Search

We propose a multivariate dynamical adjustment (MDA) modeling approach to assess the strength of baroreflex and cardiopulmonary couplings from spontaneous cardiovascular variabilities. Open loop MDA (OLMDA) and closed loop MDA (CLMDA) models were compared. The coupling strength was assessed during progressive sympathetic activation induced by graded head-up tilt. Both OLMDA and CLMDA models suggested that baroreflex coupling progressively increased with tilt table inclination. Only CLMDA model indicated that cardiopulmonary coupling due to the direct link from respiration to heart period gradually decreased with tilt table angles, while that due to the indirect link mediated by systolic arterial pressure progressively increased., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

In physiological conditions, heart period (HP) affects systolic arterial pressure (SAP) through diastolic runoff and Starling's law, but, the reverse relation also holds as a result of the continuous action of baroreflex control. The prevailing mechanism sets the dominant temporal direction in the HP-SAP interactions (i.e., causality). We exploited cross-conditional entropy to assess HP-SAP causality. A traditional approach based on phases was applied for comparison. The ability of the approach to detect the lack of causal link from SAP to HP was assessed on 8 short-term (STHT) and 11 long-term heart transplant (LTHT) recipients (i.e., less than and more than 2 yr after transplantation, respectively). In addition, spontaneous HP and SAP variabilities were extracted from 17 healthy humans (ages 21-36 yr, median age 29 yr; 9 females) at rest and during graded head-up tilt. The tilt table inclinations ranged from 15 to 75° and were changed in steps of 15°. All subjects underwent recordings at every step in random order. The approach detected the lack of causal relation from SAP to HP in STHT recipients and the gradual restoration of the causal link from SAP to HP with time after transplantation in the LTHT recipients. The head-up tilt protocol induced the progressive shift from the prevalent causal direction from HP to SAP to the reverse causality (i.e., from SAP to HP) with tilt table inclination in healthy subjects. Transformation of phases into time shifts and comparison with baroreflex latency supported this conclusion. The proposed approach is highly efficient because it does not require the knowledge of baroreflex latency. The dependence of causality on tilt table inclination suggests that "spontaneous" baroreflex sensitivity estimated using noncausal methods (e.g., spectral and cross-spectral approaches) is more reliable at the highest tilt table inclinations.

We checked whether the observed shift of the dominant causality from heart period (HP) to systolic arterial pressure (SAP) in supine position to the reverse causal direction in upright position could be the result of the exogenous action of respiration on both variables. A model-based approach exploiting a multivariate dynamic adjustment class was utilized to decompose HP and SAP dynamics into partial processes and cancel respiratory-related influences from HP and SAP series. Causality was assessed in the information domain through a bivariate approach based on cross-conditional entropy. After canceling respiratory-related influences we observed the same trend on causality from supine to upright position as detected from the original series, thus suggesting that respiratory influences are not responsible per se for HP-SAP causal relations.

An information domain approach to the assessment of causality was applied to the beat-to-beat variability of heart period and systolic arterial pressure to test the open loop condition along baroreflex in heart transplant recipients. The closed loop between heart period and systolic arterial pressure was detected as open at the level of the baroreflex if systolic arterial pressure is more easily predictable from heart period than vice versa according to a conditional entropy approach. We found that in short-term heart transplant (STHT) recipients (less than 2 years after transplantation) the closed loop between heart period and systolic arterial pressure was open at the level of baroreflex. Baroreflex appeared to be involved in the heart period regulation in long-term heart transplant (LTHT) recipients (more than 2 years after transplantation). The significant linear correlation of causality index on the number of months after transplantation suggests that baroreflex control recovers after transplantation.

We validated a symbolic approach to assess autonomic modulation from pulse interval (PI) and systolic arterial pressure (SAP) series obtained from an animal model of chronic heart failure (CHF). We studied three groups of rats: controls; CHF animals; CHF animals treated with spironolactone (CHF-SP), reducing sympathetic activity in CHF. Simulations confirmed that symbolic analysis captures modifications of cardiovascular regulation in the case of fast dynamics and negligible variance. While spectral indexes did not reveal any significant difference among groups, symbolic analysis pointed out that sympathetic modulation is reduced in CHF group and restored to basal values in CHF-SP one.

To compare post-resistance exercise hypotension ( PREH) and its mechanisms in normotensive and hypertensive individuals, 14 normotensives and 12 hypertensives underwent two experimental sessions: control (rest) and exercise (seven exercises, three sets, 50% of one repetition maximum). Hemodynamic and autonomic clinic measurements were taken before (Pre) and at two moments post-interventions (Post 1: between 30 and 60 min; Post 2: after 7 h). Ambulatory blood pressure ( BP) was monitored for 24 h. At Post 1, exercise decreased systolic BP similarly in normotensives and hypertensives (−8 ± 2 vs −13 ± 2 mmHg, P > 0.05), whereas diastolic BP decreased more in hypertensives (−4 ± 1 vs −9 ± 1 mmHg, P < 0.05). Cardiac output and systemic vascular resistance did not change in normotensives and hypertensives (0.0 ± 0.3 vs 0.0 ± 0.3 L/min; −1 ± 1 vs −2 ± 2 U, P > 0.05). After exercise, heart rate (+13 ± 3 vs +13 ± 2 bpm) and its variability (low- to high-frequency components ratio, 1.9 ± 0.4 vs +1.4 ± 0.3) increased whereas stroke volume (−14 ± 5 vs −11 ± 5 mL) decreased similarly in normotensives and hypertensives (all, P > 0.05). At Post 2, all variables returned to pre-intervention, and ambulatory data were similar between sessions. Thus, a session of resistance exercise promoted PREH in normotensives and hypertensives. Although this PREH was greater in hypertensives, it did not last during the ambulatory period, which limits its clinical relevance. In addition, the mechanisms of PREH were similar in hypertensives and normotensives. [ABSTRACT FROM AUTHOR]