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18  Cardiovascular Physiology And Support

        availability of metabolic substrates to support myocardial contraction   Table 4.1: Age and average blood pressures.
        and  a  decreased  ventricular  preload.  Conversely,  a  low  heart  rate  is   Average blood
        usually associated with an increase in stroke volume due to increased   Age  pressure (mm Hg)
        ventricular preload.
           Cardiac output is affected by the above-noted variables in patients of   Premature  Systolic 40–60
        all ages, but neonates are somewhat unique in that they have a limited   Full term  75/50
        ability to increase stroke volume. The difference reflects a relatively   1–6 months  80/50
        lower  compliance  of  the  neonatal  myocardium,  thereby  limiting   6–12 months  90/65
        increases in cardiac output associated with increases in preload. Also,   12–24 months  95/65
        the neonatal myocardial contractility is less responsive to sympathetic
        stimulation  due  to  differences  in  calcium  transits.  Therefore,  the   2–6 years  100/60
        neonate is much more dependent upon changes in heart rate to increase   6–12 years  110/60
        cardiac output in times of need.                        12–16 years     110/65
           Anatomy and Physiology of the Circulation            16–18 years     120/65
        The body has two circulatory systems: the pulmonary circulation and   Adult  125/75
        the systemic circulation. Normally, the two circulations are separate.
        Structurally, both circulations are similar in that they start with a single   Source: Modified from Zuckerberg AL, Wetzel RC, Shock, fluid resuscitation, and coagulation
        large  vessel  that,  through  sequential  branchings,  distributes  blood  to   disorders. In Nichols DG, Yaster M, Lappe DG, Buck JR (eds). Golden Hour: The Handbook of
                                                               Advanced Pediatric Life Support. Mosby-Year Book, 1991.
        arteries of decreasing diameter but increasing number. Ultimately the
        small arteries (arterioles) empty blood into a series of capillaries, which
        are the primary site of exchange of solutes between the intravascular   the adrenal medullae. Finally, the CNS also affects blood pressure over
        and  extravascular  compartments.  From  the  capillaries,  blood  enters   the long term by releasing vasopression (antidiuretic hormone) from the
        a  large  number  of  small  veins  (venules),  which,  through  a  series  of   posterior pituitary, which primarily has an effect by increasing water
        junctions with other venous structures of similar calibre, coalesce into   reabsorption by the renal tubules and increasing intravascular volume.
        several large venous structures that return blood to the atria.   Vasopressin  is  also  a  potent  vasoconstrictor;  however,  under  normal
           An important concept to remember is that blood always flows down   conditions, the circulating concentration of this hormone is too low to
        a pressure gradient and will always take the path of least resistance. As a   have a direct effect on vascular control.
        result, points of increased resistance in the circulatory system will result   Given the potent effects of the CNS on blood pressure regulation,
        in an increase in pressure proximal to the point of the obstruction until   it is not surprising that there are multiple mechanisms for controlling
        either the pressure is adequate to overcome the cause of the resistance   the  vasomotor  centres  in  the  brain.  These  vasomotor  centres  are
        or blood flow is diverted through an alternate pathway. The pulmonary   located  in  the  medulla  and  pons  and  include  both  a  vasoconstrictor
        and  systemic  circulations  differ  in  that  the  pulmonary  circulation  is   area and a vasodilator area. The vasoconstrictor area causes excitation
        a  high-flow,  low-resistance,  and  thus  low-pressure  system,  whereas   of  vasoconstrictor  neurons  in  the  SNS,  whereas  the  vasodilator
        the systemic circulation has much higher overall resistance and as a   centre  primarily  functions  to  cause  inhibition  of  neurons  in  the
        result has higher intraluminal pressures. Reflecting these differences in   vasoconstrictor area. The activities of these two vasomotor centres are
        pressures, the relative stiffness and thickness of the arteries are greater   affected by afferent impulses (1) from stretch receptors (baroreceptors)
        in the systemic circulation than in the pulmonary circulation. However,   located  in  the  carotid  sinus  and  the  wall  of  the  aortic  arch,  which
        in  patients  with  abnormal  connections  between  the  pulmonary  and   respond to short-term changes in pressure in these arteries; (2) from
        systemic  circulations  (e.g.,  patent  ductus  arteriosus),  the  pulmonary   low pressure receptors in the atria and pulmonary arteries that reflect
        arteries will ultimately hypertrophy in response to the higher pressures   changes  in  blood  volume;  and  (3)  from  higher  brain  centres  that
        experienced by the pulmonary vessels. The following section primarily   respond  to  stressful  stimuli  (e.g.,  pain,  alarm)  and  CNS  ischaemia.
        addresses the mechanisms that control pressure and blood flow within   Under normal conditions, the vasoconstrictor centre of the brain stem
        the systemic circulation.                              is continuously active, causing partial contraction of the blood vessels
           Control  of  pressure  and  blood  flow  in  the  systemic  circulation   and maintaining baseline vasomotor tone. This explains why rapid loss
        occurs  both  globally  and  locally.  Table  4.1  lists  ranges  for  average   of SNS activity (such as following a cervical spinal cord injury) often
        blood pressure based upon age, with the mean pressure +20% at the   results in hypotension.
        95% confidence limit. Values for females are approximately 5% lower   The  other  organ  that  has  important  global  effects  on  blood
        than for males. These paediatric blood pressure references may help   pressure is the kidney. In the kidney, the juxtaglomerular cells located
        guide  diagnosis  and  management  during  times  when  the  patient  is   in  the  proximal  arterioles  release  renin  into  the  bloodstream  in
        demonstrating signs of shock.                          response to a decrease in perfusion. Renin is an enzyme that cleaves
           The  central  nervous  system  (CNS)  and  the  kidneys  are  the   circulating plasma angiotensinogen, resulting in release of angiotensin
        primary  organs  responsible  for  global  blood  pressure  control.  The   I.  Angiotensin  I  is  subsequently  metabolised  to  angiotensin  II  by
        CNS affects systemic blood pressure both directly via the autonomic   converting  enzyme,  which  is  primarily  located  in  the  walls  of  small
        nervous system and indirectly by inducing release of humoral factors.   vessels  in  the  lung.  Angiotensin  II  has  several  effects,  including
        Increased sympathetic nervous system (SNS) activity increases blood   vasoconstriction of both arterioles and veins, resulting in an increase
        pressure by increasing cardiac contractility (as described previously)   in  vascular  resistance  and  venous  return.  It  also  decreases  salt  and
        and  by  causing  vasoconstriction  through  release  of  norepinephrine   water loss by the kidney (both by a direct effect on the kidney and by
        from  nerve  endings  that  innervate  the  vasculature.  The  latter  effect   stimulating secretion of  aldosterone by  the  adrenal  cortex),  resulting
        increases  arterial  resistance,  thus  increasing  proximal  pressures,  and   in expansion of the circulating blood volume. Ultimately, these effects
        also increases venous return to the heart—and thus cardiac output—by   cause an increase in blood pressure and renal perfusion, resulting in a
        increasing venous tone and decreasing the capacitance of the venous   negative feedback on renin release.
        system.  SNS  activation  also  increases  blood  pressure  by  stimulating   The  global  mechanisms  for  affecting  blood  flow  are  primarily
        release of the vasoconstrictors, epinephrine, and norepinephrine from   involved  in  maintaining  adequate  central  systemic  blood  pressure
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