Anesthesia w/ Cardiac Disease

Question 1

What are the determinants of cardiac performance/output?

Answer 1

• Global Oxygen Delivery (DO₂) = Cardiac Output (CO) x Oxygen Content (C_aO₂)
• Arterial Blood Pressure = CO x Systemic Vascular Resistance (SVR)
• CO = HR x Stroke Volume (SV)
• SV = End diastolic Volume (EDV) - End Systolic Volume (ESV)
• Preload
• Afterload
• Inotropy
• Lusitropy

Question 2

What is preload

Answer 2

Hemodynamic load or stretch on the myocardial wall at the end of diastole just before contraction begins

Question 3

What is afterload

Answer 3

hydraulic load on the ventricle during ejection, used to describe the force opposing ventricular ejection

Question 4

What is Inotropy

Answer 4

Contractility or intrinsic ability of the heart to generate force

Question 5

What is Lusitropy

Answer 5

Ability of the myocardium to relax following excitation-contraction coupling

Question 6

What is Degenerative Mitral Valve Disease

Answer 6

• Myxomatous mitral valve degeneration (MMVD)
  
  • Prevalence of approximately 30% in small breed dogs > 10 years of age
• Blood flow out the left ventricle
  
  • Divided between high-pressure/low compliance outflow tract and low pressure/high compliance outflow route
• ⇡LA pressure ⇢ LA dilation
• High end-diastolic pressures & volumes
  
  • Ventricular dilation & eccentric hypertrophy
• Regurgitant fraction
  
  • 50-70% of SV
• Severity of symptoms influenced by time course of MR development
  
  • Slower development of MR ⇢ compensation & gradual dilation of left atrium
  • Acute ⇢ severe pulmonary congestion

Question 7

What are the Anesthetic considerations for MMVD?

Answer 7

• Primary goals:
  
  • Maintain forward aortic flow
  • minimize regurgitant flow
• Regurgitant fraction depends on:
  
  • Size of regurgitant orifice
  • Time available for retrograde flow
  • Pressure gradient across MV
• Maintain preload during anesthesia
  
  • Hemodynamically optimize the patient prior to anesthesia
• Avoid overzealous fluid administration
  
  • Consider decreasing fluid rate by 25-50% in patients with stage B₂ MMVD or greater
• Heart Rate:
  
  • Normal or slightly above normal rate
  • Bradycardia increases the regurgitant fraction
  • Consider an anticholinergic prior to induction or as part of premedication anesthetic drug plan
• Afterload reduction optimizes forward flow
  
  • Decreases pressure gradient from LV to LA
• Avoid alpha2 agonists
  
  • Impair forward flow
  • Increase regurgitant fraction
• Patients with significant MR may benefit from inotropic support

Question 8

What is Hypertrophic Cardiomyopathy?

Answer 8

• Common in the cat, rare in the dog
• Pathophysiology:
  
  • Idiopathic concentric hypertrophy of the left ventricle
  • Increased myocardial oxygen demand & risk of myocardial ischemia
  • Development of left ventricular outflow tract obstruction due to systolic anterior motion (SAM) of MV leaflets
• HCM + SAM = HOCM
  
  • Cause of left parasternal systolic murmur
  • ≥50% of all HCM cases
• Increased Ventricular wall stiffness leads to diastolic dysfunction
  
  • Stiff LV requires increased pressure to properly fill
    
    • Increased LA pressure & dilation
• Impaired diastolic filling during early relaxation/passive ventricular filling
• Predisposed to ventricular arrhythmias, systemic thromboembolism, congestive heart failure (CHF)

Question 9

What are the anesthetic concerns of Hypertrophic Cardiomyopathy?

Answer 9

• Primary Goals with HCM/HOCM:
  
  • Optimize perfusion & minimize increases in myocardial oxygen demand
  • Optimize diastolic filling
  • Maintain relatively low heart rates
  • Avoid drugs that increase cardiac contractility
• Optimize preload & ventricular filling through adequate fluid therapy
• Afterload:
  
  • increasing afterload may decrease SAM and outflow tract obstruction
  • May benefit from an alpha2 agonist
• Consider risks/benefits of ketamine
  
  • High-doses can cause catecholamine release
  • Increase HR, BP, CO & myocardial contractility
• Hypotension occurs with relative frequency in healthy cats anesthetized with inhalation anesthetized with inhalation anesthetics
  
  • Decreases in arterial blood pressure will contribute to inadequate coronary perfusion & myocardial ischemia
  • Consider multi-modal approach for MAC reduction of inhalant
  • Positive inotropic activity can increase dynamic outflow tract obstruction & myocardial oxygen consumption
  • consider vasopressors to increase SVR for blood pressure support

Question 10

What is Dilated Cardiomyopathy?

Answer 10

• Idiopathic primary loss of myocardial contractility
• Systolic dysfunction
• Decreased ability of the heart to generate a normal stroke volume ⇢ eccentric dilation & volume overload
• Reduced ejection fraction, fractional shortening & rate of ejection
• Increased in end-systolic volume
• Predisposes to left-sided or biventricular CHF & ventricular arrhythmias
• Con be further complicated by atrial fibrillation

Question 11

What are the anesthetic considerations with Dilated Cardiomyopathy?

Answer 11

• Concerns:
  
  • Decreased systolic function
  • Increased incidence of arrhythmias
  • CHF
• Primary Goals:
  
  • Maintain systolic function with inotropic support
  • Manage arrhythmias with antiarrhythmics
  • Avoid overzealous fluid administration
• ECG monitoring before induction of anesthesia
  
  • Arrhythmias are common
• DCM + AF
  
  • Consider electro-cardioversion at the beginning of anesthesia
  • Resynchronization will increase ventricular filling & improve cardiac output
• Titrate Fluid therapy to the lowest necessary dose for hydration & ongoing losses
  
  • 3-5ml/kg/hr
• Down regulation of beta-adrenergic receptors
  
  • Receptor affinity does not change
  • More resistant to treatment with a positive inotrope
  • Dobutamine preferred over dopamine to increase contractility, SV, CO, and BP

Question 12

What is Subaortic stenosis & Pulmonic Stenosis

Answer 12

• Common congenital valvular defects in the dog
• PS: malformation of the pulmonic valve/fusion of valve leaflets
• SAS: fibromuscular nodule/ridge/ring of tissue
• Chronic systolic pressure increase ⇢ compensatory concentric hypertrophy
• Increased myocardial work
• Increased demand for coronary blood flow & myocardial oxygen delivery
• Risk for ischemia greatest during periods of tachycardia
  
  • Myocardial necrosis, replacement fibrosis ⇢ ventricular arrhythmias
• Beta-blockers prescribed for chronic medical management of SAS or PS

Question 13

What are the Anesthetic considerations with SAS and PS?

Answer 13

• Patients have poor ventricular compliance
• Maintaining adequate pre-load is critical
  
  • maintain adequate venous return by insuring a full intravascular volume to fill ventricular chamber
  • Optimize diastolic filling & CO
• avoid volume overload
• Dependent on maintenance of normal sinus rhythm & atrial kick to optimize ventricular filling
• Avoid tachycardia
• Afterload is elevated but relatively fixed
  
  • Vasodilation & reductions in vascular tone do not relieve afterload but decrease preload & coronary perfusion pressure
• Managing hypotention
  
  • Optimize volume
  • Vasoconstrictor (phenylephrine)
• Contractility well maintained generally
  
  • Positive inotropes useful in patients with ventricular dysfunction with dopamine preferred to dobutamine
• Minimize increases in pulmonary vascular resistance
  
  • Maintain a low normal P_aCO₂ in PS patients

Question 14

What is Pulmonary Arterial Hypertension (PAH)?

Answer 14

• PA pressure >25-30 mmHg abnormal
• Mechanisms of disease:
  
  • Primary PAH
  • PAH secondary to left sided heart disease
  • Pulmonary hypoxia
  • Thromboembolic disease
• Common in small/toy breeds, middle age to older
• Diagnosed by echocardiography
  
  • severity of PAH graded by assessment of tricuspid valve regurgitation velocity
• Characteristic echocardiographic findings:
  
  • Concentric RV hypertrophy
  • Dilation of main pulmonary artery

Question 15

What are the anesthetic considerations for PAH?

Answer 15

• Preoxygenate by face mask before anesthetic induction
  
  • Increase F₁O₂, P_AO₂, and P_aO₂
  • Oxygen is a pulmonary vasodilator
• Primary goals with PAH ⇢ maintain CO by:
  
  • Optimizing preload & contractility
  • Minimizing decreases in SVR
  • Avoid worsening or increasing pulmonary vascular resistance
• Increased catecholamine release ⇢systemic & pulmonary vasoconstriction
  
  • Avoid stress/pain by using pre-medications that allow good sedation & analgesia
• PAH worsened by hypercapnia & hypoxia
  
  • Continuous mechanical ventilation recommended
• Continue sildenafil perioperatively

Question 16

What are the Inotropic & vasopressor drugs for small animals? alpha/beta effects?

Answer 16