Heart rate variability

Analysis of small oscillations in heart rate, termed heart rate variability (HRV), provides a non-invasive estimate of autonomic reflex function.33 34 Typically, two major components are seen in a heart rate spectrum. The high frequency component (0-15-0-5 Hz) is said to be mediated by the parasympathetic nervous system. The low frequency component (0-04-0-15 Hz) is influenced by both the sympathetic and the parasympathetic nervous system, and is related to waves in arterial pressure mediated...

Conclusions

The low resistance of the pulmonary circulation results in relatively poor control of the distribution of blood flow. Distribution is primarily dependent on the interrelationship of transpulmonary and vascular pressures, but is modified by differences in regional conductance associated with the length of the vascular pathways and the fractal branching pattern of the pulmonary vasculature. Distribution is further modified at alveolar level by the influence of alveolar and mixed venous gas...

Central nervous system cardiovascular centres

The critical central nervous system (CNS) cardiovascular control centres are located in the medulla oblongata and lower pons.2 The centres for circulatory control are in close proximity to those regulating respiration, and together both comprise the CNS areas crucial for survival of the organism. The centres for circulatory control have two major divisions, the vasomotor and the cardiac regions, which supply nervous innervation to the peripheral vasculature and heart, respectively.3 4 The two...

Effects of anaesthesia on blood volume distribution

From a physiological point of view, the vascular system can be divided into two parts. One is the low pressure system that comprises all postarteriolar vessels, the right heart, the pulmonary vascular system, and the left heart during diastole. The other is the high pressure system, which includes the left ventricle during systole, systemic arteries, and arterioles. The low pressure system holds about 85 of the total intravascular blood volume, one third of which is in the intrathoracic...

Autoregulation

Autoregulation refers to the ability of the cerebral circulation to maintain CBF at a relatively constant level in the face of changes in CPP by altering cerebrovascular resistance (CVR) (Fig. 7.13). Although autoregulation is maintained irrespective of whether changes in CPP arise from alterations in MAP or ICP, autoregulation tends to be preserved at lower levels when falls in CPP are the result of increases in ICP rather than decreases in MAP Fig 7.12 Relationship of astrocytes to oxygen and...

Role of pulmonary vasodilators in pulmonary hypertension

The most obvious effect of pulmonary hypertension is that it increases the right ventricular pressure, workload, and oxygen consumption, and so may lead to right ventricular failure. As the blood flow to the right ventricle occurs during both systole and diastole it is important to maintain a high systemic pressure to minimise myocardial ischaemia. A second problem resulting from a high pulmonary artery pressure is that it may cause high pressure pulmonary oedema by increasing the pressure in...

Narrowing of the pulmonary vessels

The presence of pulmonary hypertension in patients with ARDS was first documented in 1977.72 In the acute phase narrowing of the pulmonary vessels may be caused by endogenous substances, such as thromboxane A2 or B2 and prostaglandin E2, by an increase in interstitial pressure secondary to pulmonary oedema, and by alveolar hypoxia and hypercapnia secondary to respiratory failure. In the later phases of the disease, fibrosis and destruction of the pulmonary vascular bed become important. Hypoxia...

Pulmonary venous hypertension

Pulmonary venous hypertension may be caused by mediastinal lesions which compress the pulmonary veins, a myxoma or ball-valve thrombus in the left atrium, mitral or aortic valve disease, or left ventricular failure. It may also result from pulmonary veno-occlusive disease. An acute increase in pressure in the pulmonary venous system results in pulmonary congestion and a corresponding increase in pulmonary artery pressure. If the congestion is severe, pulmonary oedema may result. Chronic...

Pulmonary hypertension

Pulmonary hypertension is defined as a chronic increase in pulmonary artery systolic pressure above 30 mm Hg or a mean pressure greater than 20-25 mm Hg. From a consideration of Poiseuille's equation (page 187) it is apparent that pulmonary hypertension may be expected to occur in the following circumstances 1 When there is a reduction in the number of vessels perfused. 2 When there is a narrowing of the vessels as a result of intimai thickening, muscle hypertrophy, vascular spasm, or a...

Cellular cardiac electrophysiology

Tmp Action Potential Phases

Loss of membrane potential maximum diastolic potential resting membrane potential change in TMP after excitation of cell spontaneous diastolic depolarisation atrial, ventricular, or Purkinje fibres with AP upstroke dependent on fast Na current most negative diastolic TMP automatic fibres stable TMP during diastole most fast response fibres SA and AV node cells with AP upstroke mostly dependent on slow Ca current electrical potential across cell membrane responsible for generation of the APs,...

Coronary anatomy

Coronary Anatomy

The right and left coronary arteries provide the entire blood supply to the myocardium. They arise from the coronary ostia in the sinuses of Valsalva, Fig 4.1 Relationship between coronary blood flow and myocardial oxygen consumption in conscious dogs. Each dog is represented by a separate symbol. A close correlation between the two variables over a wide range is evident. Reproduced with permission from Khouri EM, Gregg DE, Rayford CR. Effect of exercise on cardiac output, left coronary flow...

Fluid balance in the lung

Thoracic Extra Anatomic Air

No review of the pulmonary circulation would be complete without a brief consideration of its role in the control of water and solute transfer within the lung, and of the mechanisms causing pulmonary oedema. Electron micrographs show that the basement membranes of the alveolar epithelium and endothelium appear to be fused over the thin portion of the interalveolar septum, where most gas exchange is presumed to take place, but that they are separated in the thick part of the septum by the...

Control of the renal circulation

Dopamine And Angiotensin Kidney

The kidneys receive approximately 20 of cardiac output. They are capable of increasing flow even further, although they constitute less than 0.5 of the total body weight. This marked renal blood flow is well in excess of that required to provide renal tissue with sufficient oxygen and nutrients. Therefore, renal blood flow is regulated to maintain an optimum delivery of filtrate to the nephrons and adequate reabsorption of fluid back into the vascular system. The factors that control renal...

Microcirculatory transport and the bloodbrain barrier

Blood Brain Barrier Components

Endothelial cells in cerebral capillaries contain few pinocytic vesicles, and are sealed with tight junctions, with no anatomical gap. Consequently, Fig 7.4 Relative effects of PaCO2 on cerebral blood flow CBF and voume CBV . Hyperventilation is aimed at reducing CBV in patients with intracranial hypertension, but may be detrimental because of its effects on CBF. Note that the slope of CBF reactivity to PaCO2 is steeper than that for CBV about 25 per kPa PaCO2 vs 20 per kPa PaCO2, respectively...

Endothelial control of coronary vascular tone

Muscle Endothelium Adenosine

The coronary vascular endothelium plays a major role in regulating vasomotor tone in health and disease.12 It modulates the contractile activity of the underlying smooth muscle through the secretion and synthesis of substances with different biological activities in response to a variety of pharmacological agents for example, acetylcholine, substance P, catecholamines and physical stimuli for example, blood flow, pulsatile flow, shear stress . Endothelium-derived relaxing factors Since the...

Other humoral substances

There are several other endogenous substances that can be released into the circulation and affect the heart and vasculature. These include adenosine, histamine, the plasma kinins kallidin and bradykinin , serotonin, and endothelins. Adenosine is a ubiquitous endogenous nucleotide that stimulates A , A2-, and A3- receptors to produce haemodynamic actions.11 It has inhibitory effects on cardiac impulse conduction through the atrioventricular node negative dromotropic effect . Adenosine is also a...

Indices of ventricular contractility

Maximum rate of change of ventricular pressure dP dtmax dP dtmax IP IP developed pressure at dP dtmax minus end diastolic pressure Left ventricular dP dtmax end diastolic pressure Pre-ejection period PEP Ejection phase parameters Ejection fraction EF Velocity of circumferential fibre shortening Vf Ejection time ET , PEP ET Peak systolic flow velocity, flow acceleration, time to peak flow Doppler techniques End systolic pressure volume relationship left ventricular elastance Emax End systolic...

Autonomic nervous system

Paravertebral Anatomy

The cardiovascular control centres of the brain stem ultimately activate deactivate the autonomic nervous system ANS which provides innervation of cardiac and vascular smooth muscle. The ANS represents the efferent or motor component of cardiovascular control and consists of two complementary divisions the sympathetic and parasympathetic nervous systems SNS and PNS, respectively . The SNS and PNS are commonly considered physiologically to be antagonistic, producing opposite effects on...

Acknowledgements

This work was supported by the Anesthesiology Research Training Grants GM 08377. 1 Mitchell GF, Pfeffer JM, Pfeffer MA. The heart and conduit vessels in hypertension. Med clin North Am 1997 81 1247-71. 2 Dampney RA, Goodchild AK, Tan E. Identification of cardiovascular cell groups in the brain stem. Clin Exp Hypertens A 1984 6 205-20. 3 Hilton SM, Spyer KM. Central nervous regulation of vascular resistance. Annu Rev Physiol 1980 42 399-441. 4 Alexander RS. Tonic and reflex functions of...

Cerebral blood volume physiology and potential for therapeutic intervention

Intracranial Contents

Most of the intracranial blood volume of about 200 ml is contained in these venous sinuses and pial veins, which constitute the capacitance vessels of the cerebral circulation reduction in this volume can buffer rises in the volume of other intracranial contents the brain and CSF . Conversely, Fig 7.2 Venous sinuses of the brain, showing drainage of the superior sagittal sinus into the right jugular vein via the transverse and sigmoid sinus. Note that the first extracranial tributary of the...

Changes in ventricular performance related to ageing

As age itself is rarely a contraindication to surgery, anaesthetists are increasingly encountering very old patients undergoing major surgery that puts formidable stress on the cardiovascular system. It is therefore important to understand the changes in cardiac function associated with ageing.121 On consideration of the high prevalence of clinically latent coronary artery disease and hypertension in elderly people, it is not easy to separate the effects of ageing from other pathological...

Variations in myocardial perfusion

Coronary Flow And Aortic Pressure

Blood supply to the heart is affected by ventricular contraction and relaxation. Any myocardial stress is expected to alter underlying myocardial geometry and, in turn, geometry of intramyocardial vessels. This may affect vascular resistance and flow. Forces acting on a myocardial segment may include interactions between myofibres and adjacent vessels, intramyocardial tissue fluid pressure, cavity pressure transmitted as radial stress, myofibre force transmitted tangentially, and pericardial...

Metabolic pathways and moiety conserved cycles

Tumor Resistance

It is a characteristic property of all living cells, including heart muscle, to provide an environment in which complex chemical reactions can proceed quickly at relatively low temperatures and low substrate concentrations. The efficient transfer of energy occurs via enzyme catalysed metabolic pathways, at the centre of which are moiety conserved cycles that is, a cycle in which the concentration of the participating intermediaries neither increases nor decreases . Moiety conserved cycles...

Reentry of excitation

Intraatrial Reentrant Tachycardia

The excitatory wavefront emanating from the SA node continues until all of the heart has been activated and become completely refractory. If for some reason a group of fibres is not activated by the propagating impulse, and it returns by another pathway to excite them, this process is termed ''re-entry of excitation. It is also termed circus movement or reciprocation, and extrasystoles caused by re-entry reciprocal or echo beats. Three basic criteria for ascribing abnormal beats or rhythm to...

Effects of lung volume on pulmonary vascular resistance

Oxygen Tension Venous Blood

The pulmonary vascular resistance is calculated by dividing the driving pressure pulmonary artery minus left atrial pressure by the flow, so it includes the resistance of all the vessels between the right and left heart. The importance of the differentiation between intra- and extra-alveolar vessels is that changes in lung volume exert opposing effects on the two sets of vessels. Expansion of the lung will occur when transpulmonary pressure is increased, whether this is produced by a reduction...

Pressure autoregulation

Autoregulation Coronary Hypertensive

Flow is locally controlled in certain vascular beds by a process termed autoregulation36 Fig. 6.5 . By definition, autoregulation is the ability of an organ to maintain a relatively constant blood flow in the presence of changes in arterial perfusion pressure. The kidneys, brain, and heart are organs that exhibit autoregulation, whereas the skin and lungs are organs with minimal ability for autoregulation. Fig 6.5 Pressure autoregulation in the normal coronary circulation. Note that during...

Pulmonary vasodilator drugs

Conventional vasodilator drugs have three main disadvantages in patients with pulmonary hypertension 1 Most pulmonary vasodilators also dilate the systemic vascular bed and so decrease aortic pressure. This results in a decrease in coronary perfusion which may lead to myocardial ischaemia when right ventricular stroke work and oxygen consumption are increased by 2 The decrease in pulmonary artery pressure may decrease the perfusion of non-dependent zones and so increase alveolar dead space. 3...

Reflex control of the circulation

Baroreceptor Reflex Arc

Cardiovascular reflexes represent rapidly acting mechanisms to control the circulation using the central and autonomic nervous systems.16 Reflex control of the circulation can be initiated either from within the cardiovascular system intrinsic reflexes or from other organs or systems extrinsic reflexes . Intrinsic reflexes are the most important short term regulators of arterial pressure. These reflexes are produced by changes in arterial pressure or special chemical stimuli. Alterations in...

Automaticity phase 4 diastolic depolarisation

Pacemaker Current

Pacemaker fibres in the SA node and latent also subsidiary or secondary pacemakers spontaneously depolarise during phase 4 towards threshold for regenerative excitation -termed automaticity. The SA nodal action potential with underlying currents is shown in Fig 3.7.20 From a maximum diastolic potential of - 50 to - 60 mV, the cell undergoes slow diastolic phase 4 depolarisation. This phase merges smoothly with the AP upstroke phase 0 , and there is no distinct AP overshoot, phase 1, or phase 2...

Coronary flow reserve

Tachycardia And Increased Contractility

The pressure-flow relationship during maximum coronary vasodilation is almost straight D, Fig 4.8 . Maximum flow at any given pressure is determined primarily by the cross-sectional area of the resistance vessels. The difference between autoregulated A, Fig 4.8 and maximally dilated D, Fig 4.8 flow is the reserve capacity for vasodilation. Coronary flow reserve CFR is defined as the ratio of CBF during maximal coronary vasodilation to CBF under resting conditions.162 Depending on the technique...

Anaesthetic arrhythmic potential

Anaesthetic drugs can have both pro- and antiarrhythmic actions.33 34 38 Proarrhythmic actions include the facilitation of catecholamine mediated ventricular arrhythmias, suppression of normal automaticity in primary and secondary pacemaker fibres, and depression of AV conduction. Examples of antiarrhythmic actions include suppression of abnormal automaticity and DAD triggered activity, and ventricular tachyarrhythmias in canine models of myocardial ischaemia and infarction. Discussion of these...

Ventricular function curves and the Frank Starling law of the heart

Frank Starling Law

Clinical evaluation of the overall pump function of the heart is usually based on the ventricular function curve, which relates a measure of ventricular performance cardiac output, stroke volume, or stroke work as a dependent variable to a measure of ventricular preload, such as ventricular filling pressure, end diastolic volume, end diastolic diameter, cross sectional area, or end diastolic wall stress.9 10 78 The ventricular function curve describes the fundamental dependence of ventricular...

Afterload mismatch

Afterload Cardiac Output

As a result of the immediate upregulation in force generation in response to acute increases in afterload, the steady state relationship between stroke work and end diastolic volume remains insensitive to wide alterations in afterload.85 This is true as long as there is a sufficient preload reserve only then does the ventricular performance generated stroke volume or stroke Fig 2.12 Guyton's diagram showing right and left ventricular function curves. At steady state, the output of both...