Shock
is often defined clinically by a low blood pressure. However,
it is not the measure of the blood pressure that is the
problem; it is what that low measurement does to tissue
perfusion. Therefore, shock is decreased tissue perfusion.
More precisely, shock is decreased tissue
oxygenation. The goal of treatment should
be to balance oxygen delivery with oxygen consumption
to maintain tissue oxygenation.
Principles
of tissue oxygenation
Oxygen supply
-
Delivered oxygen (FiO2)
-
Hemoglobin
-
Cardiac output
Oxygen demand
-
Activity
-
Body temperature
It may be difficult to remember all of the
components of oxygen delivery and consumption when a patient
is in distress. The concept of the Ventilation-Perfusion
train might be helpful. The Ventilation-Perfusion train
uses the analogy of a train carrying passengers from one
station to another. The station on the left side (station
“A”) is where the passengers board. If they
cannot get there they cannot board. The same concept is
true of oxygen in the body. If oxygen can’t get
in the lungs, then it can’t be transported to the
tissues.
Next, people go from station “A”
to the car on the train. If this train pulls up with just
one small car, then very few people can be transported.
The concept holds true for hemoglobin in the blood. If
the hemoglobin level is low, then very little oxygen will
be transported. Next, there must be a locomotive to pull
the cars. This is like the cardiac output in the body.
Lastly, if the train brought 100 people to station “B”
and the need was for 200, it still didn’t meet the
need. In the body, delivered oxygen must meet oxygen consumption
or tissue ischemia results.
In order to manage shock, all of these variables
will need to be optimized. The lungs can be optimized
by providing good pulmonary hygiene before the patient
decompensates. During the acute crisis, appropriate oxygen
therapy should be initiated to “start” the
ventilation-perfusion train.
Hemoglobin levels need to be monitored and
maintained. Pay attention to how much of the patient’s
blood is lost to venopuncture. Critically-ill patients
can lose as much as 850cc of blood to venopuncture during
their ICU stay.
Decreased cardiac output can adversely affect
oxygen delivery. Many patients have previous cardiac disease
that can limit cardiovascular response to shock. Hemodynamics
may have to be optimized to maintain an acceptable cardiac
output.
Oxygen consumption must be decreased as
much as possible in order to achieve a balance. Two factors
that nurses can manage at the bedside are: 1) the patient’s
activity level, and 2) fever. Activity can be decreased
with reassurance, pain control, or sedation. Antipyretics
and cooling blankets can be used to restore normal body
temperature and decrease oxygen consumption.
Treatments that increased blood pressure
(vasopressors), and oxygen saturation (high-flow oxygen)
can cause adverse effects that actually contribute to
further decompensation. By manipulating all of the variables
of oxygen delivery and balancing them with oxygen consumption
the patient will experience fewer adverse effects of treatment
and maintain tissue oxygenation.