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Pathophysiology of the Cardiovascular System and Neonatal Hypotension

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Pathophysiology of the Cardiovascular System and Neonatal Hypotension Pathophysiology of the Cardiovascular System and Neonatal Hypotension Sandra L. Shead, RNC-NIC, MSN, CNS, NNP-BC Continuing Nursing Education BSTRACT (CNE) Credit A A total of 2.5 contact Hypotension is common in low birth weight neonates and less common in term newborns and is hours may be earned as CNE credit associated with significant morbidity and mortality. Determining an adequate blood pressure in for reading the articles in this issue neonates remains challenging for the neonatal nurse because of the lack of agreed-upon norms. Values identified as CNE and for completing an online posttest and evaluation. for determining norms for blood pressure at varying gestational and postnatal ages are based on To be successful the learner must empirical data. Understanding cardiovascular pathophysiology, potential causes of hypotension, and obtain a grade of at least 80% on assessment of adequate perfusion in the neonatal population is important and can assist the neonatal the test. Test expires three (3) years from publication date. Disclosure: nurse in the evaluation of effective blood pressure. This article reviews cardiovascular pathophysiology The author/planning committee as it relates to blood pressure and discusses potential causes of hypotension in the term and preterm has no relevant financial interest or affiliations with any commercial neonate. Variation in management of hypotension across centers is discussed. Underlying causes and interests related to the subjects pathophysiology of hypotension in the neonate are described. discussed within this article. No commercial support or sponsorship was provided for this educational Keywords: neonatal hypotension; definition; physiology; cardiovascular system; preterm; management; activity. ANN/ANCC does not assessment; cerebral; autoregulation; blood pressure endorse any commercial products discussed/displayed in conjunction with this educational activity. The Academy of Neonatal Nursing is accredited as a provider of continuing nursing education by the American Nurses Credentialing Center’s Commission on Accreditation. AINTAINING EFFECTIVE BLOOD resistance (the forces acting against blood flow Provider, Academy of Neonatal pressure for adequate tissue and from the heart). Maintaining adequate organ Nursing, approved by the California M organ perfusion in the neonate is critical perfusion is dependent on cardiac output and Board of Registered Nursing, Provider #CEP 6261; and Florida for optimum outcomes and is dependent on systemic vascular resistance. Cardiac output, Board of Nursing, Provider #FBN several physiologic mechanisms which may be or systemic blood flow, is determined by the 3218, content code 2505. compromised in the ill or premature neonate. heart rate and stroke volume. Heart rate is The purpose of this article is to review the cardiovascular pathophysiology To have an understanding of the complexities influenced by stimulation of the heart’s con- of blood pressure and discuss the of the neonatal cardiovascular system and the duction system and the autonomic nervous potential causes of hypotension in the term and preterm neonate. potential causes of hypotension, it is essential system, both of which determine the intrinsic to review the circulatory changes that occur rate of the heart. The higher the heart rate, during the transition from intrauterine to the greater the cardiac output. Stroke volume extrauterine life, the many mechanisms that is the amount of blood ejected from the control blood pressure in the body, and the heart with each contraction. Stroke volume is differences in the term and preterm neonate. determined by the amount of preload, after- load, and contractility of the heart.1 Preload is the volume of blood in the PHYSIOLOGY OF BLOOD ventricles prior to contraction at the end of PRESSURE CONTROL diastole when the ventricle has filled. Preload Cardiac Output can be reduced in ill or premature infants Blood pressure is the amount of blood as a result of an elevated heart rate or low ejected from the heart and is the product of circulating blood volume.2 Afterload is the cardiac output (the amount of blood ejected pressure against which the heart must pump from the left ventricle) and systemic vascular during systole and is influenced by systemic Accepted for publication October 2014. N EONATAL N ETWORK VOL. 34, NO. 1, JANUARY/FEBRUARY 2015 © 2015 Springer Publishing Company 31 http://dx.doi.org/10.1891/0730-0832.34.1.31 vascular resistance.1 Systemic vascular resistance is the resis- blood pressure. Baroreceptors are nerve endings located in tance of the systemic circulation against blood flow from the the carotid arteries and the aortic arch. Baroreceptors send heart and is determined by vessel diameter and blood viscos- impulses to the brain which stimulates the sympathetic ity. Blood vessel diameter is influenced by blood vessel vaso- nervous system to vasoconstrict or vasodilate arteries to alter dilation and vasoconstriction. The greater the blood vessel blood flow in response to stretch of the arterial walls or a diameter, the less the systemic vascular resistance, and the fall in blood pressure (Figure 1).3 Baroreflex sensitivity is smaller the blood vessel diameter, the greater the systemic impaired in newborns and to a greater extent in premature vascular resistance. Blood viscosity increases when the blood newborns because of immaturity of the vagal system which contains a greater amount of red blood cells versus plasma can contribute to cardiovascular instability.5 Baroreflex sensi- (polycythemia), therefore increasing the systemic vascular tivity improves with increasing postmenstrual age.6 resistance. Afterload is influenced in the newborn at birth by After blood is ejected from the heart, there are long- and increased pulmonary vascular resistance and elimination of short-term mechanisms that control arterial blood pressure in the low-resistance placental circulation, which is replaced by the body. Blood pressure regulation is maintained by neural a higher-resistance systemic circulation. mechanisms of the autonomic nervous system and humoral Contractility is the force of contraction of the heart or mechanisms of the kidneys and pituitary gland consisting inotropy.3 Contractility can be reduced in the premature mainly of the renin-angiotensin system and vasopressin.1 neonate because of fewer cardiac muscle fibers and an imma- Neural mechanisms providing short-term control of arterial ture myocardium.2 Cardiac output is increased in the mature blood pressure include the sympathetic and parasympathetic heart by increasing heart rate and stroke volume. Neonates nervous systems. The sympathetic nervous system consists of lack the ability to increase stroke volume and rely primarily nerve fibers located on each side of the spinal column that on increased heart rate to compensate for low cardiac output.4 extend into the blood vessels and the heart. The sympathetic nervous system controls blood pressure by causing constric- Neural Control tion of the arteries and arterioles. Constriction of the arter- The autonomic system consists of the sympathetic and ies and arterioles increases peripheral vascular resistance, parasympathetic nervous systems which stimulate the heart which raises the amount of blood flow to the heart and con- to regulate heart rate and contractility. The sympathetic sequently causes a temporary rise in arterial blood pressure. nervous system acts on the contractile filaments of the heart A significant fall in blood pressure is immediately followed to increase or decrease the force of contraction of the heart by autonomic nervous system signals that increase the rate muscle separately from preload and afterload.1 and contractility of the heart and contraction of systemic The autonomic nervous system responds quickly to vessels, which increases the amount of blood flow back to hypotension by acting on the baroreceptors to maintain the heart. Conversely, a temporary decrease in blood pressure FIGURE 1 n In response to a drop in blood pressure, the autonomic nervous system acts on baroreceptors to send impulses to the brain which stimulates the sympathetic nervous system to vasoconstrict or vasodilate to alter blood flow. baroreceptors on aorta and carotid sinus send information about changes in BP to cardiovascular center in medulla cardiovascular center arteriole sympathetic nerves sympathetic innervate the nerves innervate myocardium the sino-atrial node Abbreviation: BP blood pressure. N EONATAL N ETWORK 32 JANUARY/FEBRUARY 2015, VOL. 34, NO. 1 can be caused by parasympathetic stimulation of the vagus and acting as a potent vasoconstrictor of arterioles through- nerve, which contains nerve fibers that extend into the heart. out the body.3 Parasympathetic vagal stimulation of the heart reduces the heart rate and slightly decreases muscle contractility, conse- Autoregulation quently causing a temporary decline in blood pressure.3 The local nutrient needs of the body tissues drive the amount of blood flow they receive. In response to the tissue Humoral Control needs, cardiac output increases to some degree but is not The mechanism for short-term regulation, as occurs in able to meet high tissue demands for nutrients. High tissue response to a sudden drop in blood pressure or a sudden demands occur with strenuous activity which in the term increase in physical activity, responds quickly but is ineffec- infant can occur with crying.3 Increased tissue demands can tive at maintaining blood pressure over a longer
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