Cardiac Physiology Flashcards
Define Cardiac output and its determinants
Volume of blood ejected by the heart per unit of time. Usually expressed as HR x SV with units L/min
What is Fick’s Principle?
VO2 = (CO x Pa-oxygen content of art) - (CO x Pv-oxygen content of vein)
CO = VO2/Pa-Pv
can be used to calculated CO as VO2 = 3.5mLO2/kg/min
SV is a function of what factors?
Preload, Afterload and contractility
Define Preload, how is it approximated?
Myocardial sarcomere length just prior to contraction
Approximated by EDV (or worse by EDP)
because we are interested in the length using a surrogate with dimensions (volume) is better than the force (pressure) which creates that length
How can preload clinically be estimated?
EDV measured by echo
EDP (worse estimate) by CVC for RVEDP and PAC for LVEDP
What are the two main determinants of preload?
When preload is defined in terms of volume (EDV):
- Pressure filling the ventricle
- Compliance of the vetricle
What factors effect the pressure filling the ventricle?
CO (forward pressure)
Mean Systemic Filling Pressure (MSFP) -which has its own determinants: total venous blood volume and venous vascular compliance
Intrathoracic pressure (effect on venous return)
Atrial pressure/Atrial kick (Atrial contractility and rhythm, valve competence, ventricular ESV, ventricular compliance)
right atrial pressure (less accurately CVP)
What factors effect the compliance of the ventricle?
a. Pericardial compliance (walls and contents)
b. Ventricular wall compliance (duration of diastole, wall thickness, lusitropic properties, ESV (Afterload)
describe the effect of IPPV on the RV and pulmonary circulation
IPPV increases intrathoracic pressure (ITP)
Raised ITP is transmitted to central veins and the RA and decreases RV preload
Raised ITP is transmitted to the pulmnary arteries and increases pulmonary vascular resistance which leads to increased RV afterload
net effect is deccreased preload and increased afterload decreasing RV SV
describe the effect of IPPV on the LV and systemic circulation
IPPV increases intrathoracic pressure (ITP)
Raised ITP decreased preload by decreasing pulmonary venous pressure
Raised ITP decreases afterload by decreasing LV transmural pressure
What net effect does IPPV have on CO and myocardical O2 demand
Decreases both
How does IPPV decrease afterload
Afterload = wall stress
Laplaces law: wall stress = (LV transmural pressure x r)/LV wall thickness
in spontaneous breathing (NPV) LV transmural pressure is LVP - ITP so 90mmHg - -10mmHg = 100mmHg
in IPPV with PEEP of 10 LV transmural pressure is 90mmHg - 10mmHg = 80mmHg
using laplaces law there is decreased wall stress
What is normal coronary blood flow? How does this change with exercise? How does myocardial oxygen extraction change with increased myocardial work?
- Normal is ~250ml.min-1 (~5% of resting CO)
- May increase 4x during strenuous exercise
- Myocardial work may increase up to 9x, though as myocardial oxygen extraction is unchanged efficiency is actually improved during exercise.
What are the determinants of coronary blood flow?
CBF = (PAorta−PCavity(or RAP)) / CVR
CBF= coronary blood flow
CVR= Coronary vascular resistance
Coronary perfusion pressure: The difference between aortic root pressure and the greater of RAP or intracavity pressure
-Note that the pressure gradient is usually Aorta-Cavity rather than Aorta-RA
This is because the pressure in the ventricle acts as a Starling resistor - coronary flow is independent of RAP whilst RAP
What are the two broad categories of control of coronary blood flow?
- Autoregulation
- Autonomic control
Describe how CBF is autoregulated
- Myogenic autoregulation
Increasing transmural pressure increases the leakiness of smooth muscle membranes, depolarising them
Resistance increases proportionally to pressure, such that flow remains constant - Metabolic autoregulation
Anaerobic metabolism results in production of vasoactive mediates such as lactate and adenosine, which stimulate vasodilation and therefore increase flow (and oxygen delivery).
This is the predominant means for autoregulation in the heart
Typical myocardial oxygen extraction is 70% and raising this further is difficult
Therefore, increasing oxygen supply requires an increase in blood flow.
Describe the autonomic regulation of coronary blood flow
Direct effects include:
Parasympathetic and sympathetic innervation of coronary vessels, with release of ACh or NA and A decreasing or increasing coronary blood flow
Indirect effects
Are more important than direct effects
Are related to autoregulation occurring with changing levels of myocardial work in response to parasympathetic or sympathetic stimuli
What are the two phases and six stages of the cardiac cycle?
- Diastole
-Isovolumetric Ventricular Relaxation
-Rapid Ventricular Filling
-Slow Ventricular Filling
(The cycle begins here).
-Atrial Contraction - Systole
- Isovolumetric Ventricular Contraction
- Ejection
draw a wiggers diagram
https://partone.litfl.com/cardiac_cycle.html
describe the slow ventricular filling stage of the cardiac cycle
- MV+TV open, AV+PS closed
- The ventricle is relaxed completely and fills slowly
- The ventricles have been mostly filled during rapid ventricular filling and so the pressure gradient is reducing.
- The pressure in each ventricle is almost zero
- Arterial pressure is falling, as it is end-diastole
- CVP is slowly rising as the ventricle and atria fill
- This period occurs after the y descent.
- The ECG will show the beginnings of a P-wave at the end of this phase
How much ventricular filling is contributed by atrial contraction?
10% of the ventricular filling at rest, but up to 40% in tachycardia
describe the atrial contraction stage of the cardiac cycle
- Arterial pressure is still falling
- The CVP waveform demonstrates the a wave as atrial -contraction also causes blood to reflux into the SVC
- The ECG will show the PR interval
describe the isovolumetric ventricular contraction stage of the cardiac cycle
- Ventricular pressure rises, and the AV valves close. This gives rise to the first heart sound, S1.
- As ventricular pressure is still less than systemic vascular pressure, the semilunar valves remain closed
- Arterial pressure is still falling
- The CVP waveform shows the C (closure) wave, as the tricuspid valve herniates back into the RA during ventricular contraction. There is a similar spike in LA pressure as the mitral valve also bulges back into the LA.
- The ECG will show the remainder of the QRS or the start of the QT interval
- Atrial repolarisation occurs at this stage, but is typically masked by ventricular depolarisation
describe the ejection stage of the cardiac cycle
When ventricular pressure exceeds arterial pressure, the semilunar valves open and ejection occurs. Initial ejection is rapid, but as ventricular pressure falls and systemic pressure rises the gradient falls ejection becomes slower.
During ejection:
- Arterial pressure rises rapidly, and is slightly less than ventricular pressure during this stage
- The CVP waveform shows the x descent, as the shortening RV pulls the RA down, rapidly lowering CVP
- The ST segment shows on the ECG as the ventricles are fully depolarised, though the T wave may appear in late ejection