Heart Structure & Blood Flow
In a normal dog, blood leaving the left ventricle immediately enters which large vessel?
The aorta
What mechanical event of the heart corresponds to the first heart sound (S1)?
Closure of the AV valves (mitral and tricuspid) at the start of systole
What is the normal pacemaker of the heart?
The sinoatrial (SA) node
A dog under anesthesia becomes significantly bradycardic due to high vagal tone. Which commonly used drug class (and an example) is given to block vagal effects and raise heart rate?
Anticholinergics, e.g., atropine (or glycopyrrolate)
What is the main function of arteries in the circulation?
To carry blood away from the heart under high pressure to the tissues
What is the main job of the right side of the heart?
To pump deoxygenated blood to the lungs for oxygenation
Is a typical physiologic systolic ejection murmur in a young, athletic dog usually heard during systole or diastole?
During systole
On a standard ECG, which part corresponds to atrial depolarization?
The P wave
β-blockers (Class II antiarrhythmics) generally have what effect on:
Heart rate, and
AV node conduction?
They decrease heart rate and slow AV node conduction
What is the primary role of capillaries?
To allow exchange of gases, nutrients, and wastes between blood and tissues
Put these chambers in the correct order of blood flow starting with blood returning from the body:
Right ventricle, left atrium, right atrium, left ventricle.
Right atrium → right ventricle → left atrium → left ventricle
A dog with mitral regurgitation has blood leaking back into the left atrium when the ventricle contracts.
How does this typically affect left atrial size over time?
It usually causes left atrial enlargement (dilation) over time
On an ECG, the QRS complexes are normal in shape but occur very slowly, with long pauses between them.
What overall rhythm description fits
Bradycardia
A dog with ventricular tachycardia is treated with lidocaine.
What ion channel does lidocaine primarily block, and in which part of the heart is it most effective?
It blocks fast Na⁺ channels and is most effective in ventricular myocardium and Purkinje fibers
Which Starling force favors movement of fluid out of capillaries into the interstitium: capillary hydrostatic pressure or plasma oncotic pressure?
Capillary hydrostatic pressure
A dog has chronic severe tricuspid regurgitation. Over time you notice jugular distension and ascites.
Which heart chamber is primarily failing, and why does this cause jugular distension and fluid accumulation in the abdomen?
Right ventricle is failing; tricuspid regurgitation increases right-atrial and venous pressures, backing up into the systemic venous system. This elevates jugular venous pressure and capillary hydrostatic pressure in abdominal organs, leading to fluid leaking into the peritoneal cavity (ascites).
On auscultation you hear a loud, holosystolic murmur at the left apex that starts with S1 and ends around S2. The dog also has a left atrial enlargement on imaging.
Name the most likely valvular lesion and briefly explain the timing of the murmur.
Most likely degenerative mitral valve disease causing mitral regurgitation. The murmur is systolic because regurgitant flow from LV to LA occurs during ventricular contraction—from S1 (AV valves closing and systole starting) until just before S2 (semilunar valves closing).
On a dog’s ECG, the P waves are normal, and QRS complexes are normal, but the PR interval is consistently prolonged beyond normal.
What conduction abnormality does this represent, and where is the delay occurring?
First-degree AV block; the delay is in conduction through the AV node/AV junction.
Match each Vaughan-Williams class with its primary cardiac target and give one veterinary-relevant example drug:
Class I, Class II, Class III, Class IV.
Class I – Na⁺ channel blockers; example: lidocaine or quinidine
Class II – β-adrenergic blockers; example: atenolol
Class III – K⁺ channel blockers (prolong repolarization); example: amiodarone or sotalol
Class IV – Ca²⁺ channel blockers; example: diltiazem
Compare the typical pattern of edema you expect with right-sided heart failure versus left-sided heart failure in dogs, and briefly explain the main hemodynamic reason for each.
Right-sided failure: systemic venous congestion → increased systemic capillary hydrostatic pressure → peripheral edema and ascites.
Left-sided failure: pulmonary venous congestion → increased pulmonary capillary hydrostatic pressure → pulmonary edema.
A dog is diagnosed with severe pulmonic stenosis.
Explain how this lesion affects:
Right-ventricular pressure and wall thickness
Blood flow to the lungs
The risk for right-sided heart failure
Pulmonic stenosis creates a fixed outflow obstruction, so right-ventricular pressure increases and the RV develops concentric hypertrophy. Forward flow to the lungs may be reduced, especially during exercise. The chronic pressure overload predisposes the dog to right-sided heart failure with signs like jugular distension and ascites.
A medium-sized dog has a diastolic decrescendo murmur best heard over the left heart base. You also note bounding pulses and wide pulse pressure.
Which valve lesion best explains this, and why does it cause bounding pulses?
Aortic regurgitation. During diastole, blood leaks back from the aorta into the LV, lowering diastolic pressure while systolic pressure remains high due to large stroke volume. The combination of high systolic and low diastolic pressure creates wide pulse pressure and bounding pulses.
You are asked to distinguish a supraventricular tachycardia (SVT) from a ventricular tachycardia (VT) on ECG.
List two ECG features that suggest SVT and two that suggest VT.
SVT: usually narrow QRS complexes, and P waves may be present/related to QRS (often 1:1 or with consistent relationship).
VT: wide and bizarre QRS complexes, often no preceding P waves or AV dissociation (P waves independent of QRS).
A Doberman with dilated cardiomyopathy has ventricular tachycardia and poor systolic function. You want to control the arrhythmia without significantly worsening contractility.
Name one antiarrhythmic drug or class that is relatively safe in this situation and briefly explain why.
Sotalol (Class III with some β-blocking) or a pure Class III agent is often chosen because it prolongs repolarization and reduces ventricular arrhythmias with less negative inotropic effect than many other options; lidocaine (Class Ib) IV is also useful acutely because it targets ventricular myocardium without major depression of contractility at therapeutic doses.
A dog has both severe hypoalbuminemia and right-sided heart failure.
Explain how the combination of decreased plasma oncotic pressure and increased venous pressure dramatically increases the risk of edema/ascites compared to either problem alone.
Decreased plasma oncotic pressure reduces the inward pulling force that keeps fluid in the vessels, while increased venous pressure raises capillary hydrostatic pressure pushing fluid out. Together, there is weaker force pulling fluid in and a stronger force pushing it out, so much more fluid leaves capillaries than the lymphatics can return, leading to marked edema and ascites.