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Cardiovascular Physiology And Support 19

and thus assuring adequate perfusion to organs with high metabolic and neurogenic. Each of these forms of shock can be present in the
needs, including the heart and CNS. The vasoconstricting effects of paediatric perioperative surgical patient, and it is imperative that each
sympathetic nervous activity and circulating humoral agents result type of shock be adequately treated prior to an elective procedure. One
in a decrease in vascular compliance (the same as an increase in may encounter those occasions when the shock may not be completely
vascular resistance). The cumulative effect of increasing vascular tone resolved, however, and surgery becomes unavoidable or emergency
contributes to the body’s total vascular resistance. The total vascular surgery is required. During these types of patient presentations, the
resistance is one factor that affects pulse pressure, or the difference understanding of each form of shock needs to be understood and, if
between systolic and diastolic blood pressures. The other factor is possible, treated before surgery ensues and further complications arise.
the stroke volume output of the heart. Increases in vascular resistance Hypovolaemic Shock
primarily are reflected by increases in diastolic pressure, whereas Hypovolaemic shock is the most common form of shock in the paedi-
increases in cardiac output typically result in an increase in both the atric population and results primarily from a decreased intravascular
systolic and the diastolic pressure. Typically, the diastolic pressure is volume, causing a diminishing venous return and consequently, a
two-thirds to three-fourths of the systolic pressure. Changes in pulse decreased preload. Neonates and young infants have a relatively set
pressure can be a valuable indicator of circulatory derangements. stroke volume due to the immaturity of their cardiac muscle, which
Whereas the globally active mechanisms primarily affect systemic results in the compensation mechanism of an increased heart rate when
blood pressure, local control mechanisms are primarily involved in the preload decreases.
controlling blood flow to individual organs and tissues. Both metabolic In the typical African clinic, it would be common to see a paediatric
and myogenic mechanisms may be involved in local control of blood patient who has a 2- or 3-day history of diarrhoea or vomiting and
flow. Myogenic control reflects the ability of the vascular smooth presents in hypovolaemic shock with cool, pale extremities; decreased
muscle to constrict in response to increased wall stretch. The myogenic peripheral perfusion (>4 seconds); and decreased urine output. In
mechanism allows local autoregulation of blood flow that is somewhat general, the blood pressure decrease seen in adult patients who have lost
independent of upstream pressures. The physiologic importance of 15–25% of their intravascular volume does not occur in the paediatric
the myogenic response is debatable, but it may provide a means for population, and blood pressure alone is an insensitive indicator of
preventing local hyperperfusion and tissue oedema during periods of dehydration in children due to their ability to increase their heart rate
elevated systemic blood pressure. Likely, the more important control (Table 4.2). The SNS discharge attempts to compensate for the loss in
mechanism is metabolic control, which enables the local vasculature to intravascular volume, but when the acidosis persists and overcomes the
respond to changes in local tissue demand. vasoconstriction, capillary leak may occur as well.
Two theories have been proposed to explain how increases in The paediatric patient may develop tachypnea in an effort to decrease
tissue metabolic demand can affect blood flow. The first theory the acidosis that is produced due to the low tissue perfusion occurring
is that a vasodilator substance (e.g., adenosine, carbon dioxide, during the shock phase. Lethargy and decreased responsiveness to
histamine, or similar) is produced by tissues in response to local pain occur secondary to decreased cerebral perfusion and low oxygen
decreases in the availability of oxygen or another metabolite. Of the delivery. These findings associated with a drop in heart rate and blood
proposed substances, adenosine is a likely candidate. Once released, pressure are ominous signs, and immediate action needs to be quickly
the vasodilator agent is believed to diffuse locally and induce dilatation pursued. The aetiology of the shock needs to be determined. The most
of upstream arterioles. The resultant increase in local blood flow would common causes of hypovolaemic paediatric shock include trauma,
increase the local supply of oxygen and other metabolites to the tissues, burns, peritonitis, severe vomiting, and diarrhoea, and in some cases,
thus creating a negative feedback mechanism. The other theory is that hyperthermia with decreased intake, which is common with malaria.
local decreases in oxygen tension are directly responsible for causing
local vasodilation. This response is based upon the requirement of
vascular smooth muscle for oxygen to maintain active contraction. Table 4.2: Clinical effects of dehydration based upon percentage of body weight
Thus, in response to local decreases in oxygen tension, the vascular decrease.
smooth muscle of the local upstream arterioles would relax, resulting Dehydration
in an increase in blood flow and oxygen delivery to the tissues in need. (% body weight) Clinical observation
Of course, local vasodilation of downstream blood vessels is of no • Increase in heart rate (10–15% above baseline)
use if perfusion is limited due to vasoconstriction of more proximal
arteries. Local activation of vasodilator responses cannot affect the tone • Dry mucous membranes
of proximal arterioles. However, as downstream vessels dilate, blood 5% • Concentration of the urine
flow velocity in the upstream vessels is increased. The endothelial • Poor tear formation
cells lining arterioles have the ability to sense increases in flow
velocity as shear stress. As shear stress increases, endothelial cells • Decrease in skin turgor
release vasodilator substances locally, thereby resulting in relaxation
of the adjacent vascular smooth muscle. The most important of these • Oliguria
vasodilator agents is the endothelial-derived relaxing factor, nitric 10% • Soft, sunken eyes
oxide. Thus, in response to increases in tissue metabolic need, both • Sunken anterior fontanelle
local and upstream vessels dilate, resulting in increasing blood flow to
meet metabolic demands.
• Decrease in blood pressure, tachycardia, tachypnoea
Shock and Clinical Implications in the • Poor tissue perfusion and acidosis
Paediatric Surgical Patient 15% • Delayed capillary refill
Shock is defined as a severe pathophysiological alteration in the normal
homoeostatic processes of oxygen delivery and cellular metabolism Source: Modified from Zuckerberg AL, Wetzel, RC. Shock, fluid resuscitation, and coagulation
that, if untreated and prolonged, can lead to major changes in these disorders. In Nichols DG, Yaster M, Lappe DG, Buck JR (eds). Golden Hour: The Handbook of
processes and cellular death. The traditional classifications of shock in Advanced Pediatric Life Support. Mosby-Year Book, 1991.
the paediatric population include: hypovolaemic, septic, cardiogenic,