Wiggers Diagram Explained: Cardiac Cycle Masterclass

Four parameters alter the PV loop predictably: (1) Increased preload shifts point 1 up/right along EDPVR, increasing stroke volume with modest LVEDP rise and increased pulse pressure; (2) Increased afterload raises points 2/3 along ESPVR, decreasing stroke volume (except in aortic stenosis/HOCM where gradients persist); (3) Increased contractility steepens ESPVR, shifting point 3 up/left and increasing both stroke volume and blood pressure; (4) Decreased compliance steepens EDPVR, reducing stroke volume and blood pressure. Importantly, these parameters cannot be changed independently in real physiology.

The pressure-volume loop changes predictably with alterations in cardiac function: (1) Increased contractility shifts the loop leftward, increasing stroke volume and ejection fraction while decreasing end systolic volume; (2) Increased afterload shifts the loop upward and rightward, decreasing stroke volume while increasing end systolic volume due to higher pumping resistance; (3) Increased preload stretches the loop rightward, increasing stroke volume by providing more blood volume for ejection. These graphical representations allow clinicians to infer cardiac function and identify underlying pathophysiology from hemodynamic data.

Pressure-volume loops are graphical representations plotting left ventricular pressure against volume throughout one cardiac cycle, showing how changes in preload, afterload, and contractility affect cardiac function; these loops provide critical clinical measurements including stroke volume (end-diastolic volume minus end-systolic volume), ejection fraction (stroke volume divided by end-diastolic volume), and blood pressures (systolic and diastolic), with the area inside the loop representing stroke work performed by the heart during each heartbeat.

In left ventricular pressure-volume loops, contractility changes pivot the end-systolic pressure-volume relationship (ESPVR) line around the zero-pressure point, with increased contractility producing taller and wider PV boxes (higher end-systolic pressure and stroke volume) while decreased contractility produces smaller boxes; arterial elastance changes pivot the arterial elastance line around end-diastolic volume, creating a trade-off where increased elastance raises end-systolic pressure but decreases stroke volume, and vice versa; preload changes roll the entire arterial elastance line horizontally, with increased preload shifting it rightward to increase both end-systolic pressure and stroke volume, while decreased preload shifts it leftward to decrease both parameters.

PV loops describe how preload, afterload, and inotropy affect cardiac output. The loop shape changes with each variable: preload (starting ventricular volume before contraction) shifts the loop upward along the end-systolic pressure-volume relationship; afterload (pressure against which heart pumps) causes the heart to operate at higher volumes throughout the cycle with increased left ventricular pressure; inotropy (strength of contraction/velocity of muscle shortening) changes the slope of the end-systolic pressure-volume line, increasing cardiac output with enhanced contractility. Cardiac output is represented by the area of the PV loop. Preload and afterload are interrelated and do not function in isolation.

Positive inotropes increase contractility: dobutamine, milrinone, epinephrine. Positive chronotropes increase heart rate: atropine, epinephrine. Vasoconstrictors increase BP: norepinephrine is most important. Negative inotropes decrease contractility: non-dihydropyridine calcium channel blockers (verapamil, diltiazem) - contraindicated in heart failure. Negative chronotropes decrease heart rate: beta-blockers, non-dihydropyridine calcium channel blockers. Understanding these mechanisms is crucial for appropriate drug selection in cardiac patients.

Positive inotropic drugs increase the strength of heart contraction in weakened hearts. Catecholamines (norepinephrine, epinephrine, dopamine) naturally increase contractility through sympathetic stimulation. Pimobendan is a positive inotrope that also acts as a vasodilator, decreasing both preload and afterload while improving cardiac contractility and calcium availability. Digoxin is a cardiac glycoside derived from foxglove plant that inhibits the sodium-potassium pump, allowing sodium to leak into cells which displaces calcium and strengthens contractions. It also slows heart rate and delays AV node conduction, treating atrial fibrillation. However, it has a narrow therapeutic index and can cause arrhythmias due to increased sodium leakage. Side effects include vomiting, anorexia, diarrhea, bradycardia, and skipped QRS complexes. Hypokalemia increases toxicity risk. Plasma levels must be monitored, especially with renal impairment. Digoxin interacts with quinidine, requiring dose reduction. Vasodilators increase blood vessel diameter and are classified as arterial, venous, or mixed. Arterial vasodilators reduce afterload by dilating arteries, easing ventricular ejection. Venous vasodilators reduce preload by dilating veins, decreasing blood returning to the heart. Mixed vasodilators affect both systems. Initial use often causes hypertension, so dosing starts very low and increases gradually. Amlodipine and hydralazine are calcium channel blockers that act as arterial vasodilators, used for hypertension and reducing hypertrophic cardiomyopathy effects. Nitroglycerin is a potent venous vasodilator used as emergency treatment for congestive heart failure and pulmonary edema, shifting blood volume from pulmonary to systemic circulation. It is metabolized very quickly and administered topically with glove protection. ACE inhibitors (enalapril, captopril, benazepril) block angiotensin-converting enzyme, preventing formation of angiotensin II (vasoconstrictor) and aldosterone (causes sodium retention). They relax arteriole and vein smooth muscles, improving outcomes in congestive heart failure and left-sided heart disease.

Cardiovascular drugs are classified by mechanism: Antiarrhythmics affect electrical activity (lidocaine, a sodium influx inhibitor controlling VPCs/PVCs). Positive inotropes increase heart muscle contraction force (cardiac glycosides like digoxin/digitoxin, pimobendan, sympathomimetics). Vasodilators decrease cardiac workload by dilating vessels. ACE inhibitors prevent vasoconstriction. Diuretics reduce fluid overload. Beta blockers block sympathetic tone. Calcium channel blockers are negative inotropes decreasing contraction force. Cardiac glycosides increase calcium availability to cardiac muscle cells, enhancing contraction while slowing heart rate; digoxin has less liver metabolism and is contraindicated with renal disease, while digitoxin has more liver metabolism and is avoided with liver disease. Both have very narrow therapeutic indices requiring careful dosing. Pimobendan sensitizes heart muscle to calcium, increasing contraction while causing vasodilation; contraindicated in HCM and valvular disease. Sympathomimetics include epinephrine (stimulates all sympathetic receptors, used for cardiac arrest), dopamine (dilates visceral vessels, improves organ perfusion), and dobutamine (specifically stimulates beta-1 receptors without significant vasodilation). Vasodilators include hydralazine (blocks calcium in smooth muscle, decreasing peripheral resistance), nitroglycerin (causes venous and arterial dilation, absorbed through skin and mucous membranes), and prazosin (alpha-receptor blocker without RAAS stimulation).

Cardiac drugs are classified based on their effects on heart function. Positive inotropes, chronotropes, and dromotropes strengthen, speed up, and facilitate cardiac stimulation respectively - these are cardiac stimulants. Negative inotropes, chronotropes, and dromotropes weaken, slow down, and depress cardiac function - these are cardiac depressants. Calcium channel blockers and beta blockers are both negative inotropic, chronotropic, and dromotropic agents, meaning they relax and calm the heart.

This section covers cardiovascular medications for managing heart disease. Positive inotropes increase heart contraction force: Dobutamine (Vetmedin) is a positive inotrope and vasodilator, first-line for heart disease with narrow margin of safety requiring blood level monitoring. Pimobendan (Vetmedin) increases contractility and dilates blood vessels. Beta blockers (atenolol, propranolol, metoprolol, sodol) slow heart rate and are used for tachyarrhythmias. ACE inhibitors (benazepril, enalapril, ramipril, lisinopril) treat congestive heart failure and protein-losing nephropathies, also lowering blood pressure. Calcium channel blockers (amlodipine) treat hypertension and certain arrhythmias. Vasodilators (hydralazine, nitroglycerin, nitrates, prazosin) decrease cardiac workload. Lidocaine treats ventricular premature complexes and runs of PVCs in emergencies. Diuretics manage fluid overload: Loop diuretics (furosemide/Lasix) are first-line for congestive heart failure; spironolactone is potassium-sparing and used with Lasix to prevent hypokalemia; mannitol is an osmotic diuretic used for cerebral edema and glaucoma.

Hemodynamic monitoring uses color-coded transducers (red=arterial, blue=CVP, yellow=PA) that require zeroing (ignoring atmospheric pressure) and leveling (measuring at right atrium level, 5-6 cm below sternum). Arterial line tracings show systolic peak, dicrotic notch (aortic valve closure), and diastolic pressure. The square wave test assesses accuracy: 1.5-2 oscillations indicate proper function; fewer indicates overdamping (falsely low BP), more indicates underdamping (falsely high BP). The four determinants of cardiac output are heart rate/rhythm, preload, afterload, and contractility, assessed in this order. CVP measures right heart preload using catheters in the superior vena cava or right atrium. The CVP waveform has A wave (atrial contraction, tallest), C wave (tricuspid closure), and V wave (atrial filling). Normal CVP is 2-6 mmHg. PAWP measures left heart preload indirectly using a Swan-Ganz catheter in the pulmonary artery. The catheter has a volume-limiting balloon (1.5cc) that wedges into pulmonary vasculature. Normal PAWP is 6-12 mmHg. PA catheters are typically inserted under fluoroscopy in cardiac catheterization laboratories. All hemodynamic measurements are taken at end expiration because lungs influence venous return.

Hemodynamic tracing interpretation requires understanding normal pressure values across cardiac chambers (right atrium: 2-5 mmHg mean, 15-30 mmHg systolic; pulmonary artery: 25 mmHg systolic, 9 mmHg diastolic; pulmonary capillary wedge pressure: 9 mmHg mean; left atrium: 8 mmHg mean, 130 mmHg systolic; left ventricle: 130 mmHg systolic, 70 mmHg diastolic), recognizing that diastolic pressures of pulmonary artery, wedge pressure, left atrium, and left ventricle should be equivalent (8-12 mmHg 'golden number'), and identifying chamber types by tracing morphology—ventricular pressures show upward-sloping diastole while arterial pressures show downward-sloping diastole, with aortic pressures around 120-140 mmHg systolic and pulmonary artery pressures around 30 mmHg systolic.

This comprehensive lecture covers cardiac catheterization pressure tracing interpretation. The system uses a catheter with saline connected to a transducer to measure pressure changes in cardiac chambers and great vessels. Zeroing at the heart's center is critical for accurate readings. Right atrial tracing shows A-wave (atrial contraction), C-wave (cusp closure), and V-wave (ventricular filling). In normal patients, A-wave is higher than V-wave. Elevated A-wave indicates resistance to right atrial emptying (TR, PS, PA, PH). Elevated V-wave indicates volume overload (TR, left-to-right shunts). Right ventricular tracing has rapid upstroke, plateau, rapid downstroke, and slow rise to end-diastolic pressure. Normal RV systolic pressure is <30 mmHg and diastolic <8 mmHg. Pulmonary artery tracing has slow systolic upstroke, diastolic notch, and slow fall. A small RV-PA gradient of 5 mmHg can be normal, but gradients >10 mmHg suggest structural obstruction. Pulmonary capillary wedge pressure (PCWP) reflects pulmonary venous pressure, which reflects left atrial pressure and left ventricular end-diastolic pressure (LVEDP) when there is no mitral stenosis. Left atrial pressure tracing has a mean pressure of 6-10 mmHg, 3-4 points higher than right atrial mean pressure. In the left atrium, the V-wave is more prominent than the A-wave due to increased pulmonary venous return. A prominent A-wave suggests mitral stenosis, elevated LVEDP, or left atrial outflow obstruction. Left ventricular tracing is more rectangular with a longer, flatter plateau. LV pressure should be equal to or 5 mmHg higher than aortic pressure. Aortic pressure tracing has components: anacrotic limb, anacrotic notch, dicrotic notch, and diastolic run-off. Pulse pressure is defined as systolic minus diastolic pressure. Wide pulse pressure (>40-60 mmHg) indicates conditions that increase stroke volume or decrease arterial compliance. Cardiac causes include aortic regurgitation, PDA, aortopulmonary window, ruptured sinus of Valsalva, coarctation, and large left-to-right shunts. Narrow pulse pressure causes include aortic stenosis, low cardiac output states, and pericardial tamponade.

This lecture by Dr. K. Sivakumar, a senior pediatric cardiologist with 25 years of experience, provides comprehensive training on interpreting pressure tracings in cardiac catheterization labs. Key concepts include: (1) Pressure waves are complex periodic fluctuations in force per unit area, requiring transducers with natural frequencies three times the 10th harmonic of the recorded frequency for accurate detection; (2) Properly damped tracings show characteristic waveforms for each chamber—left ventricular tracings should have rectangular plateaus during isovolumic phases, while right ventricular tracings are triangular; (3) Common artifacts include rapid tachycardia causing frequency response deterioration, under-damping showing tall peaks and deep troughs, and catheter impact artifacts; (4) Pressure recovery phenomenon explains why elderly patients with calcific aortic stenosis may show lower catheterization gradients than echocardiographic estimates due to smaller ascending aortas; (5) Atrial pressure waves include a-waves from atrial contraction, v-waves from venous filling, x-descents from tricuspid valve closure, and y-descents from valve opening, with respiratory variations showing inspiratory decline and expiratory rise in normal physiology.

Cardiac catheterization is a gold-standard diagnostic procedure that measures hemodynamic parameters including right ventricular pressure (15-25 mmHg systolic, 3-12 mmHg diastolic), pulmonary artery pressure (15-25 mmHg systolic, 8-15 mmHg diastolic), and pulmonary capillary wedge pressure (6-15 mmHg) to assess cardiac function, with cardiac output measured via Fick principle (4-6.5 L/min) or thermodilution, and valve stenosis severity quantified using Gorlin formula (aortic valve area = CO / (44.33 × HR × period × √mean gradient)).