FOR CARDIAC SURGERY


Cardiac Cycle: Definitions & Equations


•  Systole = isovolumic ventricular contraction & ejection


•  Diastole = isovolumic ventricular relaxation & filling


•  Cardiac output = stroke volume × heart rate


→ volume of blood pumped by each ventricle per minute


Figure 16-1. Frank–Starling relationship.



•  Stroke volume = amount of blood pumped out of each ventricle with each contraction


•  Cardiac reserve = difference between cardiac output at rest & the max volume of blood the heart is capable of pumping per minute


•  Preload = volume of blood in ventricle before systole, used to estimate left ventricular end diastolic volume (LVEDP)


•  Starling’s law = contractility depends on muscle fiber length


•  Afterload = resistance to ejection of blood by each ventricle


•  Coronary perfusion pressure (CPP) = aortic diastolic BP – LVEDP


•  Left ventricular wall tension → Law of Laplace: T = p × r/(2 × t) where T = wall tension, p = pressure, r = radius, t = wall thickness


•  Fick equation:

Cardiac output (C.O.) = O2 consumption/([arterial O2 content] – [venous O2 content])



COMMON DISEASE STATES AFFECTING THE HEART



Determinants of Myocardial Perfusion


Supply: CPP, HR, PaO2, coronary artery diameter


Demand: Myocardial O2 consumption, HR, LV wall tension, contractility, conduction, relaxation



Hypertension (HTN)


•  Definition: >140/90 or 130/80 in high-risk pts


•  Essential HTN (1° HTN)—no definable cause (95% of pts)


•  2° HTN: Iatrogenic (meds), renal, aortic coarctation, pheochromocytoma, adrenocortical hormone excess, thyroid hormone abnormal, estrogen therapy, Cushing’s disease


•  Consequences of HTN


• Organ damage: Ventricular hypertrophy, systolic dysfunction, CAD, stroke, abd aortic aneurysm, aortic dissection


• Hypertensive crises: HTN encephalopathy—headache, blurred vision, confusion, somnolence, coma


•  Treatment: Diuretics, sympatholytic agents (β-blockers/α-2 agonists/α-1 antagonists), vasodilators, (Ca-channel blockers, ACE inhibitors, ARBs), nitrates


•  Anesthetic considerations


• Monitoring: BP cuff vs. arterial line as indicated


• Goal: Keep BP within 20% of baseline


Valvular Disease


Mitral Stenosis


Causes: Rheumatic fever, congenital stenosis


Pathophysiology


•  ↑ LA pressure → pulmonary edema, LV hypertrophy


•  Atrial fibrillation may result from LA dilation, LA thrombus from stasis of flow


•  Develop pulmonary HTN


•  Atrial kick provides 40% of LV filling


•  Stroke volume is fixed


Clinical feature


•  High-pitched “opening snap” followed by low-frequency diastolic rumble


Classification


•  Mild = valve area of ≤2 cm2; critical = valve area ≤1 cm2


Treatment


•  Medical therapy; balloon mitral valvuloplasty; open mitral commissurotomy; mitral valve replacement


Anesthetic management


•  Maintain sinus rhythm (atrial kick provides 40% vent filling)


•  Maintain preload & SV to avoid drop in SVR


•  Maintain normal HR (to allow time for filling)


•  Prevent ↑ in PVR (avoid hypoxia, hypercarbia, acidosis)


Mitral Regurgitation


Causes: Myxomatous disease (mitral valve prolapse [MVP]), ischemic heart dz, heart failure, annular dilation, endocarditis, rheumatic heart dz, hypertrophic cardiomyopathy (SAM), myocardial infarction (necrotic papillary muscle, ruptured chordae)


Pathophysiology


•  Severity determined by


• Systolic pressure gradient between LV and LA


• Systemic vascular resistance opposing forward LV blood flow


• Left atrial compliance


• Duration of regurgitation with each systole


•  Regurgitant fraction = volume of MR/total LV stroke volume (>0.6 = severe)


•  Acute MR: ↑ pulmonary pressure & pulmonary congestion


•  Chronic MR: ↑ LA size & compliance


Clinical features


•  Apical holosystolic murmur radiating to axilla


Treatment


•  Medical therapy; mitral valve repair/replacement


Anesthetic management


•  Maintain HR normal or high


•  Avoid myocardial depression


•  Avoid ↑ SVR (can worsen regurgitation)


•  Initiate prophylaxis against endocarditis


•  PA catheter v waves increase as regurgitant fraction increases


Aortic Stenosis


Causes: Bicuspid AV, senile degenerative disease, rheumatic fever


Risk factors: Male gender, hypercholesterolemia, smoking


Pathophysiology


•  Blood flow across valve is obstructed during systole


•  Concentric LV hypertrophy


•  Dependence on atrial kick to fill stiff ventricle


•  Stroke volume is fixed


•  Compression of subendocardial vessels → ischemia


Symptoms & severity


•  Angina—median survival 5 yrs


•  Syncope—median survival 3 yrs


•  Congestive heart failure—median survival 2 yrs


Clinical features


•  Harsh, holosystolic, crescendo–decrescendo murmur


Classification


•  Mild = valve area <2.5 cm2, moderate = 0.7–1.2 cm2, critical <0.7 cm2


Treatment


•  Percutaneous balloon valvuloplasty, percutaneuous, transapical, or open aortic valve replacement


Anesthetic management


•  Maintain sinus rhythm (atrial kick provides 40% of preload)


•  Maintain HR slow to normal (allow time for ventricular filling)


•  Avoid ↓ SVR (will ↓ CO because of fixed SV)


→ because of this, severe AS is a relative contraindication to spinal anesthesia


•  Initiate prophylaxis against endocarditis


•  Avoid myocardial depression as stroke volume is fixed


•  Consider arterial line placement for severe AS


•  Consider percutaneous pacing capability in case of cardiac arrest (chest compressions usually ineffective)


Aortic Regurgitation (AR)


Causes: Leaflet abnormalities (rheumatic heart dz, endocarditis, bicuspid valve), dilation of aortic root (aortic aneurysm/dissection, Marfan syndrome, syphilis, cystic medial necrosis)


Pathophysiology


•  Acute = surgical emergency—sudden ↑ LV diastolic pressure rise backs up to pulm circulation causing pulm congestion, acute pulm HTN, & edema


•  Chronic—LV compensates with dilation & hypertrophy → heart failure


Clinical features


•  Bounding pulses


•  Austin Flint murmur—turbulent flow across mitral valve during diastole due to AR jet


Treatment


•  Asymptomatic—nifedipine, ACE inhibitor, diuretics


•  Symptomatic—aortic valve replacement


Anesthetic management


•  Maintain sinus rhythm


•  Maintain normal to high normal heart rate


•  Avoid ↑ SVR (will worsen regurgitant fraction)


•  Avoid myocardial depression


•  Initiate prophylaxis against endocarditis


•  Consider vasodilators (nitroprusside) to ↓ afterload


Pulmonic Stenosis


Causes: Congenital deformity, carcinoid heart disease


Classification


•  Mild: Pressure gradient <40 mm Hg, moderate 40–80 mm Hg, severe >80 mm Hg


Treatment


•  Balloon valvuloplasty; valve replacement


Pulmonic Regurgitation


Causes: Annular dilation 2° enlarged pulm artery in pulm HTN, congenital/carcinoid heart dz


Tricuspid Stenosis


Causes: Congenital, rheumatic heart dz, right atrial tumor, endocarditis


Tricuspid Regurgitation


Causes: Congenital, endocarditis, carcinoid heart dz, secondary event from mitral valve or left-sided heart dz


Hypertrophic Cardiomyopathy (HCM)


Causes: Genetic, mixed, acquired


Pathophysiology


•  LV outflow obstruction (asymmetrical hypertrophic septum interferes with LV ejection)


•  LVH & RA enlargement, ↑ myocardial O2 consumption


→ subendocardial ischemia


Clinical features


•  Mitral regurgitation from SAM (systolic anterior motion of anterior mitral leaflet)


•  ↑ risk of sudden death


Anesthetic management


•  Maintain slow HR (to allow for ventricular filling)


•  Maintain sinus rhythm


•  Maintain low to normal contractility (can cause/exacerbate SAM)


•  Maintain preload & afterload


•  Treatments include β-blockers, verapamil, pacing, ICD, surgical myectomy



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Aug 28, 2016 | Posted by in ANESTHESIA | Comments Off on FOR CARDIAC SURGERY

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