Mean Arterial Pressure Fluctuations Research Articles

Lower dynamic cerebral autoregulation following acute bout of low-volume high-intensity interval exercise in chronic stroke compared to healthy adults.

Fluctuating arterial blood pressure during high-intensity interval exercise (HIIE) may challenge dynamic cerebral autoregulation (dCA), specifically after stroke after an injury to the cerebrovasculature. We hypothesized that dCA would be attenuated at rest and during a sit-to-stand transition immediately after and 30 min after HIIE in individuals poststroke compared with age- and sex-matched control subjects (CON). HIIE switched every minute between 70% and 10% estimated maximal watts for 10 min. Mean arterial pressure (MAP) and middle cerebral artery blood velocity (MCAv) were recorded. dCA was quantified during spontaneous fluctuations in MAP and MCAv via transfer function analysis. For sit-to-stand, time delay before an increase in cerebrovascular conductance index (CVCi = MCAv/MAP), rate of regulation, and % change in MCAv and MAP were measured. Twenty-two individuals poststroke (age 60 ± 12 yr, 31 ± 16 mo) and twenty-four CON (age 60 ± 13 yr) completed the study. Very low frequency (VLF) gain (P = 0.02, η2 = 0.18) and normalized gain (P = 0.01, η2 = 0.43) had a group × time interaction, with CON improving after HIIE whereas individuals poststroke did not. Individuals poststroke had lower VLF phase (P = 0.03, η2 = 0.22) after HIIE compared with CON. We found no differences in the sit-to-stand measurement of dCA. Our study showed lower dCA during spontaneous fluctuations in MCAv and MAP following HIIE in individuals poststroke compared with CON, whereas the sit-to-stand response was maintained.NEW & NOTEWORTHY This study provides novel insights into poststroke dynamic cerebral autoregulation (dCA) following an acute bout of high-intensity interval exercise (HIIE). In people after stroke, dCA appears attenuated during spontaneous fluctuations in mean arterial pressure (MAP) and middle cerebral artery blood velocity (MCAv) following HIIE. However, the dCA response during a single sit-to-stand transition after HIIE showed no significant difference from controls. These findings suggest that HIIE may temporarily challenge dCA after exercise in individuals with stroke.

Non-intubated deep paralysis: a new anaesthesia strategy for vocal cord polypectomy

BackgroundVocal cord polyp is common otorhinolaryngological disease, traditionally treated by vocal cord polypectomy under a supporting laryngoscope with general anaesthesia. Although it is safe and controllable, it would cause some anaesthesia complications. Moreover, the complex process of general anaesthesia may significantly reduce surgical efficiency. Avoiding these problems remains an important issue.MethodsAll patients were subjected to the standard non-intubated deep paralysis (NIDP) protocol consisting of four phases. An emergency plan was launched when NIDP cannot be implemented successfully. Patient characteristics, blood gas and monitoring data were collected during NIDP. Data concerning satisfaction, complications and duration of anaesthesia and recovery were collected to assess its effectiveness.ResultAmong 20 enrolled patients, the success rate of NIDP was 95%. Only one patient failed in completing NIDP. Blood gas analysis revealed that the partial pressure of oxygen and carbon dioxide was maintained at safe levels. Monitoring during NIDP revealed fluctuations in mean arterial pressure between 110 and 70 mmHg, and the heart rate was stable at 60–100 beats per minute. The duration of anaesthesia and postoperative recovery were 13.0 ± 2.84 and 5.47 ± 1.97 min, respectively. All patients and surgeons were satisfied with NIDP, and no complications were detected before discharge.ConclusionNIDP can be safely applied to patients and can replace general anaesthesia in vocal cord polypectomy. It can significantly reduce the duration of anaesthesia and postoperative recovery. No anaesthesia complications occurred without intubation, and patients and surgeons were satisfied with NIDP.Trial registrationsThis single-centre, prospective study was registered on clinicaltrial.gov (NCT04247412) on 30th July 2020.

CHRONOBIOLOGICAL ASPECTS OF ARTERIAL PRESSURE LEVEL IN SCHOOLCHILDREN WITH ARTERIAL PREHYPERTENSION

Introduction. Arterial pressure has significant diurnal fluctuations, reaching its minimum and maximum values at certain hours of the day. For a complete characterization of arterial hypertension in recent years, the method of daily monitoring of arterial pressure is most often used, which makes it possible to determine the forms of the disease (labile and stable), circadian rhythms of arterial pressure and its variability. The use of this method in children may be even more important than in adults.Aim: to evaluate the chronobiological aspects of the level of arterial pressure in schoolchildren with arterial prehypertension.Materials and methods. 54 school-age children were examined, who were divided into the following groups: the main group consisted of 24 children with arterial prehypertension (high blood pressure) and the control group - 30 children with normal blood pressure.Chronobiological aspects of the level of arterial pressure in schoolchildren were assessed using daily monitoring of arterial pressure. The 24-hour blood pressure device provided long-term recording of blood pressure fluctuations during normal 24-hour activity and during patient’s sleep.Statistical processing of the obtained data was carried out using Microsoft Office Excel and Statistica 10.0 applications. Research results. When evaluating the characteristics of time indices of blood pressure in the examined children, it was found that children with prehypertension most often had an increase in systolic blood pressure at night.Comparing the features of daily blood pressure fluctuations between children with prehypertension and children from the control group, certain differences were identified that were observed in the curves of systolic blood pressure and diastolic blood pressure, especially at night.When assessing the variability of the average daily and average night levels of systolic and diastolic blood pressure in children by groups, it was found that the variability of systolic and diastolic blood pressure in children with prehypertension was probably lower than in children from the control group.Additionally, the features of the daily fluctuations in mean arterial pressure were evaluated, because it simultaneously takes into account the daily fluctuations of both systolic and diastolic blood pressure. The T-test showed that the mean arterial pressure level was probably higher in children with prehypertension (p<0,05) compared to children from the control group.Conclusions.1. There is a phenomenon of a decrease in blood pressure variability in children with prehypertension (especially at night), which can be regarded as a risk factor for the development of arterial hypertension.2. Based on the presence of a probable difference in the indicators of daily fluctuations in mean arterial pressure among children from the control group and children with prehypertension, the latter have an increase in the dynamic load on the vascular wall, which can lead to a slowdown in the blood flow velocity in the capillary bed and also act as a predictor of arterial hypertension.

The efficacy of different alveolar recruitment maneuvers in holmium laser lithotripsy surgery under general anesthesia using a laryngeal mask

BackgroundAlveolar recruitment maneuvers (ARMs) is an important part of lung-protective ventilation strategies (LPVSs), but the optimal duration and interval Remain unclear.MethodsPatients:252 patients who underwent holmium laser lithotripsy surgery and meet inclusion criteria were included and randomized into three groups based on the duration and frequency of ARMs (Regular, one 30 s ARM (RARMs); Improved and intermittent, three 10s ARMs (IARMs); and Control (C), no ARMs).Interventions: Groups R and I received ARMs at 20 cmH2O pressures every 30 min. All patients received the same anesthesia and mechanical ventilation. Measurements:Outcomes included heart rate and mean arterial pressure changes during ARMs and postoperative pulmonary complications (PPCs) within the first 7 postoperative days.Main resultsIncidences of PPCs in groups R(7.1%) and I (5.0%)were slightly lower than those in group C (8.9%).This indicated the potential to reduce lung injury. Heart rate and mean arterial pressure fluctuations during ARMs were significantly higher in groups R and I than in group C (P < 0.01). The rate of blood pressure decrease was significantly higher in group R than in group I (P < 0.01).ConclusionsIARMs can reduce cycle fluctuations than RARMs in patients Undergoing holmium laser lithotripsy surgery with laryngeal mask general anesthesia. Low tidal volume ventilation and low PEEP combined with ARM did not significantly reduce the incidence of PPCs in healthy lung patients, but tended to reduce lung injury.Trial registrationThe study was registered on the Chinese Clinical Trial Registry.(ChiCTR2000030815,15/03/2020). This study was approved by the ethics committee of Chengdu Fifth People’s Hospital with approval number(2020–005(Study)-1).

Evaluation of the causal effects between dopamine infusion changeover and fluctuations in mean arterial pressure in neonates

ObjectiveTo evaluate whether changing dopamine infusions every 12 hours and preparing these infusions 30 min before administration reduces blood pressure fluctuations in preterm and term neonates.DesignThis was a retrospective study...

Impact of Mean Arterial Pressure Fluctuation on Mortality in Critically Ill Patients

The purpose of this study was to investigate the association between mean arterial pressure fluctuations and mortality in critically ill patients admitted to the ICU. Retrospective cohort. All adult ICUs at a tertiary care hospital. All adult patients with complete mean arterial pressure records were selected for analysis in the Multiparameter Intelligent Monitoring in Intensive Care II database. Patients in the external cohort were newly recruited adult patients in the Medical Information Mart for Intensive Care III database. None. The records of 8,242 patients were extracted. Mean arterial pressure fluctuation was calculated as follows: (mean nighttime mean arterial pressure - mean daytime mean arterial pressure)/mean arterial pressure. Patients were divided into two groups according to the degree of mean arterial pressure fluctuation: group A (between -5% and 5%) and group B (<-5% and >5%). The endpoints of this study were ICU and hospital mortality. Patients in group A (n = 4,793) had higher ICU and hospital mortality than those in group B (n = 3,449; 11.1% vs 8.1%, p < 0.001 and 13.8% vs 10.1%, p < 0.001, respectively). After adjusting for other covariates, the mean arterial pressure fluctuations between -5% and 5% were significantly correlated with ICU mortality (odds ratio, 1.296; 95% CI, 1.103-1.521; p = 0.002) and hospital mortality (odds ratio, 1.323; 95% CI, 1.142-1.531; p < 0.001). This relationship remained remarkable in patients with low or high Sequential Organ Failure Assessment scores in the sensitive analysis. Furthermore, external validation on a total of 4,502 individuals revealed that patients in group A still had significantly higher ICU (p < 0.001) and hospital mortality (p < 0.001) than those in group B. The reduced mean arterial pressure fluctuation (within -5% and 5%) may be associated with ICU and hospital mortality in critically ill patients.

GW29-e0820 Mean arterial pressure fluctuation may predict mortality in critically ill patients

GW29-e0820 Mean arterial pressure fluctuation may predict mortality in critically ill patients

Dynamic cerebral autoregulation is preserved during isometric handgrip and head-down tilt in healthy volunteers.

In healthy humans, cerebral blood flow (CBF) is autoregulated against changes in arterial blood pressure. Spontaneous fluctuations in mean arterial pressure (MAP) and CBF can be used to assess cerebral autoregulation. We hypothesized that dynamic cerebral autoregulation is affected by changes in autonomic activity, MAP, and cardiac output (CO) induced by handgrip (HG), head‐down tilt (HDT), and their combination. In thirteen healthy volunteers, we recorded blood velocity by ultrasound in the internal carotid artery (ICA), HR, MAP and CO‐estimates from continuous finger blood pressure, and end‐tidal CO 2. Instantaneous ICA beat volume (ICABV, mL) and ICA blood flow (ICABF, mL/min) were calculated. Wavelet synchronization index γ (0–1) was calculated for the pairs: MAP–ICABF, CO–ICABF and HR–ICABV in the low (0.05–0.15 Hz; LF) and high (0.15–0.4 Hz; HF) frequency bands. ICABF did not change between experimental states. MAP and CO were increased during HG (+16% and +15%, respectively, P < 0.001) and during HDT + HG (+12% and +23%, respectively, P < 0.001). In the LF interval, median γ for the MAP–ICABF pair (baseline: 0.23 [0.12–0.28]) and the CO–ICABF pair (baseline: 0.22 [0.15–0.28]) did not change with HG, HDT, or their combination. High γ was observed for the HR–ICABV pair at the respiratory frequency, the oscillations in these variables being in inverse phase. The unaltered ICABF and the low synchronization between MAP and ICABF in the LF interval suggest intact dynamic cerebral autoregulation during HG, HDT, and their combination.

Impairment of cerebral autoregulation in pediatric extracorporeal membrane oxygenation associated with neuroimaging abnormalities.

.Extracorporeal membrane oxygenation (ECMO) is a life-supporting therapy for critically ill patients with severe respiratory and/or cardiovascular failure. Cerebrovascular impairment can result in hemorrhagic and ischemic complications commonly seen in the patients supported on ECMO. We investigated the degree of cerebral autoregulation impairment during ECMO as well as whether it is predictive of neuroimaging abnormalities. Spontaneous fluctuations of mean arterial pressure (MAP) and cerebral tissue oxygen saturation () were continuously measured during the ECMO run. The dynamic relationship between the MAP and fluctuations was assessed based on wavelet transform coherence (WTC). Neuroimaging was conducted during and/or after ECMO as standard of care, and the abnormalities were evaluated based on a scoring system that had been previously validated among ECMO patients. Of the 25 patients, 8 (32%) had normal neuroimaging, 7 (28%) had mild to moderate neuroimaging abnormalities, and the other 10 (40%) had severe neuroimaging abnormalities. The degrees of cerebral autoregulation impairment quantified based on WTC showed significant correlations with the neuroimaging scores (; ). Evidence that cerebral autoregulation impairment during ECMO was related to the patients’ neurological outcomes was provided.

Tight neurovascular coupling in the spinal cord during nociceptive stimulation in intact and spinal rats

Functional magnetic resonance imaging (fMRI) is based on neurovascular coupling, which allows inferring neuronal activity from hemodynamic changes. Spinal fMRI has been used to examine pain processes, although spinal neurovascular coupling has never been investigated. In addition, fluctuations in mean arterial pressure (MAP) occur during nociceptive stimulation and this may affect neurovascular coupling. The objective of this study was to examine neurovascular coupling in the rat spinal cord during nociceptive stimulation while MAP was manipulated by cervical spinal transection, which prevents nociception-related MAP increases. Six male Wistar rats were anesthetized with isoflurane (1.2–1.5%). Local field potentials (LFP) and spinal cord blood flow (SCBF) were recorded concurrently in the lumbar enlargement, where activity was evoked by electrical stimulation of the sciatic nerve. In intact conditions, stimulation of graded intensity produced proportional changes in SCBF and LFP that were paralleled by similar changes in MAP. However, spinal transection almost abolished MAP changes (p<0.001), while SCBF and LFP responses were not significantly affected (p>0.3) and remained similarly coupled before and after spinal transection. This indicates that spinal hemodynamic changes reflect neuronal activity even when large fluctuations in MAP occur. This contrasts with results from previous studies on cerebral neurovascular coupling and suggests that spinal autoregulation might allow better adaptation to sudden MAP changes than cerebral autoregulation. Although assessment of the coupling between spinal neuronal activity and BOLD signal remains to be investigated, this study supports the use of spinal fMRI, based on the tight coupling between SCBF and LFP.

ANESTHETIC MANAGEMENT OF ENDOSCOPIC ENDONASAL SURGERIES IN PEDIATRIC ONCOLOGY

Treatment of children with tumors of the nasal cavity and the base of the skull is an urgent issue in both surgery and anesthesiology. First of all, it is related to the anatomical and physiological characteristics of children, as well as to the localisation of tumors in the anatomical zone surrounded by important anatomical structures (internal carotid arteries and their branches, cavernous sinus, orbit). Currently, such tumors are removed by the endoscopic endonasal method. Considering anesthetic support of such surgeries, the issues of upper respiratory tract protection, correction of systemic hemostasis disorders, adequate analgesia, prevention of postoperative complications (postoperative nausea and vomiting, postoperative cognitive dysfunction (delirium), delayed bleeding) are the most importrant. Adequate anesthetic support in pediatric ENT surgery is the most important factor determining the patient’s safety and psychophysiological comfort, improving the quality of treatment and hospitalization. Materials and methods . From July 2016 to March 2017 27 endoscopic endonasal surgeries were performed. The age of patients was from 39 days to 17 years, the median age was 6.6 years. Types of surgeries: 18 out of 27 (66.7%) cases — resection of the tumor, 9 out of 27 (33.3%) — biopsy. Multicomponent anesthesia was conducted according to the protocol presented. The parameters of hemodynamics, the incidence of complications in the perioperative and postoperative periods were recorded. Results . Hemodynamically stable course of anesthesia without significant fluctuations in mean arterial pressure and heart rates was noted at all the stages of surgeries. The terms of observation of patients after surgery in the intensive care ward — from 1 to 3 days, median — 1.2 days. There were no cases of delayed postoperative hemorrhage. Episodes of excitement, postoperative delirium were registered in 4 (14.8%) cases. Postoperative nausea and vomiting were observed in 3 (11.1%) cases. Conclusion . The anatomical and physiological features of pediatric patients determine the tactics of anesthesia, and only taking into account these features, it is possible to achieve patient’s safety during the perioperative and postoperative periods, to improve the quality of treatment and to shorten the periods of hospitalization.

Steady-state tilt has no effect on cerebrovascular CO2 reactivity in anterior and posterior cerebral circulations.

What is the central question of this study? We investigated the effects of superimposed tilt and hypercapnia-induced cerebral arteriolar dilatation on anterior and posterior cerebrovascular CO2 reactivity using hyperoxic rebreathing in human participants. What is the main finding and its importance? The main findings are threefold: (i) cerebrovascular CO2 reactivity in the anterior and posterior cerebrovasculature is unchanged with tilt; (ii) cerebral autoregulation is unlikely responsible due to unchanging cerebrovascular resistance reactivity between positions; and (iii) cerebral blood flow is not pressure passive during tilt as it is with pharmacological or lower body negative pressure-induced changes in mean arterial pressure, suggesting that sympathetic activation or balanced transmural pressures during head-down tilt regulate cerebral blood flow. Cerebral autoregulation is a protective feature of the cerebrovasculature that maintains relatively constant cerebral perfusion in the face of static and dynamic fluctuations in mean arterial pressure (MAP). However, the extent that the cerebrovasculature can autoregulate in the face of superimposed steady-state orthostasis-induced changes in MAP (e.g. head-up and head-down tilt; HUT and HDT) and CO2 -mediated arteriolar dilatation is unknown. We tested the effects of steady-state tilt on cerebrovascular CO2 reactivity in the middle and and posterior cerebral artery in the following five body positions: 90 deg HUT, 45 deg HUT, supine, 45 deg HDT and 90 deg HDT on a tilt table during a modified hyperoxic rebreathing test. Absolute and relative cerebrovascular CO2 reactivity [cerebral blood velocity (CBV)/CO2 ], cerebrovascular resistance (CVR) reactivity (CVR/CO2 ) and MAP reactivity (MAP/CO2 ) were quantified using linear regression. Mean arterial pressure was significantly elevated in 90 deg HDT compared with other positions during baseline steady-state tilt (P < 0.01). Absolute CBV/CO2 and CVR/CO2 were greater in the middle cerebral artery than the posterior cerebral artery (P < 0.01) in all body positions, but relative measures were not different (P = 0.143 and P = 0.360, respectively), nor was there any interaction with tilt position. In addition, there was no difference in absolute (P = 0.556) and relative MAP/CO2 (P = 0.308) between positions. Our data demonstrate that cerebral blood flow remains well regulated during superimposed steady-state orthostatic stress and dynamic changes in the partial pressure of end-tidal CO2 during rebreathing. Cerebral autoregulation is likely not the mechanism responsible, but rather sympathetic nervous system activation or a balanced cerebrovascular transmural pressure with HDT maintains resting cerebral blood flow and cerebrovascular CO2 reactivity during rebreathing.

Hemodynamic Changes in Patients Undergoing Carotid Endarterectomy under Cervical Block and General Anesthesia

The objective of this study was to assess differences in hemodynamic stability for patients undergoing carotid endarterectomy (CEA) under general anesthesia (GA) as compared with cervical block anesthesia (CBA). An institutional review board-approved, retrospective review of 651 patients from 1998 to 2012 undergoing CEA was performed: 254 patients underwent CEA under CBA and 397 under GA. Comprehensive chart review including preoperative, postoperative, and 30-day follow-up was conducted. Patients were monitored continuously intraoperatively and for 24hr after surgery. All intraoperative vasoactive and antihypertensive medications administered were recorded. Both groups were similar in age, incidence of coronary artery disease, hypertension, and renal failure. There was a preponderance of female patients, with a high incidence of chronic obstructive pulmonary disease, diabetes mellitus, and nicotine abuse in the GA group. Symptomatic patients predominated the GA group (54% vs. 41%; P=0.0018). Of the symptomatic patients, 78% experienced transient ischemic attacks in the GA group vs. 64% in the CBA group. Evaluation of hemodynamic stability under GA versus CBA revealed that no significant hemodynamic changes occurred in 34.5% of CBA patients vs. only 14.4% in the GA group. Under GA, incidence of hypotension was 17.84% as compared with 0.52% under CBA (P<0.001). Under GA, patients had far more hemodynamic fluctuations with 41% of patients demonstrating >3 intraoperative fluctuations in mean arterial pressure of >20% vs. 20% in the CBA group (P<0.001). Under GA, 51% of patients required vasopressors alone vs. 36% under CBA (P<0.0002). Antihypertensive medications alone were required in 63% of patients in the GA group vs. 73% in the CBA group (P=0.0085). Thirty percent of all patients required both vasopressors and antihypertensives during surgery; 23% under CBA vs. 34% under GA (P=0.0457). There were two postoperative hematoma's CBA cohort. The GA cohort developed postoperative complications (myocardial infarction, 4; stroke, 6; and hematoma, 9), however, these complications were not statistically significant compared with the CBA group. For patients undergoing CEA, CBA resulted in less hemodynamic fluctuations and fewer intraoperative vasoactive medication requirements as compared with GA.

The effects of head‐up and head‐down tilt on cerebrovascular CO2 reactivity in anterior and posterior cerebral circulations

Cerebral autoregulation is a protective feature of the cerebrovasculature that maintains relatively constant cerebral perfusion in the face of static and dynamic fluctuations in mean arterial pressure (MAP). However, gravity‐dependent shifts in blood volume distribution during head‐up and head‐down tilt (HUT, HDT) have profound effects on venous return, cardiac output and MAP. Further, cerebral blood flow (CBF) is highly sensitive to the influence of changes in arterial CO2 (PaCO2) on arteriolar diameter. We tested the effects of steady‐state tilt on cerebral blood velocity (CBV) and CO2 reactivity in the middle and posterior cerebral arteries (MCA, PCA) using hyperoxic rebreathing and transcranial Doppler ultrasound in various positions on a tilt table. Following initial testing in supine position, subjects were positioned randomly and tested in each of four positions: 90° HDT, 45° HDT, 45° HUT and 90° HUT. There were no differences in steady‐state MCA or PCA CBV across positions. Absolute and relative CO2 reactivity slopes were calculated for the MCA and PCA using linear regression. Absolute reactivity was greater in the MCA than the PCA in all 5 positions, but there was no interaction between reactivity and tilt. Our data demonstrate that CBF is maintained through cerebral autoregulation in the face of superimposed steady‐state orthostatic stress and dynamic changes in PaCO2.

Influence of age and sex on the pressor response following a spontaneous burst of muscle sympathetic nerve activity

The sympathetic nervous system is critical for the beat-to-beat regulation of arterial blood pressure (BP). Although studies have examined age- and sex-related effects on BP control, findings are inconsistent and limited data are available in postmenopausal women. In addition, the majority of studies have focused on time-averaged responses without consideration for potential beat-to-beat alterations. Thus we examined whether the ability of muscle sympathetic nerve activity (MSNA) to modulate BP on a beat-to-beat basis is affected by age or sex. BP and MSNA were measured during supine rest in 40 young (20 men) and 40 older (20 men) healthy subjects. Beat-to-beat fluctuations in mean arterial pressure (MAP) were characterized for 15 cardiac cycles after each MSNA burst using signal averaging. The rise in MAP following an MSNA burst was similar between young men and women (+2.64 ± 0.3 vs. +2.57 ± 0.3 mmHg, respectively). However, the magnitude of the increase in MAP after an MSNA burst was reduced in older compared with young subjects (P < 0.05). Moreover, the attenuation of the pressor response was greater in older women (+1.20 ± 0.1 mmHg) compared with older men (+1.72 ± 0.2 mmHg; P < 0.05). Interestingly, in all groups, MAP consistently decreased after cardiac cycles without MSNA bursts (nonbursts) with the magnitude of fall greatest in older men. In summary, healthy aging is associated with an attenuated beat-to-beat increase in BP after a spontaneous MSNA burst, and this attenuation is more pronounced in postmenopausal women. Furthermore, our nonburst findings highlight the importance of sympathetic vasoconstrictor activity to maintain beat-to-beat BP, particularly in older men.

Interactions between heart rate variability and pulmonary gas exchange efficiency in humans

The respiratory component of heart rate variability (respiratory sinus arrhythmia, RSA) has been associated with improved pulmonary gas exchange efficiency in humans via the apparent clustering and scattering of heart beats in time with the inspiratory and expiratory phases of alveolar ventilation, respectively. However, since human RSA causes only marginal redistribution of heart beats to inspiration, we tested the hypothesis that any association between RSA amplitude and pulmonary gas exchange efficiency may be indirect. In 11 patients with fixed-rate cardiac pacemakers and 10 healthy control subjects, we recorded R-R intervals, respiratory flow, end-tidal gas tension and the ventilatory equivalents for carbon dioxide and oxygen during 'fast' (0.25 Hz) and 'slow' paced breathing (0.10 Hz). Mean heart rate, mean arterial blood pressure, mean arterial pressure fluctuations, tidal volume, end-tidal CO(2), and were similar between pacemaker and control groups in both the fast and slow breathing conditions. Although pacemaker patients had no RSA and slow breathing was associated with a 2.5-fold RSA amplitude increase in control subjects (39 +/- 21 versus 97 +/- 45 ms, P < 0.001), comparable (main effect for breathing frequency, F(1,19) = 76.54, P < 0.001) and reductions (main effect for breathing frequency, F(1,19) = 23.90, P < 0.001) were observed for both cohorts during slow breathing. In addition, the degree of (r = 0.36, P = 0.32) and reductions (r = 0.29, P = 0.43) from fast to slow breathing were not correlated to the degree of associated RSA amplitude enhancements in control subjects. These findings suggest that the association between RSA amplitude and pulmonary gas exchange efficiency during variable-frequency paced breathing observed in prior human work is not contingent on RSA being present. Therefore, whether RSA serves an intrinsic physiological function in optimizing pulmonary gas exchange efficiency in humans requires further experimental validation.

Effects of cerebrovascular pressure reactivity-guided optimization of cerebral perfusion pressure on brain tissue oxygenation after traumatic brain injury*

To evaluate the concept of a cerebrovascular pressure reactivity-guided optimal cerebral perfusion pressure after traumatic brain injury by analyzing the relationship between optimal cerebral perfusion pressure and brain tissue oxygen. Prospective observational cohort study. Neurosurgical intensive care unit of a university hospital. Thirty-eight patients after head injury. Continuous computerized monitoring of mean arterial pressure, intracranial pressure, and brain tissue oxygen for 5.3 +/- 2.6 days. The index of cerebrovascular pressure reactivity was calculated as a moving correlation coefficient between spontaneous low-frequency fluctuations of mean arterial pressure and intracranial pressure. Optimal cerebral perfusion pressure was defined as the cerebral perfusion pressure level with the lowest average index of cerebrovascular pressure reactivity. Optimal cerebral perfusion pressure could be identified in 32 of 38 patients. Median optimal cerebral perfusion pressure was between 70 and 75 mm Hg (range, 60-100 mm Hg). Below the level of optimal cerebral perfusion pressure, brain tissue oxygen decreased in parallel to cerebral perfusion pressure, whereas brain tissue oxygen reached a plateau above optimal cerebral perfusion pressure. Optimal cerebral perfusion pressure correlated significantly with the cerebral perfusion pressure level, where brain tissue oxygen reached its plateau (r = .79; p < .01). Average brain tissue oxygen at optimal cerebral perfusion pressure was 24.5 +/- 6.0 mm Hg. This study supports the concept of cerebrovascular pressure reactivity-based individual optimal cerebral perfusion pressure. Driving cerebral perfusion pressure in excess of optimal cerebral perfusion pressure does not yield improvements in brain tissue oxygen after head injury and should be avoided, whereas cerebral perfusion pressure below optimal cerebral perfusion pressure may result in secondary cerebral ischemia.

Relationship between breath-synchronous arterial pressure and heart rate variations during orthostatic stress.

It has recently been shown that the phase relationship between respiration-induced changes in arterial pressure (AP) and heart rate (HR) are different in supine and upright postures. We wanted to further analyse the coupling between respiration, arterial blood pressure and HR in the time domain, and how this coupling was altered during orthostatic stress. Nine healthy subjects were studied. Respiration-induced changes in AP and HR were recorded during frequency- and volume-controlled breathing. This was done during supine rest with and without lower body negative pressure (-50 mmHg) (LBNP). All experiments were performed after beta1-blockade. Responses were averaged breath-by-breath to enhance the time resolution and to eliminate noise. The respiration-induced changes in arterial pulse pressure (PP) were different between control and LBNP: The peak in PP during the respiratory cycle occurred 0.9 +/- 0.8 (mean +/- SD) s before the onset of inspiration during supine control and 0.8 +/- 2.1 s after the onset of inspiration during LBNP (P = 0.03). These changes in the timing of peak PP significantly distorted the cyclic systolic AP and mean AP fluctuations during LBNP. Despite the altered AP response with LBNP, HR fluctuations closely correlated in time with respiration in all conditions, albeit with a significantly reduced amplitude during LBNP (-49%, P = 0.01). The results points to a lack of coupling between AP and HR during paced breathing and thus suggest that respiratory sinus dysrhythmia at least, to a large extent, is independent of the arterial baroreflex.

Role of cardiac output in mediating arterial blood pressure oscillations.

The objective of this study was to determine the role of cardiac output in mediating spontaneous fluctuations in mean arterial pressure (MAP) conscious dogs. Dogs were chronically instrumented to monitor MAP and cardiac output. Atrioventricular (AV) block was induced, and left ventricular and right atrial electrodes were implanted. After recovery, MAP was observed for 5 min under two conditions: 1) normal variation in heart rate and cardiac output via triggering the ventricular stimulator with each atrial depolarization (effectively reversing the AV block, AV-linked stimulation) and 2) computer control of ventricular rate to maintain cardiac output constant on a by-beat basis at the same level as observed during normal variations in heart rate and cardiac output. When cardiac output was held constant, large-amplitude, low-frequency oscillations in MAP were readily apparent. Spectral analysis by fast Fourier transform revealed that during constant cardiac output the power observed at low frequencies in the MAP spectrum represented 95.0 +/- 2.7% of the total power compared with 75.5 +/- 4.6% during normal variations in heart rate and cardiac output (P < 0.05). In addition, when cardiac output was held constant, the power observed at higher frequencies markedly decreased from 24.5 +/- 4.6% of total power during AV-linked stimulation to only 5.0 +/- 2.7% of total power during constant cardiac output (P < 0.05). We conclude that low-frequency oscillations in MAP are due to changes in peripheral resistance, whereas a significant amount of high-frequency changes in MAP stems from spontaneous changes in cardiac output.

Deterministic-chaotic and periodic properties of heart rate and arterial pressure fluctuations and their mediation in piglets

Only the simultaneous analysis of periodic and nonlinear properties of heart rate fluctuations (HRF) can describe completely this complex physiological process. Up to now there is, apart from a study of our own, no systematic and correlative investigation using both parameter groups, also not in early development. Thus, we tried to describe in this manner these properties of HRF, the corresponding mean arterial pressure fluctuations (MAPF) and respiratory movements (RM) and their mutual relations in neonatal pig. In 6 term newborn piglets, periodic properties of HRF, RM, and MAPF were analyzed by spectral and coherence analysis, and deterministic-chaotic properties by calculation of correlation dimension (CD), Lyapunov exponent (LE), and construction of phase space plots. The assumption of deterministic chaotic components was supported by Theiler's test for nonlinearity, by always positive leading LEs, and by the results of a nonlinear deterministic model. These analyses were done in sleep states, general anaesthesia, hypoxic hypoxia, in ventilated state, and during cholinergic and additional beta-adrenergic blockade. In all experimental states, HRF and MAPF have periodic and nonlinear, very probably deterministic-chaotic properties, but in different relations. In anaesthetized piglets, periodic properties of HRF and MAPF dominate. In hypoxia the decreasing LE and CD of HRF and CD of MAPF were connected with increasing MAPF power density. Cholinergic blockade caused a decreased overall HRF and MAPF power and a decreasing LE and CD, but beta-adrenergic blockade decreased a small part of power density of both in 0.02-0.08 Hz only. The results of CD, LE, Theiler's test and the low dimensional deterministic model data suggested mainly deterministic-chaotic properties in the nonlinear part of HRF and MAPF. Already in neonatal piglets, both periodic and nonlinear, very probably deterministic chaotic properties of HRF and MAPF exist which change both during hypoxia and cholinergic blockade. They are partly cholinergically and--to a small extent--also beta-adrenergically mediated. The decrease of nonlinear complexity of HRF and MAPF during hypoxia suggests characteristic pathological change even in early development.