Cardiac Output
Cardiac Output (in liters/minute) is defined as the amount
of blood ejected from the ventricle (primarily the left
ventricle) in a minute. Cardiac output is the term that is
used when discussing the pumping effectiveness and
ventricular function of the heart – the cardiac performance.
Stroke Volume
Stroke volume (SV) is the amount of blood ejected from the
left ventricle each time the ventricle contracts. Stroke
volume is the difference between end-diastolic volume
(EDV), the amount of blood in the left ventricle at the end
of diastole, and end-systolic volume (ESV), blood volume in
the left ventricle at the end of systole. Normal stroke volume
is 60 to 100 ml/beat.
SV = EDV – ESV
Preload
Preload refers to the amount of myocardial fiber stretch at
the end of diastole. Preload also refers to the amount of
volume in the ventricle at this phase. It is very difficult to
actually measure fiber length or volume at the bedside. It
has been clinically acceptable to measure the pressure
required to fill the ventricles (LVFP) as a measure of left
ventricular end diastolic volume (LVEDV) or fiber length.
Afterload
Afterload refers to the resistance, impedance, or pressure
that the ventricle must overcome to eject its blood volume.
Afterload is determined by a number of factors, including:
volume and mass of blood ejected, the size and wall
thickness of the ventricle, and the impedance of the
vasculature. In the clinical setting, the most sensitive
measure of afterload is systemic vascular resistance (SVR)
for the left ventricle and pulmonary vascular resistance
(PVR) for the right ventricle. In reality, the resistance of the
vascular system is derived from the measurements of cardiac
output (CO) and mean arterial pressure (MAP). The
formulas for calculating afterload look at the gradient
difference between the beginning (inflow) of the circuit and
the end (outflow) of the circuit.
Review of Hemodynamic Monitoring
