Cardiac Output by Nancy Braudis.
My name is Nancy Braudis and I am a clinical nurse specialist in the Cardiac ICU at Children’s
Hospital Boston. The topic today is cardiac output. These slides represent our practice
here at Children’s Hospital Boston, and you may have to modify for your own individual
institutions.
Blood Flow During the Cardiac Cycle.
Blood flow during the cardiac cycle. The pumping action of the heart consists of the contraction
and relaxation of the heart muscle. Each contraction and relaxation is one cardiac heart cycle.
During the contraction of the heart, called systole, the blood is pumped out of the ventricles
and into the circulation. At this time, the mitral and tricuspid valves are closed and
the aortic and pulmonic valves are open. The closing of the mitral and tricuspid valves
creates the first heart sound, also known as S1. During relaxation of the heart, called
diastole, blood fills the ventricles. At this time the mitral and tricuspid valves are open
and the aortic and pulmonic valves are closed. The closing of the aortic and pulmonic valves
creates the second heart sound, also known as S2.
Cardiac Metabolis.
Cardiac metabolism. The oxygen supply of the heart is delivered by the coronary arteries.
70 to 75% of the oxygen from the coronary arteries is used immediately by the heart
muscles, leaving little oxygen reserves. Increased energy needs of the heart can only be met
by increasing coronary blood flow. Oxygen consumption by the heart increases during
exercise and fever. In the immediate postoperative period, fever can have a negative impact on
the function of the heart.
Cardiac Output.
Cardiac output is the volume of blood ejected from the heart
in one minute, expressed as liters per minute. Normal cardiac output in children is 200 mls/kilogram.
Cardiac index most often used in children is obtained by dividing the cardiac output
by the body’s surface area. A normal cardiac index is 3.5 to 4.5 liters per minute per
meter squared. Children have a much higher cardiac output than adults.
Cardiac output. Cardiac output is the heart rate multiplied by stroke volume. Heart rate
affects cardiac output by rate and rhythm. An abnormal rhythm within the heart can result
in a 20 to 30% decrease in cardiac output. Stroke volume is the amount of blood ejected
during the contraction of the heart. Stroke volume is affected by preload, contractility,
and afterload. Preload is the pressure generated in the left ventricle prior to contraction.
Contractility is the strength of the contraction of the ventricles. Afterload is a resistance
to the ejection of blood from the left ventricle. Efforts to maximize all of these factors will
improve cardiac output in the postoperative patient.
Cardiac Saturations.
In the normal heart, greater than 95% oxygenated blood is circulated from the left side of
the heart out to the body. A little more than 20% of the oxygen is extracted by the body
and returned to the right side of the heart. Obtaining the saturation from a catheter in
the heart gives valuable information related to cardiac output and intercardiac shunting.
The best location to obtain a mixed venous saturation is from a pulmonary artery catheter
or an internal jugular catheter. A low saturation from the internal jugular catheter or the
pulmonary artery catheter may be an indication of low cardiac output.
A high saturation from the pulmonary artery catheter maybe an indication of intercardiac shunting.
Meaning, that oxygenated blood from the left side of the heart is mixing with blood from the right side of the heart.
Point of Clarification.
The explanation for that is actually pretty simple.
And I've tried to diagram this here in a very, very crude way by indicating on the left side of this diagram in red lines
the arterial side of the circulation, on the right hand of this cartoon with the blue lines
the venous circulation and the arrow pointing downward coming off this represents gas exchange
in the capillaries. Basically, the principle is simple. If the capillaries suck out a certain
fixed amount of oxygen to supply the needs of the tissue, the amount of oxygen that is
left over in the systemic venous side is going to be determined by how much O2 went into
the capillary bed. What determines how much O2 goes into the capillary bed is very simple,
its two factors – one is oxygen content to the lungs, which is related to both arterial
oxygen saturation and hemoglobin, and the other is of course the amount of blood flow
into the capillary bed, which is, roughly speaking cardiac output or systemic blood flow. So
the greater the amount of systemic blood flow, or the higher the amount of O2 content, all
other things being equal in terms of oxygen consumption, the higher the systemic venous
content, also referred to as mixed venous saturation.
And so as you can see as cardiac output falls, again given a certain amount of O2 consumption
mixed venous saturation will fall. Similarly if arterial saturations or hemoglobin is less
there will also be less oxygen left over for the systemic venous circuit. Just as importantly
if O2 consumption goes up, and one could imagine this being a clinically relevant factor in
a patient who is febrile or very active, all other things being equal mixed venous content
will tend to go down.
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