Cardiac Physiology and Pharmacology

16 Cardiac Physiology and Pharmacology




THE CARDIOVASCULAR SYSTEM plays a dominant role within the human body: a centrally located “powerhouse” provides oxygenation and nutrition via an extensive network of vessels and capillaries throughout the body. All other organ systems depend on its normal development and function. At birth, and especially in the first few hours of life, the heart and the vascular system have to adapt to the extrauterine conditions. Prematurity, congenital defects, complications during labor and delivery, and many other factors can prevent or delay the necessary changes and cause significant morbidity.


A thorough understanding of the fetal circulation, the changes at birth, and the age-specific characteristics is important for the safe management of neonates, infants, and especially the growing number of preterm and small-for-gestational-age (SGA) babies who come to our diagnostic suites and operating rooms. Given the complex embryology and difficult transition from fetal to extrauterine life, it is amazing that more than 90% of neonates are delivered without any special interventions and that congenital heart defects occur in only 7 to 10 of every 1000 live births.1 (A detailed discussion of the embryologic development is beyond the scope of this chapter; the interested reader is referred to the excellent review by Van Praagh2 or Langman’s classic embryology textbook.3)


Congenital heart defects are among the most common birth defects. In the United States, approximately 32,000 babies are born every year with congenital heart disease (CHD); a significant number require urgent interventions in the catheterization laboratory or surgical procedures during the neonatal period. In addition, CHD is often associated with other, noncardiac anomalies, and many of these children will present for procedures outside the cardiac operating room. Pediatric anesthesiologists have to be able to classify and recognize the pathophysiologic effects of CHD on the cardiovascular system of the neonate or infant and the potential impact of anesthesia and surgical manipulations.


With recent advances in surgical techniques, critical care, and anesthesia management, 85% of all infants with CHD are now expected to reach adulthood. Anesthesiologists will increasingly encounter children with “repaired” or “palliated” CHD presenting for noncardiac procedures. Chapter 21 addresses specific long-term problems and anesthetic considerations for various repaired heart defects, but a few conditions deserve additional discussion: the basic changes in the exercise physiology of repaired heart defects, the characteristics of the so-called Fontan physiology after single ventricle palliation, and the altered physiologic responses in the transplanted heart.


Many conditions require pharmacologic support with cardiovascular drugs, some of which can have significant age-specific effects. Well-controlled drug studies in infants and children are rare. Dosing is often based on long-standing experience or extrapolation from adult data. Understanding the basic pharmacology of the most commonly used cardiovascular drugs and the special considerations for infants and children is essential for successful perioperative care. This chapter will help the pediatric anesthesiologist to understand the complexity of the neonatal cardiovascular system, the implications of CHD, and basic pharmacologic considerations and will provide the necessary tools to develop a safe management plan.



Cardiovascular Physiology



Fetal Circulation


In utero, the placental gas exchange provides the fetus with relatively oxygenated blood; the partial pressure of oxygen (Po2) in the umbilical vein is approximately 30 mm Hg, and in the umbilical arteries it is approximately 16 mm Hg. The fetal lungs are fluid filled and only minimally perfused (10% to 15% of the cardiac output). The normal postnatal circulation can be principally described as a serial circuit: Two pumps, the right ventricle [RV] and left ventricle [LV], support two different resistance systems, the pulmonary and systemic vasculatures, one after the other. In contrast, the fetal circulation is better explained by the concept of a parallel circuit: Both ventricles provide systemic blood flow and a variety of fetal shortcuts or connections allow for mixing of oxygenated and deoxygenated blood (Fig. 16-1).4,5 Oxygenated blood from the placenta returns via the umbilical vein to the portal venous system, where 30% to 50% of the blood flow is shunted across the ductus venosus to the inferior vena cava (IVC), bypassing the liver and thereby maintaining higher oxygenation and velocity. The rest of the umbilical venous blood passes through the hepatic microcirculation into the suprahepatic IVC.



The IVC blood entering the right atrium (RA) is a mixture of bloodstreams with different velocities and saturations: the low-velocity, deoxygenated venous return from the lower body and hepatic veins and the high-velocity, oxygenated umbilical venous blood from the ductus venosus. Valve-like tissue in the RA (eustachian valve) and the Chiari network preferentially direct the high-velocity blood-stream from the IVC across the foramen ovale into the left atrium (LA), bypassing the RV and pulmonary vessels. In the LA, the oxygenated blood mixes with the minimal amount of venous return from the pulmonary circulation and is then ejected by the LV into the ascending aorta and the major vessels of the aortic arch. This blood, with a saturation of 65% to 70%, provides the oxygenation for the growing heart and brain.


Most of the venous return from the superior vena cava (SVC) and about 20% of the IVC blood flow (mainly the low-velocity, deoxygenated part) reach the RV and are pumped into the pulmonary artery (PA), where the high pulmonary resistance in the nonexpanded lung redirects 90% of the blood flow into the descending aorta via the ductus arteriosus. The bulk of the blood flow in the descending aorta is generated by the RV, with minor contributions from the LV. The blood has a saturation of only 55% to 60%; two thirds of it returns to the placenta for oxygenation, and the rest is distributed to the intestines, the kidneys, and the lower part of the body (Fig. 16-2).


image

FIGURE 16-2 Fetal circulation in the late-gestation lamb. A, The numbers indicate the percentage of oxygen saturation. Oxygen saturation is greatest in the inferior vena cava (IVC), representing flow that is primarily from the placenta. The saturation of the blood in the heart is slightly greater on the left side than on the right side. B, The course of the circulation. The numbers represent the percentage of combined ventricular output. Some of the return from the IVC is diverted by the crista dividens in the right atrium (RA) through the foramen ovale into the left atrium (LA), where it meets the pulmonary venous return (PV), passes into the left ventricle (LV), and is pumped into the ascending aorta. Most of the ascending aortic flow goes to the coronary, subclavian, and carotid arteries, with only 10% of combined ventricular output passing through the aortic arch (indicated by the narrowed point in the aorta) into the descending aorta (AO). The remainder of the IVC flow mixes with return from the superior vena cava (SVC) and coronary veins (3%), passes into the RA and right ventricle (RV), and is pumped into the pulmonary artery (PA). Because of the increased pulmonary resistance, only 7% of the blood passes through the lungs (PV), with the rest passing through the ductus arteriosus (DA) to the AO and then to the placenta and lower half of the body.


(Modified from Rudolph AM. Congenital diseases of the heart. Chicago: Year Book Publishers; 1974, p. 1-48; and from Freed MD. Fetal and transitional circulation. In: Fyler DC, editor. Nadas’ pediatric cardiology. Philadelphia: Mosby-Year Book; 1992. p. 57-61.)


The fetal circulation has to support a growing fetus in a relatively cyanotic atmosphere (highest oxygen saturation, 65% to 70%). This difficult task is further complicated by the parallel circuit, which creates increased workload for the RV, and the limitations of the fetal shortcuts, which add additional volume load by incomplete shunting of oxygenated and deoxygenated blood. Initially, understanding of the fetal circulation was based mainly on experimental animal data, but recent advances in ultrasound technology have made it possible to assess and monitor fetal cardiovascular parameters, especially stroke volume and cardiac output, under various conditions throughout the gestational period. RV stroke volume has been found to increase from about 0.7 mL at 20 weeks to 7.6 mL at 40 weeks, and LV stroke volume increases from 0.7 mL to 5.2 mL. The combined fetal cardiac output of both ventricles is estimated to be 400 to 425 mL/kg/min, with an RV dominance due to the increased volume load. At 38 weeks, the RV provides approximately 60% of the combined cardiac output (E-Table 16-1).68 Intrauterine growth restriction and placental compromise are associated with redistribution of cardiac output and relative changes in the size of the foramen ovale.9 A functional placenta, the fetal cardiovascular high-output state, greater hemoglobin concentrations, and additional alterations in oxygen binding and release (hemoglobin F, increased 2,3-diphosphoglycerate [2,3-DPG]) are all necessary to provide adequate tissue oxygenation for the developing fetus.


image




Until recently, CHD was thought to be relatively well tolerated in utero, but growing evidence suggests that fetal cardiovascular defects can induce intrinsic autoregulatory changes in cerebral perfusion and thereby compromise brain development.10,11 Ultrasound and magnetic resonance imaging demonstrate that 30% to 50% of neonates with CHD have neurologic abnormalities before any surgical intervention.12



Transitional Circulation


At birth, a variety of humoral, biochemical, and physiologic changes occur abruptly. First, the placental circulation is eliminated shortly after the lungs expand. Second, expansion of the lungs to a normal functional residual capacity (FRC) results in an optimal geometric relationship of the pulmonary microvasculature. Third, air entering the lungs causes the alveolar Pco2 to decrease and the alveolar Po2 to increase. These three factors act in concert to markedly reduce pulmonary vascular resistance (PVR).5,13,14 The net effect is a considerable increase in pulmonary blood flow, which augments pulmonary venous return to the left heart. Along with elimination of the placenta and the low-resistance umbilical circulation, the LV is suddenly subjected to increased volume and afterload (Table 16-1). Typically, LV end-diastolic pressure, and thus LA pressure, increases enough to exert hydrostatic pressure on the septum primum, resulting in functional closure of the foramen ovale. In contrast to the increased stress for the LV, the RV is relatively unloaded by the transition to extrauterine life.


TABLE 16-1 Hemodynamic Changes at Birth



























Right Ventricle Left Ventricle
Decreased afterload: Increased afterload:
Decreased pulmonary vascular resistance Placenta eliminated
Ductal closure Ductal closure
Decreased volume load: Increased volume load:
Eliminated umbilical vein return Increased pulmonary venous return
Output diminished 25% Output increased almost 50%
  Transient left-to-right shunt at ductus

The three fetal connections (ductus arteriosus, ductus venosus, and foramen ovale) close over a variable period. The ductus arteriosus has functionally (but not anatomically) closed in 58% of normal full-term infants by 2 days of life and in 98% by day 4.15 Although many substances such as eicosanoids have been implicated, initial constriction probably occurs primarily in response to the increased arterial oxygen tension16,17 and the reduction in circulating prostaglandins that follow separation of the placenta.18 The response to oxygen is age dependent: Term neonates usually demonstrate effective constriction of the smooth muscles in the ductal tissue when exposed to oxygen, whereas preterm infants poorly respond and often require medical (prostaglandin inhibitor) or even surgical therapy. Additional catecholamine-induced changes in PVR and systemic vascular resistance (SVR) and other substances such as acetylcholine contribute to ductal closure. Within 2 to 3 weeks, functional constriction is followed by a process of ductal fibrosis, leaving a band-like structure, the ligamentum arteriosum.19,20 With ligation of the umbilical vein, the portal pressure falls, triggering functional closure of the ductus venosus. This process rarely requires more than 1 to 2 weeks; by 3 months only fibrous tissue, the ligamentum venosum, is left.


The foramen ovale is functionally closed when the LA pressure exceeds the pressure in the RA, but it remains anatomically patent in most infants, in 50% of children younger than 5 years of age, and in 25% to 30% of adults.21 Echocardiographic studies have confirmed right-to-left shunting via the foramen ovale in healthy infants emerging from general anesthesia, and this can be a significant cause of persistent arterial desaturation at that time despite ventilation with 100% oxygen.22



Neonatal Cardiovascular System


Compared with the adult, the neonatal myocardium is immature and incompletely developed (Table 16-2). Differences in cytoarchitecture and metabolism account for many of the functional limitations. The neonatal heart contains fewer muscle cells and more connective tissue than the adult myocardium. Contractile elements add up to only 30% of the total cardiac mass, in contrast to 60% in the adult.23 The ratio of surface area to mass and that of water to collagen content are larger. There are fewer myofibrils within the muscle cells, and they tend to be less organized (i.e., not parallel to the long axis of the cell). The sarcoplasmic reticulum and the T-tubule network, both important components of rapid and effective calcium regulation, are incompletely developed, and the immature myocardium relies substantially on the calcium flux through the sarcolemma to initiate and terminate contraction.2426 One likely practical consequence is a greater degree of contractile dysfunction in the infant exposed to substances that decrease extracellular ionized calcium, such as citrate (blood products) and albumin; there is also increased sensitivity to volatile anesthetics and calcium channels blockers.


TABLE 16-2 Characteristic Differences between the Immature and the Adult Myocardium



















  Immature Myocardium Adult Myocardium
Cytoarchitecture

Metabolism

Function


ATP, Adenosine triphosphate; CO, cardiac output; SR, sarcoplasmic reticulum.


Data from Mossad EB, Farid I. Vital organ preservation during surgery for congenital heart disease. In: Lake CL, Booker PD, editors. Pediatric cardiac anesthesia. 4th ed. Philadelphia: Lippincott, Williams & Wilkins; 2005, p. 266-90; and DiNardo J, Zwara DA. Congenital heart disease. In: DiNardo J, Zwara DA, editors. Anesthesia for cardiac surgery. 3rd ed. Malden, Mass.: Blackwell Publishing; 2008. p. 167-251.


Reduced numbers of underdeveloped mitochondria and maturational differences in various signaling pathways and related messenger systems are also characteristic of the neonatal myocardium. Immature mitochondrial enzyme activity for fatty acid transport may explain the primary use of carbohydrates and lactates as energy sources and might be a reason for the greater anaerobic tolerance and faster recovery after periods of ischemia. A variety of developmental changes in contractile proteins occur from fetal through early postnatal life, including changes in pH, calcium sensitivity, and adenosine triphosphate (ATP) hydrolyzing activity. The key features of the immature cardiac function are summarized in Table 16-2.


The increased amount of non-contractile tissue in the neonate results in poor ventricular compliance and limited response to increased preload. Compliance of both ventricles progressively increases during fetal life and the postnatal period so that maximal stroke volume occurs at a significantly reduced atrial pressure in the neonate compared with the fetus (Figs. 16-3 and 16-4).2729 The extraordinarily high metabolic rate of the neonate (oxygen consumption, 6 to 8 mL/kg/min, compared with 2 to 3 mL/kg/min in the adult) requires a proportional increase in cardiac output. The neonatal heart functions at close to maximal rate and stroke volume just to meet the basic demands for oxygen delivery.30,31 The cardiac output is commonly described as being primarily heart rate (HR) dependent due to a fixed stroke volume, but echocardiographic studies in human fetuses and neonates clearly demonstrate the capacity to increase stroke volume (Fig. 16-5).32 In fact, the neonate employs both tachycardia and stroke volume adjustments simply to meet metabolic demand. On the other hand, neonates exhibit exquisite sensitivity to pharmacologic agents that produce negative inotropic or chronotropic effects. At birth both ventricles are equal in mass and connected via a common septum. Increased pressures in one ventricle lead to a septal shift and decreased compliance of the opposite ventricle, causing a reduction in cardiac output. Neonates and infants often present with biventricular failure as a result of this interventricular dependence.





Immature autonomic regulation of cardiac function persists throughout the neonatal period. Both sympathetic and parasympathetic innervation of the heart can be demonstrated at birth. However, evidence suggests that development of the sympathetic nervous system is incomplete at both the postganglionic nerve-receptor level and the receptor-effector level.33 The sympathetic system reaches maturity by early infancy, whereas the parasympathetic system reaches maturity within a few days after birth.34 The relative imbalance of these two components of the autonomic nervous system at birth may account for the clinical observation that neonates are predisposed to exhibit marked vagal responses to a variety of stimuli.



Pulmonary Vascular Physiology


At birth, pulmonary vascular development is incomplete. Lung sections demonstrate diminished numbers of arterioles, and the arterioles exhibit thick medial muscularization (Fig. 16-6).3537 The pulmonary vasculature matures during the first few years of life. During this period, arterioles proliferate faster than alveoli, and the medial smooth muscle thins and extends more distally in the vascular tree. PVR continues to decrease as long as pulmonary mechanics and alveolar gas composition remain favorable, with a significant decrease occurring immediately after birth due to lung expansion and oxygenation. Progressive remodeling of the pulmonary vasculature facilitates further decreases in PVR (assuming normal physiology) during the first 2 to 3 months of life; by 6 months of age, the PVR has almost reached the level seen in healthy adults.37


image

FIGURE 16-6 Peripheral pulmonary artery development. The normal pattern of pulmonary vascular development and that of a 2-year-old child with pulmonary vascular changes accompanying a large ventricular septal defect (VSD). Rabinovitch characterized the pulmonary vasculature morphometrically in three respects: vessel thickness, muscular extension, and the ratio of alveoli to arteries seen on lung biopsy specimens. The normal neonate exhibits thick vascular smooth muscle, but this extends only as far as the arterioles accompanying the respiratory bronchiole. In neonates, the alveoli/artery ratio is 20 : 1. In the first few months of life, the vessels thin substantially and proliferate relative to the alveoli, so that by the age of 2 years, the normal child has an alveoli/artery ratio of 12 : 1 and thin muscles extending to the arteries associated with alveolar ducts. In the normal adult, the alveoli/artery ratio is 6 : 1 and muscle extends all the way to the arteries in the alveolar wall. In contrast, in the 2-year-old child with a large VSD, the vessel numbers are markedly diminished (alveoli/artery ratio, 25 : 1), and persistent neonatal muscle thickness extends all the way to the alveolar wall. AD, Artery at alveolar duct; AW, artery at alveolar wall; RB, respiratory bronchiole; TB, artery at terminal bronchiole.


(From Steven JM, Nicolson SC. Congenital heart disease. In: Miller RD, series editor. Atlas of anesthesia. vol 7: Pediatric anesthesia [Greeley WJ, volume editor]. Orlando, Fla.: Harcourt Publishers; 1998, p. 6.6; modified from Rabinovitch M, Haworth SG, Castaneda AR, et al: Lung biopsy in congenital heart disease: a morphometric approach to pulmonary vascular disease. Circulation 1978;58:1107-22.)


The fetal pulmonary vasculature is extremely reactive to a number of stimuli. Hypoxia, acidosis, increased levels of leukotrienes, and mechanical stimulation can cause significant and prolonged increases in PVR (e.g., reactive pulmonary hypertension). On the other hand, acetylcholine, histamine, bradykinin, prostaglandins, β-adrenergic catecholamines, and nitric oxide are strong vasodilators.35 In the first days of life, many pathophysiologic conditions can trigger severe and sustained increases in PVR38,39 and prevent the normal adjustment to extrauterine life (E-Table 16-2). The acute load imposed on the RV can induce diastolic dysfunction and promote right-to-left shunting via the foramen ovale. Once PVR exceeds the SVR, a right-to-left shunt develops via the ductus arteriosus and patent foramen ovale. This situation is called persistent fetal circulation,40 and it can result in a life-threatening hypoxemia that may require inhaled nitric oxide,4145 sildenafil,46 or extracorporeal support (i.e., extracorporeal membrane oxygenation)47,48 (see Chapter 19) to provide oxygenation and sustain life.


image



E-TABLE 16-2 Conditions Prolonging Transitional Circulation









Pulmonary vascular occlusive disease (PVOD) is a term used to describe structural changes in the pulmonary vasculature after longstanding exposure to abnormal pressures and flow patterns in utero and after birth. Lung biopsies demonstrate thickened muscle layers in the small pulmonary arteries, intimal hyperplasia, scarring, and thrombosis as well as a decreased number of distal (intraacinar) arteries.36 Over time, these changes lead to progressive and finally irreversible obstruction of pulmonary blood flow with increases in PVR and PA pressures. The highly muscularized pulmonary arteries are also extremely reactive to pulmonary vasoconstrictors, which can easily trigger a pulmonary hypertensive crisis.


Many cardiac defects are associated with abnormal pulmonary flow patterns and can be categorized into three basic groups:



The muscle tone in the pulmonary arteries is regulated by numerous factors, and various therapeutic interventions can be used to manipulate the PVR (Table 16-3)14:



TABLE 16-3 Manipulations of Pulmonary Vascular Resistance (PVR)









Increasing PVR Decreasing PVR



Fio2, Fraction of inspired oxygen; FRC, functional residual capacity; PEEP, positive end-expiratory pressure.


In summary, the pulmonary vasculature undergoes a complex maturation process that can be influenced by a multitude of external factors and congenital heart defects. Persistent fetal circulation and PVOD are examples of inadequate adaptation and development. In cases of increased PVR, ventilator strategies using greater inspired oxygen concentrations, lung volumes close to the FRC, and interventions aiming for a Pao2 greater than 60 mm Hg, a Paco2 of 30 to 35 mm Hg, and a pH of 7.5 to 7.6 can improve pulmonary blood flow.



Incidence and Prevalence of Congenital Heart Disease


CHD can be defined as “a gross structural abnormality of the heart or intrathoracic great vessels that is actually or potentially of functional significance.”49 This definition covers a wide array of defects, which are among the most common congenital malformations. However, the precise incidence of CHD, both collectively and by individual anatomic subset, varies depending on definition, method of case identification, and epoch (E-Table 16-3). Including all categories of CHD, large epidemiologic surveys place the prevalence anywhere between 4 and 50 cases per 1000 live births.5053 When stratified according to trivial, moderate, and severe forms, the incidence for moderate and severe forms of CHD is relatively consistent at about 6 per 1000 live births.


image




Anatomic diagnoses within the population of infants with CHD vary according to the method used to identify cases. In 2002, Hoffman and colleagues compiled 62 epidemiologic studies published after 1955 and investigated the potential causes for the wide variability in the reported incidence of CHD.54 More recent studies based mainly on prenatal and postnatal echocardiographic screening data often include a large number of trivial lesions (e.g., tiny VSDs, nonstenotic bicuspid aortic valve, “silent” PDA) for which no interventions may be required; other data collections, such as the New England Regional Infant Cardiac Program (NERICP), a registry of children with CHD who died or required catheterization or surgery during the first year of life, are clearly biased toward more severe forms of CHD.1


The increasing availability of prenatal diagnostic methods may exert an impact on the relative prevalence of reported lesions as well as their outcome. When fetal echocardiography is used, the apparent shift toward more complex lesions may reflect technical limitations in identifying simple defects.55 In addition, evaluation in utero skews the results because it includes fatally malformed fetuses that will not survive to term. The prevalence of CHD among spontaneous abortions reaches 20% and remains as large as 10% among stillborn infants.56 In one study, 50% of women whose children were given a prenatal diagnosis of CHD elected to terminate the pregnancy, particularly when presented with complex heart lesions.55


On the other hand, female infants with severe CHD have a 5% lower mortality rate than similarly affected male infants,1 and with increased survival rates, more females will reach childbearing age. The recurrence risk of CHD for their offspring is about 3% to 4%.57,58


A study from Canada examined the changing epidemiology of CHD with respect to prevalence and age distribution in the general population between 1985 and 2000.59 The prevalence of all categories of CHD in 2000 was 11.89 per 1000 in children (<18 years of age), 4.09 per 1000 in adults, and 5.78 per 1000 in the general population. For the subcategory of severe CHD, the prevalence was 1.45 per 1000 in children and 0.38 per 1000 in adults. In 2000, 49% of all patients with severe CHD were adults, compared with 35% in 1985, and females accounted for 57% of the adult CHD population. The prevalence of CHD increased for both children and adults between 1985 and 2000, although the increase for severe CHD was 85% in adults compared with 22% in children. The median age of all patients with severe CHD was 11 years in 1985 and 17 years in 2000, reflecting the fact that more children with CHD were surviving to adulthood (E-Fig. 16-1). Improved survival may be attributed to improved prenatal care, early diagnostic imaging, and major advances in pediatric cardiac care, particularly for those with severe CHD; these improved outcomes will continue to influence the future demographic profile. The growing number of adolescents and adults with CHD will require long-term follow-up with experienced cardiologists and access to specialized care facilities; this will require a thorough understanding of their underlying pathophysiology by all members of the adult care team, including anesthesiologists.


image





Pathophysiologic Classification of Congenital Heart Disease


CHD consists of an almost endless array of anatomic and functional variants. Many different classification systems have been introduced, some using a segmental approach to anatomic features, others by examining the amount of pulmonary blood flow (cyanotic versus acyanotic) or the common physiologic characteristics (e.g., volume versus pressure overload).6070 Several of these classifications are discussed in Chapter 14. However, certain defects are better described using the concepts of shunting (physiologic, anatomic, simple or complex), intercirculatory mixing, and single ventricle physiology, which are presented in the following sections.



Shunting


Shunting occurs when blood return from one circulatory system (systemic or pulmonary) is recirculated to the same system, completely bypassing the other circulation. For example, if deoxygenated blood from the systemic veins flows directly to the aorta, the result is a right-to left shunt with recirculation of deoxygenated blood in the systemic circulation. In contrast, redirection of oxygenated blood from the pulmonary veins to the PA causes a left-to-right shunt with recirculation of oxygenated blood within the pulmonary circulation. The terms physiologic and anatomic are often used to describe shunting. Basically, any kind of recirculation of blood within one circulatory system is called physiologic shunting. In most cases, physiologic shunting is caused by an anatomic shunt (i.e., a communication between the cardiac chambers or the great vessels), but physiologic shunting can also exist by itself, as in the classic transposition physiology.


To really understand the pathophysiology of shunting and its implications, it is important to introduce the concepts of effective and total systemic/pulmonary blood flows. Effective blood flow is the quantity of venous blood from one circulatory system that reaches the arterial system of the other circulatory system. Effective pulmonary blood flow is the volume of systemic venous blood reaching the pulmonary circulation, whereas effective systemic blood flow is the volume of pulmonary venous blood reaching the systemic circulation. Effective pulmonary blood flow and effective systemic blood flow are always equal, no matter how complex the lesions. Total blood flow, on the other hand, is the sum of recirculated and effective blood flow and a measure of the workload of the circulatory system. Total systemic and pulmonary blood flows are not equal. Even in healthy patients there is a small amount of normal physiologic shunting (e.g., thebesian cardiac veins, bronchial vessels), but with CHD the difference can be quite significant. Physiologic shunting or recirculation should be viewed as a noneffective, superfluous load added to the essential nutritive blood flow (effective blood flow).


Anatomic shunts are communications between the two circulatory systems, either within the heart or at the level of the great vessels. They can be divided into simple and complex shunts, depending on the presence of additional outflow obstructions. In simple shunts without any additional outflow obstruction, the size of the communication (the so-called shunt orifice) determines the flow characteristics. For small orifices (restrictive shunts) with large pressure gradients across the communication, the size of the opening essentially regulates the amount of shunting. Changes in SVR or PVR have little influence. In contrast, for large orifices or nonrestrictive shunts (also classified as dependent shunts), the quantity and direction of blood flow are controlled by the respective outflow resistances (i.e., the ratio of SVR to PVR) (Table 16-4 and Fig. 16-7).


TABLE 16-4 Characteristics of Simple Shunts (without Additional Outflow Obstruction)























  Restrictive
(Small Shunt Orifice)
Nonrestrictive
(Large Shunt Orifice)
Examples Small ASD, VSD, or PDA; modified Blalock-Taussig shunt Large VSD, PDA, CAVC
Pressure gradient across shunt Large Small or none
Direction and magnitude of shunt Independent of PVR/SVR PVR/SVR dependent
Influence of pharmacologic and ventilatory interventions Minimal Large

ASD, Atrial septal defect; CAVC, common atrioventricular canal; PDA, patent ductus arteriosus; PVR, pulmonary vascular resistance; SVR, systemic vascular resistance; VSD, ventricular septal defect.


Modified from DiNardo J, Zwara DA. Congenital heart disease. In: DiNardo J, Zwara DA, editors. Anesthesia for cardiac surgery. 3rd ed. Malden, Mass.: Blackwell Publishing; 2008. p. 167-251.



Complex shunts are defined by an additional outflow obstruction, which can be at various levels within the ventricle, valves, or great vessels and is often described as subvalvular, valvular, or supravalvular. These obstructions can be fixed (e.g., valvular stenosis) or variable (e.g., dynamic infundibular obstruction by muscle bundles). Shunt flow and direction are determined by the combined resistance across the outflow obstruction and the pulmonary/systemic vascular beds. For severe obstructions downstream, SVR or PVR will have little influence on the shunt. Tetralogy of Fallot is a good example of a complex shunt lesion. The amount of right-to-left shunt and therefore the amount of cyanosis are influenced by the degree and type of right ventricular outflow tract obstruction (RVOTO). This is especially evident in the setting of a dynamic infundibular obstruction, where changes in preload, contractility, and HR can lead to significant decreases in pulmonary blood flow and increased shunting (Table 16-5).


TABLE 16-5 Characteristics of Complex Shunts (with Additional Outflow Obstruction)























  Partial Outflow Obstruction Complete Outflow Obstruction
Examples TOF, VSD/PS, VSD/coarctation Tricuspid or mitral atresia
Pulmonary or aortic atresia
Shunt magnitude and direction Relatively fixed Totally fixed
Dependence on PVR/SVR ratio Inversely related to obstruction Independent
Pressure gradient across shunt Dependent on shunt orifice and degree of obstruction Dependent only on shunt orifice

PS, Pulmonary stenosis; PVR, pulmonary vascular resistance; SVR, systemic vascular resistance; TOF, tetralogy of Fallot; VSD, ventricular septal defect.


Modified from DiNardo J, Zwara DA. Congenital heart disease. In: DiNardo J, Zwara DA, editors. Anesthesia for cardiac surgery. 3rd ed. Malden, Mass.: Blackwell Publishing; 2008, p. 167-251.



Intercirculatory Mixing


The concept of intercirculatory mixing is often used to explain the unique physiology in children with transposition of the great arteries (TGA). In this cardiac defect, the aorta arises from the RV, transporting deoxygenated blood back to the right heart, and the PA originates from the LV, returning oxygenated blood to the pulmonary circulation (see Fig. 15-6). Unless there is some mixing of blood via an ASD, VSD, or PDA, this defect will result in a complete separation of the two systems, a parallel circulation with 100% physiologic shunting or recirculation of oxygenated and deoxygenated blood that is incompatible with life once the fetal ductus arteriosus has closed. Effective pulmonary blood flow (i.e., deoxygenated blood reaching the pulmonary vascular bed for oxygenation) has to be provided by some form of right-to-left shunt; effective systemic blood flow (i.e., oxygenated blood returning to the systemic circulation) must be achieved by a left-to-right shunt. Intercirculatory mixing is the combined systemic and pulmonary effective blood flow and is only a small portion of the total blood flow. The bulk of the respective systemic and pulmonary total blood flows consists of recirculated blood (Fig. 16-8). Usually the total blood flow and the volume in the pulmonary system are two to three times greater than in the systemic circulation.


image

FIGURE 16-8 Depiction of saturations, pressures, and blood flows in transposition of the great arteries with a nonrestrictive atrial septal defect134 and a small left ventricular (LV) outflow tract gradient. Intercirculatory mixing occurs at the atrial level. Effective pulmonary and effective systemic blood flows are equal (1.1 L/min/m2) and are the result of a bidirectional anatomic shunt at the atrial level. The physiologic left-to-right shunt is 9.0 L/min/m2; this represents blood recirculated from the pulmonary veins to the pulmonary artery (PA). The physiologic right-to-left shunt is 1.2 L/min/m2; this represents blood recirculated from the systemic veins to the aorta (Ao). Total pulmonary blood flow (image = 10.1 L/min/m2) is almost five times higher than the total systemic blood flow (image = 2.3 L/min/m2). The bulk of pulmonary blood flow is recirculated pulmonary venous blood. In this depiction, pulmonary vascular resistance (PVR) is low (approximately 1/35 of systemic vascular resistance [SVR]) and there is a small (17 mm Hg peak to peak) gradient from the LV to the PA. These findings are compatible with the high pulmonary blood flow depicted. LA, Left atrium; RA, right atrium; RV, right ventricle.


(From DiNardo J, Zwara DA. Congenital heart disease. In: DiNardo J, Zwara DA, editors. Anesthesia for cardiac surgery. 3rd ed. Malden, Mass.: Blackwell Publishing; 2008. p. 167-251.)


The arterial saturation (Sao2) is influenced by the volumes and saturations of recirculating and effective systemic blood flows and can be calculated with the use of the following equation:



image



Increasing the intercirculatory mixing will improve the arterial saturations, and in severely cyanotic neonates with TGA, intact ventricular septum, and inadequate atrial communication, a balloon atrial septostomy (balloon dilation of an existing patent foramen ovale or small ASD, either echo-guided at the bedside or under fluoroscopy in the catheterization laboratory) can be lifesaving. Additional measures to improve systemic and pulmonary venous saturations (e.g., blood transfusion, inotropic support, ventilatory strategies) can help to stabilize the arterial saturation.



Single Ventricle Physiology


Single ventricle physiology defines the circulation present in a wide variety of complex cardiac defects. It is characterized by complete mixing of systemic and pulmonary venous blood return at either the atrial or the ventricular level; the mixed blood is then distributed to both systemic and pulmonary circulations in parallel. The defects can consist of one anatomic single ventricle with severe hypoplasia and inflow or outflow obstruction of the other one (hypoplastic left heart syndrome [HLHS] or pulmonary atresia with intact ventricular septum) or even two well-developed ventricles with atresia of the outflow tract or severe obstruction (tetralogy of Fallot with pulmonary atresia, interrupted aortic arch). In some lesions, a PDA is the only source of systemic or pulmonary blood flow; these are called duct-dependent circulations. In others, intracardiac communications provide adequate blood flow to both circulations (Table 16-6).


TABLE 16-6 Examples of Single Ventricle Physiology







































Congenital Heart Defect Aortic Blood Flow from Pulmonary Blood Flow from
Hypoplastic left heart syndrome PDA RV
Neonatal critical aortic stenosis PDA RV
Interrupted aortic arch Proximal LV, distal PDA RV
Tetralogy of Fallot with pulmonary atresia LV PDA, MAPCAs
Pulmonary atresia with intact septum LV PDA
Tricuspid atresia 1B (VSD and PS) LV LV through VSD to RV
Truncus arteriosus LV and RV Aorta
Double inlet left ventricle, no TGA LV LV through VSD to bulboventricular foramen

LV, Left ventricle; MAPCAs, major aortopulmonary collateral arteries; PDA, patent ductus arteriosus; PS, pulmonary stenosis; RV, right ventricle; TGA, transposition of the great arteries; VSD, ventricular septal defect.


Modified from DiNardo J, Zwara DA. Congenital heart disease. In: DiNardo J, Zwara DA, editors. Anesthesia for cardiac surgery. 3rd ed. Malden, Mass.: Blackwell Publishing; 2008, p. 167-251.


Irrespective of the anatomic features, in single ventricle physiology the ventricular output (delivered by one or two ventricles) is the sum of the pulmonary and systemic blood flows. The distribution of the respective flows is directly dependent on the relative outflow resistances into the two parallel circulations. Oxygen saturations in the aorta and PA are equal. The severity and location of anatomic obstructions and the ratio of PVR to SVR determine the balance of flows to the two circulations


The following equation illustrates the various factors that influence the arterial saturation (Sao2) in a single ventricle physiology:



image



Accordingly, three major variables determine arterial saturation and the initial management options for patients with single ventricle physiology:


May 25, 2016 | Posted by in ANESTHESIA | Comments Off on Cardiac Physiology and Pharmacology
Premium Wordpress Themes by UFO Themes