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C16 Cardiology and Cardiac Surgery
Cardiac Disease
Toronto Notes 2019
Sinus Arrhythmia (SA)
• NormalPwaves,withvariationoftheP-P interval by >120 msec due to varying rate of SA node
Respiratory SA
• Seenmoreofteninyoungadults(<30yrold)
• Normal,resultsfromchangesinautonomic
tone during respiratory cycle
• Rateincreaseswithinspiration,slowswith
expiration
Non-Respiratory SA
• Seenmoreoftenintheelderly
• Can occur in the normal heart; if marked may
be due to sinus node dysfunction (e.g. in
heart disease, or after digitalis toxicity)
• Usuallydoesnotrequiretreatment
Diagnostic Catheterization
• complicationsfordiagnosticcatheterization<1%
• inadequatediagnosticproceduresoccurinfewerthan1%ofcases
• provocativepharmacologicalagentscanbeusedtounmaskpathology
■ fluid loading may unmask latent pericardial constriction
■ afterload reduction or inotropic stimulation may be used to increase the outflow tract gradient in
HCM
■ coronary vasoreactive agents (e.g. methylergonovine, acetylcholine)
■ a variety of pulmonary vasoreactive agents in primary pulmonary HTN (e.g. oxygen, calcium
channel blockers, adenosine, nitric oxide, or prostacyclin)
Contrast-Enhanced CT Coronary Angiography
• description:fastECG-synchronizedmulti-sliceCTimageacquisitioninthehearttoenablenon- invasive imaging of the coronary arterial tree
• indications:oftenusedtoassesscoronaryarteryandpreviousgraftstenosis/viabilitythatcouldnotbe seen during coronary angiography
• sensitivity=85%,specificity=90%forthediagnosisofobstructivecoronarydiseasewith>50%stenosis • contraindications:allergytocontrastdye;severerenaldysfunction
• risks:radiationexposure
Magnetic Resonance Imaging
• description:offershighspatialresolution,eliminatestheneedforiodinatedcontrast,anddoesnot involve exposure to ionizing radiation
• indications:valuableinassessmentofcongenitalcardiacanomalies,abnormalitiesoftheaorta, assessment of viable myocardium, and assessment of cardiomyopathies
• contraindications:metallicforeignbodies/implants
• risks:hazardsposedbycertainmetallicdevicesinsidepatients
CARDIAC DISEASE Arrhythmias
Mechanisms of Arrhythmias
Alterations in Impulse Formation
A. Abnormal Automaticity
■ automaticity is a property of certain cardiomyocytes to spontaneously depolarize to their threshold voltage to generate action potentials in a rhythmic fashion. Under normal circumstances only cells in the specialized conduction system (SA node, AV node, and ventricular conduction system) exhibit natural automaticity. These cells are pacemaking cells. The automaticity of these cells can become abnormally increased or decreased. In disease (e.g. post-MI ventricular ischemia) cells
in the myocardium outside the conduction system may inappropriately acquire the property of automaticity and contribute to abnormal depolarization. If these ectopic generators depolarize at a rate greater than the SA node, they assume pacemaking control and become the source of abnormal rhythm. Automaticity can be influenced by:
◆ neurohormonal tone (sympathetic and parasympathetic stimulation) ◆ abnormal metabolic conditions (hypoxia, acidosis, hypothermia)
◆ electrolyte abnormalities
◆ drugs (e.g. digitalis)
◆ local ischemia/infarction
◆ other cardiac pathology
■ thismechanismisresponsiblefortheacceleratedidioventricularrhythmandventriculartachycardia
that often occurs 24-72 h post MI
B. Triggered Activity due to Afterdepolarizations 1. Early Afterdepolarizations
■ occur in the context of action potential prolongation
■ consequence of the membrane potential becoming more positive during repolarization (e.g. not
returning to baseline)
■ result in self-maintaining depolarizing oscillations of action potential, generating a tachyarrhythmia
(e.g. new baseline voltage is greater than threshold, which automatically triggers a new action
potential after the refractory period ends)
■ basisforthedegenerationofQTprolongation,eithercongenitaloracquired,intoTorsadesdePointes