Cardiovascular physiology Flashcards
Describe Einthovens triangle in Clinical practice, also what is Einthovens law?
Einthovens triangle is made up of three vectors between Right arm (RA), Left arm (LA) and Left leg (LL):
RA – Lead I -> LA
RA – Lead II -> LL
LA – Lead III -> LL
Einthovens law dictates that the total voltage of leads I-III is 0 according to: I - II + II = 0.
What is the normal axis of an ECG, and why? Which leads may be used to quicly determine the axis?
The normal ECG axis is 0 to 90 degrees, because the vectors are weighted towards potential difference arising from the muscular left ventricle.
Lead I and aVF may be used to quickly determine the axis. If both are positive, the axis is normal. If lead I is positive, there is a left axis deviation. If aVf is positive, there is a right axis deviation. If both are negative, the axis is intermediate (?).
Draw a graph depicting the pacemaker cell action potential (membrane potential in mV (y), time in ms (x)). Which ion-flows are directly involved?
What is the electrical threshold potential?
See separate graph.
Threshold potential is at -40 mV. Three distinct phases:
Phase 4: slow influx of cations (primarily Na+, then Ca++) results in a slow depolarization. This is increased by sympathetic stimulation, decreased by parasymp.
Phase 0: Threshold potential is reached (-30 to -40 mV) and slow L-type-channels open and gives a Ca++-influx, resulting in a depolarization.
Phase 3: Repolarization occurs as Ca++-channels Close and K+-channels open, with efflux of K+ from the cell.
Draw a graph depicting the cardiac conduction system action potential. Describe phases, channels and ion-flows.
Phase 0: Rapid depolarization occurs after threshold potential is reached, owing to fast Na+-influx.
Phase 1: Na+-channels begin to Close, and K+-channels open -> short repolarization, not going below 0 mV.
Phase 2: A plateu occurs owing to opening of slow L-type Ca++-channels, which offsett repolarization by K+-channels and slows depolarization. This is the absolute refractory period, no further depolarization is possible.
Phase 3: L-type Ca++-channels Close, and K++ efflux now causes repolarization. This is part of the relative refractory period.
Phase 4: The Na+/K+-pump restores the ionic gradients by pumping 3 Na+ out in Exchange for 2 K+. The overall effect is a slow loss of positive ionic charge intracellularly.
What is the major difference in the depolarization of pacemaker and conducting heart cells?
Depolarization of pacemaker cells is primarily by slow L-type Ca++-channels, giving a slower depolarization. In conducting cells however, the opening of fast Na+-channels gives an almost vertical depolarization curve.
Draw a curve depicting the cardiac cycle, with time (seconds, x) and pressure (mmHg, y). Illustrate and describe key points.
Plot: ECG, Heart sounds (s1 + s2), LV-pressure, CVP, Aortic pressure, IVC + IVR.
A. Start of IVC. Electrical depolarization causes contraction, LV-pressure rises above CVP. Mitral valve Closes (s1).
B. End of IVC. The LV-pressure rises above aortic pressure. Aortic valve opens, blood flows into the circulation.
C. Start of IVR. LV-pressure falls below aortic pressure, and the aortic valve Closes (s2).
D. End of IVR. The LV-pressure falls below CVP, and the mitral valve opens. Ventricular filling.
Describe electrocardiographic changes in:
- Hypokalaemia
- Hyperkalaemia
- Hypocalcaemia/Long QT-syndrome
- Torsade de pointes
- Digoxin effect
- Hypothermia
- AV-block 1, 2(1), 2(2), 3
- Complete heart block
- Se sidor 249-253 i physics, pharmacology
How do you calculate mean arterial pressure (MAP)?
MAP = SBP + (2xDBP) / 3
or
MAP = DBP + (PP/3)
Define coronary perfusion pressure and how to calculate it.
Coronary perfusion pressure is the maximum pressure of the blood perfusing the coronary arteries (mmHg). It can also be defined as the pressure difference between the aortic diastolic pressure and the LVEDP (left ventricular end-diastolic pressure).
CPP = ADP (aortic diastolic pressure) - LVEDP
Define coronary blood flow. In which heart phase does it peak? Draw two sets of graphs showing aortic pressure and coronary blood flow (y) vs time (x).
Coronary blood flow reflects the balance between pressure and resistance. It is at it’s peak during diastole, when CPP is peaking as well.
CBF = coronary perfusion pressure / coronary vascular resistance.
Define central venous pressure and draw a central venous pressure waveform.
CVP is the hydrostatic pressure generated by the blood in the great veins. It can be used as a surrogate of right atrial pressure (mmHg).
Waveform:
A: caused by atrial contraction, absent in atrial fibrillation.
C: This result from the bulging of the tricuspid valve into the right atrium during ventricular contraction.
X: Descending wave, caused by downward movement of the heart during ventricular systole and relaxation of the atrium.
V: Caused by atrial filling against a closed tricuspid valve. Giant v-Waves are caused by tricuspid incompetence, and mask the “X”-descent.
Y: Descending wave is caused by passive ventricular filling after the opening of the tricuspid valve.
What is the PCWP/PAOP (Pulmonary Capillary Wedge Pressure/Pulmonary Artery Occlusion Pressure)? How is it measured?
PCWP/PAOP is an indirect estimate of left atrial pressure. A catheter is passed through the right side of the heart, into the pulmonary vessels where it measures the changing pressures.
PCWP may be used as a surrogate of the left atrial pressure and therefore LVEDP.
Give a short definition of: A. Cardiac output B. Stroke volume C. Preload D. Afterload E. Contractility
A. CO = SV x HR
B. Ejected volume of blood from LV with every contraction. Dependent on afterload, preload + contractility.
C. The initial length of the cardiac muscle fibre before contraction begins. (LVEDP for LV, CVP for RV).
D. The tension which needs to be generated in cardiac muscle fibre before shortening will occur (SVR).
E. Intrinsic ability of cardiac muscle fibres to do work with a given preload and afterload. (Extrinsic factors: preload and afterload, Intrinsic factors: autonomic nervous activity, catecholamine levels).
What does the Frank-staling law describe? Draw a graph comparing CO (l/min, y) to LVEDP (mmHg, x), with a curve for inotropy, one for normal and one for failure.
The strenght of cardiac contraction is dependent upon the initial fibre length.
Normal: LVEDP may be used as a measure of preload or initial fibre length. CO increases as LVEDP increases until a maximum is reached. This is because there is an optimal degree of overlap of the muscle filaments and increasing the fibre length increases the effective overlap and therefore contraction.
Inotropy: Draw a curve below and to the right of the ‘normal’ curve, this demonstrates that for any given LVEDP, the resultant cardiac output is greater.
Failure: Draw this curve below and to the right of the ‘normal’ curve. Highlight the fall in cardiac output at high LVEDP by drawing a curve that falls back towards baseline at these values. This occurs when cardiac muscle fibers are overstretched.
