“Cardiac Output” by Nancy Braudis, RN, MS, CPNP for OPENPediatrics
By Bryan Wright
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. Please help us improve the content by providing us with some feedback.