Electrolytes and ECG; Pressure Volume Loops; Reflex control of the heart

electrolytes and ECG

K+   Nernst equation shows that hypoK = hyperpolarization and hyperK = hypopolarization
Major effect, hower, is due to changes in ion currents

Mg++
hypoMg++ similar to hypoK+  for ECG changes

Ca++
hyperCa++  

• shortens QT
• ST segment abolished

Ischemia

• depolarization of resting membranes
• inactivates fast Na channels = slows phase 0 = slows conduction cell to cell
• local acidosis
• K leakage from cell
• arrythmias due to dispersion of refractoriness in different parts of heart
• ST segment elevation in infarct zones and depression in reciprocal leads

Arrhythmia mechanisms associated with ischemia 

• A-V Block, Bundle Branch & Fascicular Blocks 
•  Bradyarrhythmias 
• Enhanced Automaticity 
• PVCs 
• Triggered Activity 
• Polymorphic VT 
• Reentry 
• Monomorphic VT 
• Ventricular Fibrillation

Hypothermia

• Osborn or J wave: A hump-like elevation of the J-point at the onset of ST segment. 
• Caused by dispersion of endocardial to epicardial phase 1 repolarization. 
• Shivering (muscle) artifact. 
• QT prolongation.

Pressure - Volume loops

Positive inotropic agents (digoxin, dopamine, dobutamine, eicosanoids, epinephrine, inamrinone, isoprenaline, milrinone, norepinephrine, phosphodiesterase inhibitors, theophylline)


Negative inotropic agents (beta-blockers, calcium channel blockers, disopyramide, flecainide, procainamide, quinidine)

Reflex control of the heart

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Of these two sites for arterial baroreceptors, the carotid sinus is quantitatively the most important for regulating arterial pressure. The carotid sinus receptors respond to pressures ranging from 60-180 mmHg 

important to know that carotid baroreceptor activity is proportional to blood pressure.

switch from sympathetic stimulation to sympathetic inhibition due to GABA release by CVLM

summary figure  P = QR

Efferent Limb: Autonomic Responses

MABP = CO x TPR          

memory tool: alphabet rule  ABC 

PQR   or P = QR