Physiology Week 2 Flashcards
How do APs travel?
Through gap junctions
What does an ECG detect?
Extracellular voltages
What are connexons? Connexins?
Part of intercalated disks. Connexon is the complete channel, connexin is the subunit protein. Can become disrupted in HF
what are the two main determinants of how fast an AP travels?
number of gap junctions between cells and the inward current responsible for the AP upstroke.
What is the waveform mathematically?
V2-V1=AP1-AP2
OR dV/dt
What’s the difference between AP shapes in the endocardium and epicardium?
Epicardial ventricular cells have more pronounced Phase 1 (doesn’t show up on ECG) and shorter APs (v. imp for ECG). This comes from different expression of K channels
What causes an upward deflection?
A depolarization wave moving toward an electrode
What is Einthoven’s Triangle?
Where you set up the bipolar ECG limb leads. LA, RA, LL. RA is neg neg, LA neg pos, LL pos pos. Start at RA.
What are precordial leads?
placed in transverse plane to provide info in anterior to posterior direction. Can be helpful in finding where an MI is.
What is the P wave?
Atrial depolarization
What is the QRS complex?
Ventricular depolarization
What the T wave?
Ventricular repolarization
Why is there no signal from conducting tissues (SA node)?
Not enough tissue mass. The amplitude of a deflection on the ECG depends both on tissue mass and the magnitude of the derivative.
What is the QT interval?
Ventricular depolarization to repolarization, so AP duration
What is the PR interval?
atrial to ventricular depolarization, so propagation through the AV node. Should be .12 to .20 seconds
Why is conduction through AV node slow?
AV nodal upstroke are carried by LTCCs, which are slow
How does parasympathetic negative dromotropy work?
Activation of muscarinic Ach receptors go to Gi prot and inhibit AC, which leads to decreased phosphorylation of LTCCs. This decreases upstroke velocity in AV nodal cells
How does sympathetic positive dromotropy work?
Activation of beta adrenergic receptor, coupled with Gs protein, activates AC and increases phosphorylation of LTCCs. This increases upstroke velocity in AV nodal cells
How long should the QRS be?
less than .12 seconds
What is the scale of ECG boxes?
1 small box is 0.04s, 1 large box is 0.2s, 5 large boxes is 1s.
How do you find the electrical axis of the heart?
From the leads that produces the largest QRS complex.
- look for maximum positive R with no negative, this is closest.
- look for closest QRS with integral of zero (up and down balance). Axis is perpendicular to this.
- If 2 R waves are equally tall, axis is between these.
What can cause a shift to negative values for electrical axis?
LV hypertrophy. Some consider -30 to 0 to be WNL
What can cause a wide QRS?
Ventricular propagation - not going through conduction system, slow. Ventricles exciting themselves. Can be premature ventricular contractions (PVCs)
What does a first degree AV block look like?
First degree means delayed - delay between P and R. PR interval should be 120-200ms.
What does a second degree AV block look like?
Some impulses fail to propagate to ventricles, so some P waves will not be followed by QRS.
What does atrial flutter/ fibrillation look like?
P waves are random and irregular, R-R intervals are irregular.
What if two PVCs have different shapes?
Premature ventricular contractions can have diff shapes if there are multiple ectopic foci so QRS look diff every time.
What does monomorphic ventricular tachycardia look like?
Wide QRS complexes with not enough time R-R. All complexes look similar. Very dangerous. Can be from ventricular focus (one site) or reentry.
Initiation of Reentry?
A stimulus delivered late will propagate on both sides and collide.
A premature stimulus will encounter refractory tissue and be blocked only on the right side. It will propagate slowly on the left. By the time it propagates around the bottom, the right side will have recovered
This is why cardiologists consider “dispersion of repolarization“ to increase arrhythmia risk
What does ventricular fibrillation look like
Irregular, no identifiable patten. Will cause death in mins.
Why is the T wave usually upright?
because although endocardium is activated first, it repolarizes after the epicardium
WHY is the ST segment flat?
There’s no difference between the endocardium and epicardium – both declining at same time in the plateau. Must have difference between endo and epi to show up on ECG.
When would you see ST segment elevation?
ECG BRUGADA SIGN. Rare. Key thing is a much more dramatic effect on AP in either endo or epi. Example is very shortened AP in epicardium.
Acute ischemia also.
When would you see a negative T wave?
Negative T wave would come from the endocardium repolarizing first
What leads should be positive?
Lead 1 and aVF. aVR usually negative.
What does left ventricular hypertrophy look like?
Left ventricular hypertrophy give you a negative electrical axis and positive aVL.
How is arterial BP determined?
primarily by resistance in the arterioles
What is Ohm’s law?
Q=(P1-P2)/R Where Q is flow and R is resistance
For BP:
MAP=COxTPR
Which has a causal relationship, don’t rearrange: changes in CO or TPR lead physiologically to changes in MAP, but an increase in MAP does not lead to an increase in CO.
MAP - mean arterial pressure
CO - cardiac output
TPR - total peripheral resistance
What is MAP?
At rest:
MAP = 2/3DP + 1/3SP
Because diastole usually lasts longer than systole