Cardiovascular failure, or shock, can be caused by (1) depletion
of the vascular volume, (2) compression of the heart or great veins,
(3) intrinsic failure of the heart itself or failure arising from
excessive hindrance to ventricular ejection, (4) loss of autonomic
control of the vasculature, (5) severe untreated systemic inflammation,
and (6) severe but partially compensated systemic inflammation.
If the shock is decompensated, the blood pressure or the cardiac
output will be inadequate for peripheral perfusion; in compensated shock,
the perfusion will be adequate but only at the expense of excessive
demands on the heart. Depending on the type and severity of cardiovascular
failure and on response to treatment, shock can go on to compromise
other organ systems. This chapter discusses the cardiovascular and
pulmonary disorders associated with shock.
Hypovolemic shock (shock caused by inadequate circulating blood
volume) is most often caused by bleeding but may also be a consequence
of protracted vomiting or diarrhea, sequestration of fluid in the
gut lumen (eg, bowel obstruction), or loss of plasma into injured
or burned tissues. Regardless of the etiology, the compensatory
responses, mediated primarily by the adrenergic nervous system,
are the same: (1) constriction of the venules and small veins in
the skin, fat, skeletal muscle, and viscera with displacement of
blood from the peripheral capacitance vessels to the heart; (2)
constriction of arterioles in the skin, skeletal muscle, gut, pancreas,
spleen, and liver (but not the brain or heart); (3) improved cardiac performance
through an increase in heart rate and contractility; and (4) increased
sodium and water reabsorption through renin-angiotensin-aldosterone
as well as vasopressin release. The result is improved cardiac filling,
increased cardiac output (both directly by the increase in contractility
and indirectly through increased end-diastolic volumes), and increased
blood flow to organs with no or limited tolerance for ischemia (brain and heart).
The symptoms and signs of hypovolemic shock are many and can
be caused either by the inadequate blood volume or by the compensatory
responses. Some manifest themselves early, some late. One of the
earliest signs is that of postural hypotension—a fall in
the systolic blood pressure of more than 10 mm Hg that persists
for more than 1 minute when the patient sits up. It can be very useful
in patients who are suspected of being hypovolemic from either dehydration
or occult internal blood loss (eg, in a patient who might have gastrointestinal bleeding).
Other signs will have to be used, however, in very ill patients
and in injured patients, who might not tolerate changes in position.
Another early sign can make itself known when the physician has
difficulty in establishing intravenous access. In addition, the
skin might be cold and pale. The pallor, which can be detected in
all patients, including those with deeply pigmented skin, is best
detected by compressing a toe to produce blanching on ...