differences were not significant in either men or women less than 50 years old, but were substantial in both men and women more than 50 years. Twenty-four-hour urinary excretion of MBG was higher in white (2.7+/-0.2 pmol) than in African American (2.1+/-0.2 pmol) participants, and high PetCO2 was a significant independent predictor of high MBG excretion in African Americans. These data are consistent with the hypothesis that the higher resting PetCO2 in African Americans plays a role in slower urinary excretion of sodium, greater BP sensitivity to high sodium intake, and increased prevalence of chronic hypertension.

Anger inhibition potentiates the association of high end-tidal CO2 with blood pressure in women

OBJECTIVE

High resting end-tidal CO2 (PetCO2) has been shown to be an independent predictor of systolic blood pressure (SBP) in women, particularly older women. The study reported in this article investigated whether the tendency to experience, express, and/or suppress anger contributes to the association of PetCO2 and SBP in women and in men.

METHODS

The Spielberger Anger Expression Inventory was administered to 403 healthy male and female participants in the Baltimore Longitudinal Study on Aging. Resting PetCO2 was obtained by means of a respiratory gas monitor, and resting blood pressure was obtained with an oscillometric device. The associations of resting PetCO2 and the anger scales with SBP and diastolic blood pressure (DBP) were investigated using multivariate regression analyses.

RESULTS

PetCO2, as well as age and body mass index, was an independent predictor of SBP in women with low, but not high, trait anger and in women with low, but not high, anger-out. PetCO2 was not an independent predictor of SBP in men with either high or low anger. In addition, PetCO2 was not an independent predictor of DBP in either men or women.

CONCLUSIONS

The results of this study indicate that inhibition of anger potentiates the relationship of high PetCO2 with SBP in women but not men. Additional studies are needed to determine the origins of the observed gender differences and the psychophysiological pathways by which high resting PetCO2 contributes to elevated resting blood pressure in women.

End tidal CO2 is an independent determinant of systolic blood pressure in women

OBJECTIVE

Recent studies have shown that high resting end tidal CO2 (PECO2) is a marker for blood pressure sensitivity to high sodium intake by normotensive humans. The present study investigated the association of resting PECO2 with resting blood pressure in participants in the Baltimore Longitudinal Study on Aging (BLSA).

DESIGN

PECO2 of 314 healthy participants (153 men and 161 women, aged 21-79 years) was measured by a respiratory gas monitor during 25 min of seated rest, and blood pressure every 5 min by an automated oscillometric system. The independent associations of PECO2 and other variables with systolic (SBP) and diastolic (DBP) blood pressure were analysed via multiple regression. Sex differences in age-associated changes in PECO2 and in the role of pulmonary capacity in the PECO2 -blood pressure relationships were also studied.

RESULTS

Resting PECO2 was an independent predictor of SBP in women (beta = 0.215; P<0.0015; overall r2 = 0.27; P<0.0001), and accounted for more than 10% of the variance in SBP in women over age 50 years. No such associations of PECO2 with SBP of men, or with DBP of men or women, were observed. PECO2 was lower in younger women (36.0+/-0.9 mm Hg) than in younger men (40.4+/-2.0), but higher in older women (37.1+/-0.5) than in older men (35.5+/-0.4).

CONCLUSIONS

PECO2 is an independent determinant of resting SBP in women, especially those aged 50 years or more. The origin of individual differences in resting PECO2 and its possible role in the development of chronic hypertension remain to be clarified.