Haemodynamics Flashcards
The gorlin formula utilises the Coeficient of orefice contraction in the formula. What is this, and why is it used?
The coefficient of orifice contraction makes
allowance for the fact that fluids moving through an orifice tend
to stream through its middle so that the physiological orifice is
smaller than the physical orifice. The velocity coefficient allows
for the fact that not all of the pressure gradient is converted to
flow because some of the velocity is lost to friction within the
valve.
What is the Gorlin equation?
A=F/(CcCv √2gh), where Cc and
Cv are the coefficients of orifice contraction and velocity loss,
respectively.
What are limitations of the Gorlin equation?
The coefficients for orifice contraction and velocity loss have never truly been established. Gorlins used an empirical constant to make their calculated mitral valve areas align better with actual valve areas obtained at autopsy or surgery.
For the other 3 valves, not even an empirical constant has been developed. Thus, the coefficients for the aortic, pulmonic, and tricuspid valves have been assumed to be 1, a theoretical impossibility.
list 7 ways dampening can occur;
- air bubbles
- blood clot in cather
- blood in the catheter
- contrast in the tubing
- luminal compromise (“kink”)
- Excessively long tubing
- catheter tip obstruction
What is the principle behind the Fick method of CO estimation
The Fick method relies on this principle that blood flow (cardiac output) is inversely proportional to the extent of oxygen extraction, that is, the difference in oxygen concentration between arterial and venous blood and the rate of oxygen uptake in the lungs.
What assumptions are made when using the Fick method?
1) As blood circulates, oxygen is extracted by the tissues at the capillary level.
2) The degree of oxygen extraction is inversely proportional to the rate of oxygen delivery
3) blood flow (cardiac output) is inversely proportional to the extent of oxygen extraction
4) pulmonary blood flow (PBF) is equal to systemic blood flow (SBF) in the absence of an intracardiac shunt
what is the “assumed fick” method
Fick method determines CO based on O2 consumption. Oxygen consumption is best measured from a metabolic hood or cart. Because of limited metabolic cart availability, the oxygen consumption is more commonly estimated as 3 mL O 2 /kg or 125 mL/min/m 2 (generating an “assumed Fick” measurement).
What degree of error can we see in the assumed Ficks method compared to the non assumed
These estimated O2 consumption values may differ as much as 40% compared with the measured oxygen consumption when using a metabolic hood
Why are the atrial a waves different between the left and right atrium ? What are the a waves compared to ?
The pressure waveforms are strongly influenced by the specific pressure-volume relationship (also known as compliance) of the chamber in which the pressure is measured. The a wave is usually smaller than the v wave in the right atrium while the reverse tends to be true for the left atrium because the right atrium can easily decompress through the SVC and inferior vena cava (IVC), whereas the left atrium is constrained posteriorly by the pulmonary veins. While valvular regurgitation is often cited as one cause for large v waves, a highly compliant atrium may accommodate a large amount of volume from acute valvular regurgitation and produce only small v waves.
what does mean PWCP match and why? What are exceptions to this?
With the normally low resistance of the pulmonary circulation, pulmonary artery diastolic pressure matches mean PCWP.
This is not the case under circumstances of elevated pulmonary vascular resistance (hypoxemia, pulmonary embolism, chronic pulmonary hypertension).
PCWP is often not sensitive enough to accuratelly determine left atrial pressures when we are looking to assess the function of the mitral valve and regurgitation. WHy?
1) the signal is more dampened due to the distance and significant vasculature between the the wedge and the atrium
2) “over wedging” of the balloon catheter causes an excessively damping signal leading to falsely low values, whereas
3) “under wedging” with a signal contaminated in part by higher PA pressure leads to falsely high pressure readings.
What is Cc and Cv int he Gorlin formula?
Explain these
- Cc is the coefficient of orifice contraction, which accounts for the fact that fluids tend to move through the center of an orifice, generating a physiologic orifice that is smaller than the anatomic orifice.
- C v is the velocity coefficient, which allows for the pressure gradient not being fully converted to flow because some of the velocity is lost to friction.
What is the Gorlin formula
A=F/(Cc×Cv√2gh)
What is the Torricelli formula?
V=√2𝑔ℎ
Describe the Torricelli formula;
The volume of flow (F) across an orifice equals the area of the orifice (A) times the velocity of flow (V): F = A × V, and accordingly, the orifice area can be calculated as A = F/V. F equals the cardiac output, and V can be calculated from the transvalvular gradient based on a special case of the Bernoulli principle called Torricelli’s law:
V=√2𝑔ℎ
