Fluctuations In Cardiac Output Research Articles

How to give a spider a heart attack: Evaluating cardiac stress reactions of Trichonephila and Argiope spiders

AbstractAll animal species, from arthropods to vertebrates, must deal with occasional stressors in their lives, though most research on this has been focused on vertebrates. Meanwhile, our understanding of stress reactions in arthropod species like spiders is nascent. In the United States, a non‐native orb‐weaving spider, Trichonephila clavata (‘jorō’ spider), is spreading as is its already‐established cousin in the United States, T. clavipes (golden silk spider). Prior study has revealed how these two species have a unique behavioural reaction to physical stressors, whereby they remain in a thanatosis state for a prolonged period compared with other species. Here, we investigate the physiological stress reactions of these Trichonephila spiders by evaluating how each species' dorsal vessel contractions (heart rates) become elevated after being subjected to a non‐lethal stressor. For comparison, we also evaluate two similarly‐sized orb weavers, Argiope aurantia (garden spider) and A. trifasciata (banded garden spider). We record baseline heart rates of inactive, resting, spiders in our lab, then restrain them under an electronic, ‘optocardiographic’, sensor for 10 min, to record their ‘stressed’ heart rates. Argiope aurantia has a pronounced heart rate elevation, for reasons unknown. We observe that all spider heart rates increase during restraint, though each has a species‐specific pattern of elevation over time under restraint. Notably, heart rates of both Trichonephila spiders are less variable under stress, since they tend not to struggle during restraint. Meanwhile, both Argiope spiders frequently struggle, leading to marked fluctuations in cardiac output. The stress reactions of Trichonephila spiders could be characterized as ‘even‐tempered’, which may factor into their ability to live in habitats with frequent disturbances.

The Peripartum Anesthesiologist

The association between pregnancy and cardiomyopathy has been evident since 1870. Moreover, the incidence of peripartum cardiomyopathy (PPCM) is thought to range from 1 in 2,400 to 1 in 15,000 pregnancies; however, a more recent review suggests an incidence of one in 3,000 to 4,000 pregnancies in western countries. Peripartum cardiomyopathy is more prevalent in older multiparous patients of African descent who have multiple gestations, chronic hypertension or preeclampsia, and have received prolonged tocolytic therapy. There are four principal diagnostic features of PPCM: the development of heart failure in the last month of pregnancy or within five months of delivery; absence of an identifiable cause; absence of underlying heart disease prior to the last month of pregnancy; and evidence of left ventricular systolic dysfunction. Recently, echocardiographic criteria were established, including an ejection fraction 45% and an end-diastolic dimension [ 2.72 cm m. Basic principal treatment of PPCM includes a reduction of both preload and afterload along with the administration of inotropic therapy. Severe cases may require mechanical support and consideration for heart transplantation. Delivery of the fetus is usually indicated when the cardiac failure is intractable. In this month’s edition of the Journal, Ituk et al. report on the peripartum anesthetic management and outcomes of 25 women with dilated cardiomyopathy four of whom were designated as having PPCM. With an average age of 32 and a history of hypertension and diabetes, these patients represent a higher risk group than the overall obstetrical population, which is in keeping with previous reports of PPCM risk factors. This challenging group of patients was optimally managed by a multidisciplinary team with a distinct knowledge of peripartum care a team that followed their patients’ clinical progress at regular multidisciplinary case conferences. Fetal delivery in PPCM is dictated by the obstetric indications and occurs in consultation with the anesthesia team. Vaginal delivery offers minimal blood loss, hemodynamic stability, and avoidance of additional Cesarean delivery-related surgical stresses with an overall reduced chance of postoperative complications. Regional anesthesia is advantageous for reducing preload and afterload and minimizing the fluctuations in cardiac output associated with labour. Cesarean delivery is best reserved for indications such as breech presentation, fetal distress, or failure to progress. Prognostic information of PPCM is limited and often confounded by the inclusion of patients with heart failure of differing etiologies. The reported mortality ranges from 18-50% and it increases as a function of age, ejection fraction 25%, multiparity, African American ethnicity, and delayed diagnosis. The recurrence rate of 50-100% is influenced by the severity of PPCM during the index pregnancy and by left ventricular systolic function at the beginning of pregnancy. Echocardiography may be helpful in delineating future risk in patients considering future pregnancies. Long-term prognosis is related to recovery of left ventricular function with 80% of women recovering partially or completely, and the remaining 20% either dying or undergo cardiac transplantation. In our view, the obstetric anesthesiologist functions as a vital member of the interdisciplinary team caring for women with complicated obstetric issues and, as such, A. Munro, MD R. B. George, MD (&) Department of Women’s & Obstetric Anesthesia, IWK Health Centre, Dalhousie University, 5850/5980 University Avenue, P.O. Box 9700, Halifax, NS B3K 6R8, Canada e-mail: rbgeorge@dal.ca

Cardiac output is not a significant source of low frequency mean arterial pressure variability

Spontaneous mean arterial pressure (MAP) variability may be mainly due to fluctuations in cardiac output (CO) and total peripheral resistance (TPR). While high frequency (HF ∼ 0.25 Hz) oscillations in MAP are ultimately driven by respiration, the source of low frequency (LF ∼ 0.1 Hz) fluctuations has not been fully elucidated. It is known that CO buffers these oscillations, but there is no evidence on its potential role in also generating them. The main goal was to determine whether CO is a source of LF variability in MAP. Six dogs were chronically instrumented to obtain beat-to-beat measurements of CO and MAP while the dogs were fully awake and at rest. A causal dynamic model was identified to relate the fluctuations in CO to MAP. The model was then used to predict the MAP fluctuations from the CO fluctuations. The CO fluctuations were able to predict about 70% of the MAP oscillations in the HF band but showed no predictive value in the LF band. Hence, respiration induces CO fluctuations in the HF band that, in turn, cause MAP oscillations, while TPR fluctuations appear to be the dominant mediator of LF fluctuations of MAP. CO is not a significant source of these oscillations, and it may only be responsible for dampening them, likely through the baroreflex.

Transient Increases In Blood Pressure After Spontaneous Bursts Of Muscle Sympathetic Nerve Activity: Contributions Of Cardiac Output And Systemic Vascular Conductance

Previously, a latency of 5.5 seconds between the occurrence of a burst of muscle sympathetic nerve activity (MSNA) and the subsequent peak arterial blood pressure (BP) response was reported. Although this BP increase was thought to be primarily due to peripheral vasoconstriction, beat-to-beat vascular conductance measures were not made. Furthermore, a transient cardio-acceleration was observed following MSNA bursts. However, whether an increase in cardiac output contributes to the BP response is unknown. PURPOSE: To assess the beat-to-beat fluctuations in cardiac output (CO) and systemic vascular conductance (SVC) following a spontaneous burst of MSNA. METHODS: In 7 young men, BP (finger photoplethysmography), stroke volume (SV; Modelflow), heart rate (HR; ECG) and MSNA (microneurography) were continuously measured during 30 minutes of supine rest. Beat-to-beat fluctuations in BP, CO (HR × SV) and systemic vascular conductance (SVC; CO/MAP) were characterized for 15 cardiac cycles following each individual MSNA burst using signal averaging to identify the temporal pattern for each variable and a peak response was calculated. RESULTS: A significant rise in diastolic BP was observed following each burst of MSNA with a peak increase of 4.35 ± 0.5 mmHg (+6.6 ± 0.7%; p<0.05) occurring at a latency of 5.4 ± 0.2 s. Within the first 2 cardiac cycles following an MSNA burst, the immediate (1.7 ± 0.05 s) increase in BP was associated with a 0.49 ± 0.10 L (+10 ± 0.03%; p<0.05) increase in CO. SVC was also transiently increased with CO during this initial period but then subsequently decreased by -0.005 ml·min-1mmHg-1 (-8.1 ± 0.4%; p<0.05), reaching a maximum reduction around the time of the peak increase in BP. CONCLUSIONS: These preliminary findings indicate that spontaneous bursts of MSNA consistently produce elevations in BP that appear to be mediated by alterations in both CO and SVC. The initial rise in BP is associated with an increase in CO that transitions to peripheral vasoconstriction, noted by the significant fall in SVC at the time of the peak BP response. Thus, it appears that both central and peripheral adjustments contribute to the transient rise in BP following a spontaneous burst of MSNA.

Cardiovascular disease in pregnancy

Cardiac disease continues to be an important cause of maternal morbidity and mortality and is now the leading cause of maternal deaths in the United Kingdom. This is largely because of extensive haemodynamic changes which occur during pregnancy, namely the increase in blood volume, fluctuations in cardiac output, fall in systemic vascular resistance and hypercoagulable state. High risk periods include the end of the second trimester, during labour and the immediate postpartum period. Prognosis depends on the specific cardiac condition, the patient's functional class, presence of cyanosis, prior history of cardiac events or arrhythmia and the degree of systolic dysfunction. Conditions where pregnancy should be include Eisenmenger's syndrome, pulmonary hypertension, complex cyanotic congenital heart disease, Marfan syndrome with aortic root dilatation and those with severe left ventricular dysfunction. Women with heart disease should be thoroughly evaluated and counselled before and during pregnancy. Multidisciplinary care is essential for successful maternal and foetal outcomes.

Model-Based Assessment of Cardiovascular Autonomic Control in Children with Obstructive Sleep Apnea

To quantitatively assess daytime autonomic cardiovascular control in pediatric subjects with and without obstructive sleep apnea syndrome (OSAS). Respiration, R-R intervals, and noninvasive continuous blood pressure were monitored in awake subjects in the supine and standing postures, as well as during cold face stimulation. Sleep disorders laboratory in a hospital setting. Ten pediatric patients (age 11.4 +/- 3.6 years) with moderate to severe OSAS (obstructive apnea-hypopnea index = 21.0 +/- 6.6/1 h) before treatment and 10 age-matched normal control subjects (age 11.5 +/- 3.7 years). Spectral analysis of heart rate variability revealed that high-frequency power was similar and the ratio of low- to high-frequency power was lower in subjects with OSAS vs control subjects. The closed-loop minimal model allowed heart rate variability to be partitioned into a component mediated by respiratory-cardiac coupling and a baroreflex component, whereas blood pressure variability was assumed to result from the direct effects of respiration and fluctuations in cardiac output. Baroreflex gain was lower in subjects with OSAS vs control subjects. Under orthostatic stress, respiratory-cardiac coupling gain decreased in both subject groups, but baroreflex gain decreased only in controls. The model was extended to incorporate time-varying parameter changes for analysis of the data collected during cold face stimulation: cardiac output gain increased in controls but remained unchanged in OSAS. Our findings suggest that vagal modulation of the heart remains relatively normal in pediatric subjects with OSAS. However, baseline cardiovascular sympathetic activity is elevated, and reactivity to autonomic challenges is impaired.

Thrombosis in the critically ill neonate: incidence, diagnosis, and management

Among children, newborn infants are most vulnerable to development of thrombosis and serious thromboembolic complications. Amongst newborns, those neonates who are critically ill, both term and preterm, are at greatest risk for developing symptomatic thromboembolic disease. The most important risk factors are inflammation, DIC, impaired liver function, fluctuations in cardiac output, and congenital heart disease, as well as exogenous risk factors such as central venous or arterial catheters. In most clinically symptomatic infants, diagnosis is made by ultrasound, venography, or CT or MRI angiograms. However, clinically asymptomatic vessel thrombosis is sometimes picked up by screening investigations or during routine imaging for other indications. Acute management of thrombosis and thromboembolism comprises a variety of approaches, including simple observation, treatment with unfractionated or low molecular weight heparin, as well as more aggressive interventions such as thrombolytic therapy or catheter-directed revascularization. Long-term follow-up is dependent on the underlying diagnosis. In the majority of infants, stabilization of the patients' general condition and hemodynamics, which allows removal of indwelling catheters, renders long-term anticoagulation superfluous. Nevertheless, in certain types of congenital heart disease or inherited thrombophilia, long-term prophylaxis may be warranted. This review article focuses on pathophysiology, diagnosis, and acute and long-term management of thrombosis in critically ill term and preterm neonates.

Comparison of the USCOM ultrasound cardiac output monitor with pulmonary artery catheter thermodilution in patients undergoing liver transplantation

The aim of the study was to compare the standard technique of cardiac output determination by pulmonary artery catheter thermodilution (PAC-TD) with a noninvasive ultrasound Doppler monitor (USCOM Pty., Ltd., Coffs Harbour, Australia) in surgery for liver transplantation. We wished to determine if the degree of accuracy would allow the ultrasound cardiac output monitor (USCOM) to be used as an alternative monitor in a clinical setting in which wide fluctuations in cardiac output could be expected. This was a prospective method comparison study, with 71 paired measurements obtained in 12 patients undergoing liver transplantation in a university teaching hospital. Bland-Altman analysis of the 2 techniques showed a bias of 0.39 L/minute, with the USCOM cardiac output lower compared with that of PAC-TD. The bias was small and did not vary with the magnitude of the cardiac output. The 95% limits of agreement were -1.47 and 2.25 L/minute. There was good repeatability for USCOM measurements, with a repeatability coefficient of 0.43 for USCOM versus 0.77 for PAC-TD. We conclude that USCOM is acceptable for the clinical determination of noninvasive cardiac output, particularly in situations in which tracking changes over time is more important than knowing the precise value. However, the utility of USCOM is limited by its inability to measure pulmonary artery pressure.

Cardiovascular disease in pregnancy

Heart disease continues to be an important cause of maternal morbidity and mortality. This is largely because of the extensive haemodynamic changes that occur during pregnancy, namely the increase in blood volume, fluctuations in cardiac output, fall in systemic vascular resistance and the hypercoagulable state. High-risk periods include the end of the second trimester, during labour and the immediate postpartum period. Prognosis depends on the specific cardiac condition, the patient's functional class, presence of cyanosis, history of cardiac events or arrhythmia and the degree of systolic dysfunction. Pregnancy is contraindicated in women with Eisenmenger's syndrome, pulmonary hypertension, complex cyanotic congenital heart disease, Marfan's syndrome with aortic root dilatation, and those with severe left ventricular dysfunction. Women with heart disease should be thoroughly evaluated and counselled before and during pregnancy. Multidisciplinary care is essential for successful maternal and fetal outcomes.

Noninvasive, Automated and Continuous Cardiac Output Monitoring by Pulmonary Capnodynamics

Cardiac output monitoring is most important where cardiovascular stability is potentially threatened, such as during major surgery and in critically ill patients. However, continuous monitoring of cardiac output is still not performed routinely during anesthesia and critical care, because of invasiveness, expense, and inaccuracy of available technologies. A technique termed the capnodynamic method was tested for breath-to-breath measurement of pulmonary blood flow from lung carbon dioxide mass balance, using measured carbon dioxide elimination and end-tidal concentration. A prototype measurement system was constructed for a feasibility study in six anesthetized sheep. Large and rapid fluctuations in cardiac output were generated by repeated dobutamine and esmolol challenge. Measurements were compared with an indwelling ultrasonic flow probe placed on the ascending aorta or pulmonary artery. Cardiac output measured by the flow probe varied between zero and 8.67 l/min, with a mean of 3.50 l/min. Overall mean bias [SD of the difference] between the methods (capnodynamic - flow probe) was -0.25 [0.94] l/min, r = 0.79 (P < 0.001). During periods of stability in cardiac output of 5 min or more, mean bias was -0.20 [0.55] l/min. The method successfully indicated two cardiac arrest events, which were induced in one of the animals. The method satisfactorily tracked wide fluctuations in cardiac output in real time. The capnodynamic method may have potential for continuous noninvasive cardiac output monitoring in patients undergoing anesthesia for major surgery, and in critical care, on a routine basis.

Cardiac Disease in Pregnancy

After completing this article, readers should be able to: 1. Describe how the cardiovascular changes of pregnancy can affect the woman who has cardiac disease. 2. Review the importance of preconception counseling. 3. Discuss the purpose of a multidisciplinary team approach to the management of the woman who has cardiac disease during pregnancy. 4. Review delivery management of women who have cardiac disease. Pregnancy causes considerable physiologic change in women, particularly in their cardiovascular systems. Circulating blood volume increases dramatically, and cardiac output increases up to 50% by the end of pregnancy. Pregnant women who have cardiac disease must be able to accommodate these normal changes to have a successful pregnancy. Inability to do so can result in increased morbidity and mortality. Cardiac disease is an important cause of nonobstetric mortality during pregnancy and can result in significant morbidity. (1)(2) Approximately 10% of all maternal deaths in the United States can be attributed to cardiac disease, a number that has remained consistent for the last half century. (1) In one study of 1,000 pregnant women who had various types of cardiac disease and were followed by the same health-care team over a 10-year period, more than 75% of the women had no complications during pregnancy. (2) Among the remaining 25%, the following complications were seen most often: The overall maternal mortality rate in this group was 2.7%, and the stillbirth and spontaneous abortion rate was 7.7%. Cardiac disease covers a wide range of conditions, including congenital heart disease, acquired disease such as rheumatic valvular disease, and coronary disease. It is estimated that 1% to 3% of women either have cardiac disease entering pregnancy or are diagnosed with cardiac disease while they are pregnant. …

Cardiovascular disease in pregnancy

Heart disease continues to be an important cause of maternal mortality. This is largely because of the extensive haemodynamic changes that occur during pregnancy, namely the increase in blood volume, fluctuations in cardiac output, fall in systemic vascular resistance and hypercoagulable state. High-risk periods include the end of the second trimester, labour and the immediate postpartum period. Prognosis depends on the specific heart condition, the patient's functional class and the degree of cardiac dysfunction. Conditions that are contra-indicated in pregnancy include Eisenmenger's syndrome, primary and secondary pulmonary hypertension, complex cyanotic congenital heart disease, Marfan's syndrome with aortic root dilatation, and severe left ventricular dysfunction. Women with heart disease should be thoroughly evaluated and counselled before and during pregnancy. Multidisciplinary care is essential for successful maternal and fetal outcomes.

Anesthesia for the parturient with cardiovascular disease

Extracted from text ... REVIEW ARTICLE Southern African Journal of Anaesthesia & Analgesia - May 2003 18 Anesthesia for the parturient with cardiovascular disease by the marked fluctuations in cardiac output observed during labor. Pain and apprehension may precipitate an increase in cardiac output to as much as 45-50% over those levels seen in the late second stage of labor.6 Further, each uterine contraction serves, in effect, as an autotransfusion to the central blood volume, resulting in an increase in cardiac output of 10-25%.7 The Valsalva maneuver results in wide swings in both venous and arterial pressures, which have been asso-ciated with acute ..

Monitoring end-tidal carbon dioxide during weaning from cardiopulmonary bypass in patients without significant lung disease.

End-tidal carbon dioxide tension (PETCO(2)) changes with fluctuations in cardiac output (CO). We compared PETCO(2) to pulmonary artery blood flow (PAQt) during weaning from cardiopulmonary bypass (CPB) in normothermic patients without significant pulmonary disease. Fifteen consecutive adult cardiac surgical patients were prospectively studied during and shortly after weaning from CPB. Before separation from CPB, PETCO(2) and PAQt were measured, the latter by transesophageal Doppler echocardiography. At the time of measurements patients were normothermic, and ventilated at 6 breaths/min with tidal volumes of 10 mL/kg. After separation from CPB, thermodilution cardiac output (TDCO) was measured in addition to PAQt and PETCO(2). Regression and bias analyses were used to compare PETCO(2), PAQt, and TDCO. Seventy measurements were recorded; 31 before separation from CPB and 39 after separation from CPB. A good correlation was seen between PAQt and PETCO(2) (r = 0.88) and between TDCO and PAQt (r = 0.93; mean bias 0.03 L/min; SD 0.52 L/min). The regression analysis of PAQt on PETCO(2) showed greater variability at PETCO(2) levels > 34 mm Hg (n = 22; r = 0.14). Increases in PETCO(2) plateaued at this level, although PAQt continued to increase. When PETCO(2) was more than 30 mm Hg, all PAQt and TDCO values were >4.0 L/min (>2.0 L/min/m(2)). When PETCO(2) exceeded 34 mm Hg, all values of PAQt, and 28/29 values of TDCO were more than 5 L/min (>2.5 L/min/m(2)). One patient had TDCO of 4.69 L/min (2.39 L/min/m(2)). In normothermic patients without significant pulmonary disease, PETCO(2) is a useful index of PAQt during separation from CPB. Under the clinical settings in this study, a PETCO(2) greater than 30 mm Hg was invariably associated with a CO more than 4.0 L/min or a cardiac index >2.0 L/min/m(2).

Chest physiotherapy in mechanically ventilated children: a review.

Many physicians, nurses, and respiratory care practitioners consider chest physiotherapy (CP) a standard therapy in mechanically ventilated children beyond the newborn period. CP includes percussion, vibration, postural drainage, assisted coughing, and suctioning via the endotracheal tube. We searched the medical literature by using the key words "chest physiotherapy" and "chest physical therapy" (among others) by means of the MEDLINE and Current Contents databases. Because of the paucity of objective data, we examined all reports dealing with this topic, including studies on adult patients. For data extraction, not enough material existed to perform a meta-analysis. Despite its widespread use, almost no literature dealing with this treatment modality in pediatric patients exists. Studies with mechanically ventilated pediatric and adult patients have shown that CP is the most irritating routine intensive care procedure to patients. An increase in oxygen consumption often occurs when a patient receives CP accompanied by an elevation in heart rate, blood pressure, and intracranial pressure. CP leads to short-term decreases in oxygen, partial pressure in the blood, and major fluctuations in cardiac output. Changes in these vital signs and other variables may be even more pronounced in pediatric patients because the lung of a child is characterized by a higher closing capacity and the chest walls are characterized by a much higher compliance, thus predisposing the child to the development of atelectasis secondary to percussion and vibration. CP in mechanically ventilated children may not be considered a standard therapy. Controlled studies examining the impact of CP on the duration of mechanical ventilatory support, critical illness, and hospital stay are needed.

Hemodynamic analysis of arterial pressure oscillations in conscious rats

This study examined the contribution of rhythmic fluctuations of regional blood flow and vascular conductance to the genesis of low- (LF, 0.27–0.74 Hz) and high- (HF, 0.76–5 Hz) frequency oscillations of arterial pressure. In conscious 15-week-old male intact ( n = 11), guanethidine-sympathectomized ( n = 8) and chronically sinoaortic denervated ( n = 7) rats, arterial pressure and regional blood flow velocities (pulsed Doppler probes) were simultaneously recorded. Indices of subdiaphragmatic aortic, hindquarters and superior mesenteric conductances were calculated on a beat-to-beat basis over a 60-min period. Spectral power was calculated in the LF and HF bands using a fast Fourier transform algorithm. Transfer function analysis was also performed to calculate coherence and phase between arterial pressure and regional flows and conductances. In the LF band, spectral power of arterial pressure was decreased by approx. 85% in sympathectomized and approx. 54% in sinoaortic denervated rats. In the HF band, spectral power did not differ between the groups. In the three groups of rats, relations between arterial pressure and blood flow were characterized by a significant coherence in the HF band with little or no phase delay (synchronous oscillations). Relations between arterial pressure and vascular conductance were characterized in intact rats by a significant coherence in the LF band and a phase delay tending to π radians (opposite oscillations), whereas in both sympathectomized and sinoaortic denervated rats, coherence did not reach significance. It is concluded that LF oscillations of arterial pressure are mostly secondary to rhythmic fluctuations in the vasomotor sympathetic tone in several regional circulations. Part of these oscillations originate from the synchronizing influence of the baroreceptor reflex. The study also suggests that the respiratory (HF) oscillations of arterial pressure involve fluctuations in cardiac output of purely mechanical origin.

Flow and thermal responses of aortic baroreceptors

1. The aims of this study were 1) to determine whether the impulse activity in rabbit aortic baroreceptors (BRs) was influenced by changes in nonpulsatile flow through the aortic lumen and 2) to examine the BR and aortic arch responses to changes in temperature. 2. An in vitro aortic arch-aortic nerve preparation was used to record suprathreshold steady-state discharge in a total of 29 single-unit BRs from 12 New Zealand white rabbits. Changes in BR frequency were measured relative to control and were recorded simultaneously with aortic arch pressure, flow, temperature, diameter, and calculated wall shear stress (Sw). 3. With pressure held constant, stair-step increases in flow (3-18 ml/min) constricted the arch and evoked two types of BR responses: activation in most units (15 of 17 BRs tested) and inhibition in 2 units. The activation response appeared closely related to the changes in flow and Sw, but also appeared related to uncontrolled changes in perfusate temperature. 4. When the effects of temperature were examined more closely with pressure and flow held constant, controlled step increases in temperature (between 32 and 42 degrees C) constricted the arch and again evoked two BR responses: activation in 11 of 14 BRs tested and inhibition in 3 units. The Q10 for the activation response was 1.55 +/- 0.08 (mean +/- SE), which was not significantly different from the Q10 for activation when temperature varied with flow (1.65 +/- 0.1, P < 0.05). Thus the effect of temperature on most BRs was directionally and quantitatively similar in the presence and absence of changes in flow. 5. Last, when flow was examined over a wide range (15-515 ml/min) with temperature and pressure held constant, stair-step increases in flow failed to produce any change in BR frequency in each of 15 fibers tested (10 arches). The lack of response was not due to a functionally damaged preparation because the usual BR and aortic arch responses to pressure and to drug-evoked vasoconstriction (norepinephrine) and endothelial-mediated vasodilation (acetylcholine) were intact. 6. These results demonstrate that aortic BRs in rabbits are not sensitive to flow and thus are not likely influenced by fluctuations in cardiac output apart from associated changes in pressure. The aortic BRs are affected directly by variations in temperature and in some cases indirectly by temperature-related vasoconstriction. The effects of temperature may have important implications for neural control of the circulation when core temperature varies because of physiological and environmental stress.

EFFECTS OF RESPIRATION ON BLOOD PRESSURE AND HEART RATE VARIABILITY IN HUMANS

1. Non-invasive continuous finger blood pressure (BP) measurement and a spectral technique based on the Fourier transform (FT) were recently combined to quantify short-term fluctuations in haemodynamic variables. 2. Systolic BP (SBP) recording combined low frequency (LF, Mayer waves) plus high frequency (HF, respiratory) oscillations. The presence of HF oscillations of SBP probably reflects fluctuations in cardiac output. Heart rate (HR) also exhibited a combination of low and HF (respiratory) oscillations. The vagus nerve mediates the efferent control of the respiratory sinus arrhythmia (RSA). 3. During controlled breathing in a supine position, a change in SBP is associated with an opposite consequent change in HR. Respiratory sinus arrhythmia could therefore depend on the baroreceptor-HR response to underlying SBP oscillations. 4. The fast regulation of R-R interval by SBP through the baroreceptor-HR reflex may explain why the respiratory variations in the diastolic BP are small.

Spectral analysis of blood pressure and heart rate in conscious rats: effects of autonomic blockers

We investigated the fluctuations which underly the spontaneous variability of blood pressure and heart rate in conscious rats. Intrafemoral blood pressure was computed to generate evenly spaced signals (systolic, diastolic, mean blood pressure, heart rate) at 200 ms intervals. This equidistant sampling allowed a direct spectral analysis using a Fast Fourier Transform algorithm. Systolic blood pressure and heart rate exhibited low-frequency oscillations (Mayer waves, 20–605 mHz) and a high- frequency oscillation related to respiration (1855 mHz). The respiratory fluctuations in heart rate were almost abolished by vagal blockade (atropine). Heart rate fluctuations in the low-frequency regime were diminished by vagal blockade or cardiac sympathetic blockade (atenolol). The respiratory frequency fluctuations in systolic blood pressure were markedly increased by α-sympathetic blockade (prazosin). In contrast, the low-frequency oscillations in systolic blood pressure were reduced by α-sympathetic blockade. These data indicate that in conscious rats: (1) the heart rate oscillation with respiration is vagally mediated; (2) the heart rate fluctuation in the low-frequency range is jointly mediated by β-sympathetic and parasympathetic activities; (3) the respiratory oscillation in systolic blood pressure depends on fluctuations in cardiac output and is normally counteracted by the sympathetic tone; (4) the low-frequency oscillations in systolic blood pressure reflect the sympathetic activity to the resistance vessels.

Mayer waves in the circulation of a teleost fish

AbstractRegular oscillation in arterial blood pressures identical to mammalian Mayer waves were recorded in unanaestetized trout (Salmo gairdneri). The waves were associated with hemorrhage, were caused by fluctuations in systemic vascular resistance, and were sometimes accompanied by fluctuations in cardiac output of opposite phase. Alpha‐adrenergic blockade eliminated the phenomena; muscarinic cholinergic blockade was ineffective. The existence of Mayer waves provides evidence for a nervous control of vasomotor tone in teleosts.