9.2 Antiarrhythmic, Antihypertensive, and Beta-Adrenergic Antagonist Drugs

9.2 Antiarrhythmic, Antihypertensive, and Beta-Adrenergic Antagonist Drugs

Cardiovascular pharmacotherapy during the perioperative period requires nuanced control over myocardial electrophysiology, cardiac rhythm, and vascular tone. Selecting the correct intervention requires an exact understanding of cardiac action potential phases, ion channel dynamics, receptor-effector coupling, and metabolic pathways governing drug onset and termination.

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1. The Vaughan-Williams Classification of Antiarrhythmic Drugs

The Vaughan-Williams classification organizes antiarrhythmic agents into four primary classes based on their predominant electrophysiological mechanism on the cardiac action potential.

                      [ CARDIAC ACTION POTENTIAL & DRUG CLASSES ]

       Millivolts (mV)
            +20 |         1 (Transient outward K+ current, I_to)
                |        /---\   2 (Plateau phase: inward I_Ca,L vs outward I_Kr/I_Ks)
              0 |       /     \===================\
                |   0  /                           \
            -20 |  (Phase 0:                        \  3 (Phase 3 repolarization:
                |   Inward I_Na)                     \  outward I_Kr, I_Ks)
            -40 |     |                               \
                |    /                                 \
            -60 |   /                                   \____________________ 4 (Resting potential)
            -80 |_ /                                      Phase 4: I_K1 (and I_f in nodal tissue)
                +-------------------------------------------------------------> Time

    Class I: Sodium Channel Blockers (Phase 0 dV/dt_max)
      - Ia (Quinidine, Procainamide): Intermediate dissociation; prolongs APD & ERP
      - Ib (Lidocaine, Mexiletine): Rapid dissociation; shortens APD in ischemic myocardium
      - Ic (Flecainide, Propafenone): Slow dissociation; marked Phase 0 depression, neutral APD

    Class II: Beta-Blockers (SA/AV nodal Phase 4 slope reduction & decreased I_Ca,L)

    Class III: Potassium Channel Blockers (Inhibit Phase 3 I_Kr/I_Ks -> prolongs APD & QTc)
      - Amiodarone, Sotalol, Ibutilide, Dronedarone

    Class IV: Non-Dihydropyridine Calcium Channel Blockers (Inhibit L-type Ca2+ in SA/AV nodes)
      - Verapamil, Diltiazem

Class I: Voltage-Gated Sodium Channel Blockers ($Na_V1.5$)

Class I agents bind to the pore-forming alpha-subunit of the cardiac voltage-gated sodium channel ($Na_V1.5$), depressing the maximum rate of Phase 0 depolarization ($dV/dt_{\max}$). They are subdivided into Ia, Ib, and Ic based on their channel binding kinetics and their net effect on the action potential duration (APD) and effective refractory period (ERP):

1. Class Ia (Procainamide, Quinidine, Disopyramide):
   • Kinetics: Intermediate onset and dissociation kinetics (time constant of recovery $\tau = 1 - 8\text{ seconds}$).
   • Electrophysiology: Moderately depress Phase 0 $dV/dt_{\max}$, slowing intra-atrial, intraventricular, and His-Purkinje conduction. Uniquely, Class Ia agents also block outward delayed rectifier potassium currents ($I_{\text{Kr}}$), which prolongs Phase 3 repolarization, APD, and the ERP. On the surface ECG, they cause widening of the QRS complex and prolongation of the QT interval, carrying a risk of torsades de pointes.
2. Class Ib (Lidocaine, Mexiletine):
   • Kinetics: Very rapid association and dissociation kinetics ($\tau < 0.5\text{ seconds}$).
   • Electrophysiology: Bind with high affinity to sodium channels in the open and inactivated states, dissociating rapidly during diastole. In normal cardiac tissue at resting membrane potentials ($-90\text{ mV}$), lidocaine has minimal effect. However, in depolarized, ischemic, or rapidly firing ventricular myocardium (where channels spend extended time in the inactivated state), lidocaine binding is intense, suppressing Phase 0 and reducing abnormal ventricular automaticity. It shortens Phase 3 repolarization and APD by blocking late persistent sodium currents ($I_{\text{Na,late}}$). Class Ib agents are completely ineffective for supraventricular arrhythmias because atrial action potentials have short plateau phases, precluding significant channel inactivation.
3. Class Ic (Flecainide, Propafenone):
   • Kinetics: Very slow association and dissociation kinetics ($\tau > 10 - 30\text{ seconds}$).
   • Electrophysiology: Because dissociation is remarkably slow, channels accumulate block from cycle to cycle even at physiological heart rates. They cause marked depression of Phase 0 $dV/dt_{\max}$, profoundly slowing intracardiac conduction and broadening the QRS complex across all heart rates, with virtually no effect on APD or QT duration. In patients with prior myocardial infarction or structural heart disease, Class Ic agents dramatically increase the risk of lethal re-entrant ventricular tachycardias (demonstrated in the Cardiac Arrhythmia Suppression Trial, CAST) and are strictly contraindicated in structural cardiac pathology.

Class II: Beta-Adrenergic Antagonists

Class II agents competitively antagonize catecholamine binding at cardiac beta-1 receptors. By reducing $\text{cAMP}$ and PKA activity, they decrease the slope of Phase 4 spontaneous diastolic depolarization in the sinoatrial (SA) node and suppress calcium currents ($I_{\text{Ca,L}}$) in the atrioventricular (AV) node. This prolongs AV nodal conduction time and refractory periods, effectively terminating AV nodal re-entrant tachycardias and slowing ventricular response in atrial fibrillation.

Class III: Potassium Channel Blockers

Class III drugs block the delayed rectifier potassium currents responsible for cardiac repolarization, predominantly the rapid ($I_{\text{Kr}}$) and slow ($I_{\text{Ks}}$) components. This delays Phase 3 repolarization, prolonging the APD and ERP across atrial and ventricular myocytes. On surface ECG, this corresponds to QT interval prolongation.

Amiodarone

Amiodarone is a benzofuran derivative containing two iodine atoms that structurally resembles thyroxine ($T_4$). Although classified under Class III, it exhibits electrophysiological properties across all four Vaughan-Williams classes:

• Class I: Blocks inactivated sodium channels (fast Phase 0 kinetics).
• Class II: Non-competitively antagonizes $\alpha$ and $\beta$ adrenoceptors.
• Class III: Blocks delayed rectifier potassium channels ($I_{\text{Kr}}, I_{\text{Ks}}$), prolonging APD and ERP.
• Class IV: Blocks L-type calcium channels, depressing sinus and AV nodal conduction.

                      [ AMIOARONE PHARMACOKINETICS & TOXICITY ]
                                         |
        +--------------------------------+--------------------------------+
        |                                                                 |
[ PHARMACOKINETIC PROFILE ]                                       [ MULTI-ORGAN TOXICITY ]
- Extreme Lipophilicity                                           - Pulmonary: Interstitial alveolitis,
- Volume of Distribution ~60 L/kg (20-200)                          fibrosis (O2 toxicity hazard)
- Elimination Half-Life: 30 to 60 days                            - Thyroid: Hypothyroidism (Wolff-Chaikoff)
- Hepatic CYP3A4 / CYP2C8 Metabolism                                or Thyrotoxicosis (Jod-Basedow)
- Active Metabolite: Desethylamiodarone (DEA)                     - Corneal Microdeposits (>90% patients)
- Potent CYP2C9, CYP3A4, & P-gp Inhibitor                         - Dermatologic: Blue-grey slate skin
  (Doubles serum digoxin & warfarin levels)                       - Cardiac: Bradycardia, AV block
                                                                    (Very low Torsades risk: <1%)

• Unique Electrophysiological Safety: Despite marked QTc prolongation, the incidence of torsades de pointes with amiodarone is exceptionally low ($<1\%$, far lower than sotalol, quinidine, or ibutilide). This protection is conferred by concurrent L-type calcium and beta-blockade, which suppresses early afterdepolarizations (EADs), coupled with uniform prolongation of refractoriness across the endocardium, myocardium (M-cells), and epicardium, minimizing transmural dispersion of repolarization.
• Complex Pharmacokinetics: Amiodarone is exceptionally lipophilic, extensively sequestered in adipose tissue, skeletal muscle, liver, and lung parenchyma. Its volume of distribution ($V_d$) exceeds $5000\text{ L}$ ($>60\text{ L/kg}$). Following chronic therapy, its terminal elimination half-life ranges from 30 to 60 days. Hepatic metabolism generates the active metabolite desethylamiodarone (DEA). It is a potent inhibitor of cytochrome P450 enzymes (CYP2C9, CYP3A4) and P-glycoprotein, doubling circulating serum levels of warfarin and digoxin.
• Organ-Specific Adverse Effects:
  • Pulmonary Fibrosis: Affects up to 5% to 10% of patients on chronic therapy; causes toxic lymphocytic and eosinophilic alveolitis progressing to irreversible pulmonary fibrosis. Perioperatively, exposure to high fractions of inspired oxygen ($Fi\text{O}_2 > 0.50$) triggers massive free radical generation, precipitating acute fulminant pneumonitis and ARDS.
  • Thyroid Dysfunction: Each 200-mg tablet contains ~75 mg of organic iodine, releasing ~6 mg of free iodide daily (hundreds of times the daily requirement). It can induce amiodarone-induced hypothyroidism (AIH) via the persistent Wolff-Chaikoff effect (inhibition of thyroid hormone synthesis) or amiodarone-induced thyrotoxicosis (AIT), either via iodine-induced hyperthyroidism in underlying goiter (AIT-1 / Jod-Basedow) or destructive autoimmune thyroiditis (AIT-2).
  • Ophthalmologic: Bilateral benign corneal microdeposits (vortex keratopathy) develop in $>90\%$ of patients within months; optic neuropathy/neuritis can cause permanent visual loss.
  • Dermatologic: Slate blue-grey cutaneous hyperpigmentation in sun-exposed areas and severe photosensitivity.
  • Hepatic: Asymptomatic transaminase elevations in up to 25%; rarely cirrhosis.

Class IV: Non-Dihydropyridine Calcium Channel Blockers

Class IV agents (verapamil and diltiazem) selectively bind the alpha-1 subunit of L-type voltage-gated calcium channels ($Ca_V1.2$) in cardiac nodal and conducting tissue. They slow Phase 0 depolarization in the SA and AV nodes, prolong AV nodal conduction time and refractory periods, and depress myocardial inotropy. They are first-line for rate control of supraventricular tachyarrhythmias, but are strictly contraindicated in severe heart failure with reduced ejection fraction (HFrEF) and in Wolff-Parkinson-White (WPW) syndrome with pre-excited atrial fibrillation (blocking the AV node diverts all conduction over the accessory bypass tract, provoking fatal ventricular fibrillation).

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2. Atypical Antiarrhythmics: Adenosine and Digoxin

Adenosine

• Molecular Mechanism: Adenosine is an endogenous purine nucleoside that binds to cardiac $A_1$ adenosine receptors located in the SA node, atrial myocardium, and AV node. The $A_1$ receptor couples to the $G_i$ protein:
  1. The liberated $G_{\beta\gamma}$ dimer directly activates acetylcholine/adenosine-sensitive inward-rectifying potassium channels ($I_{\text{K,Ado}}$), driving potassium efflux and profound cellular hyperpolarization.
  2. The $G_{\alpha i}$ subunit inhibits adenylyl cyclase, lowering $\text{cAMP}$ and suppressing L-type calcium current ($I_{\text{Ca,L}}$). The combination of hyperpolarization and reduced calcium influx produces transient, complete electrophysiological conduction block at the AV node for several seconds.
• Clinical Utility: The first-line agent for the immediate termination of paroxysmal supraventricular tachycardias involving the AV node in their re-entrant circuit (AVNRT and orthodromic AVRT).
• Ultra-Short Pharmacokinetics: Adenosine is rapidly eliminated from the vascular compartment through carrier-mediated cellular uptake into erythrocytes and vascular endothelial cells, where it is converted to inosine by adenosine deaminase (ADA) or phosphorylated to AMP by adenosine kinase. Its plasma elimination half-life is $<10\text{ seconds}$. To be effective, it must be administered as a rapid intravenous bolus through a large, proximal vein (antecubital or central line) immediately followed by a rapid 20-mL saline flush.
• Drug Interactions and Clinical Traps:
  • Methylxanthines (Caffeine, Theophylline, Aminophylline): Competitive antagonists at the $A_1$ receptor; significantly higher doses of adenosine are required for efficacy.
  • Dipyridamole: Inhibits the cellular nucleoside uptake transporter, blocking clearance and prolonging adenosine action; the dose must be reduced to one-third or one-quarter (e.g., 1.5 - 3.0 mg).
  • Denervated Hearts (Cardiac Transplants): Exhibit marked hypersensitivity to adenosine due to up-regulated postsynaptic $A_1$ receptors; initial doses should be reduced to 3 mg to prevent prolonged asystolic arrest.
  • Severe Asthma: Adenosine stimulates bronchial $A_{2B}$ and $A_3$ receptors, potentially triggering severe, acute bronchospasm; it is relatively contraindicated in severe reactive airway disease.

Digoxin (Cardiac Glycoside)

• Molecular Mechanism: Digoxin binds selectively and reversibly to the extracellular domain of the sarcolemmal $\text{Na}^+/\text{K}^+$-ATPase pump, inhibiting its active enzymatic transport. Intracellular sodium ($[Na^+]_i$) rises, reducing the trans-sarcolemmal sodium concentration gradient. This attenuates the driving force for the $3\text{Na}^+/Ca^{2+}$ antiporter (NCX), slowing calcium extrusion and elevating intracellular free calcium. This calcium is sequestered into the SR by SERCA2a, augmenting subsequent systolic calcium release and delivering moderate positive inotropy.
• Autonomic Action: Central vagomimetic effect; stimulates the nodose ganglion and vagal nuclei in the medulla, increasing parasympathetic efferent outflow to the SA and AV nodes. This prolongs AV nodal refractoriness and slows conduction velocity, making it valuable for ventricular rate control in chronic atrial fibrillation.
• Narrow Therapeutic Index: Therapeutic serum concentration is $0.5 - 2.0\text{ ng/mL}$ (modern target for heart failure is $0.5 - 0.9\text{ ng/mL}$). Toxicity occurs readily at levels $>2.0\text{ ng/mL}$.
• Electrolyte Sensitization: Hypokalemia, hypomagnesemia, and hypercalcemia profoundly aggravate toxicity. Extracellular $K^+$ and digoxin compete for the same binding site on the $\text{Na}^+/\text{K}^+$-ATPase pump; hypokalemia increases digoxin binding and toxicity.
• Manifestations of Toxicity:
  • Gastrointestinal: Anorexia, nausea, vomiting, abdominal pain.
  • Neurological: Confusion, weakness, characteristic yellow-green chromatic visual halos (xanthopsia).
  • Cardiac Arrhythmias: Frequent premature ventricular contractions (PVCs), paroxysmal atrial tachycardia with variable AV block, junctional escape rhythms, and pathognomonic bidirectional ventricular tachycardia.
• Treatment: Immediate administration of Digoxin-specific Fab antibody fragments (DigiFab), gentle correction of hypokalemia and hypomagnesemia, and temporary cardiac pacing for high-grade AV block. Dialysis is ineffective due to a large volume of distribution ($V_d \approx 5 - 7\text{ L/kg}$). Avoid cardioversion if possible, as it may precipitate intractable ventricular fibrillation.

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3. Parenteral Antihypertensive Agents and Vasodilators

                       [ PARENTERAL VASODILATOR PATHWAYS ]
                                        |
        +-------------------------------+-------------------------------+
        |                                                               |
[ NITROVASODILATORS ]                                           [ CALCIUM CHANNEL BLOCKERS ]
Sodium Nitroprusside (SNP) & Nitroglycerin (GTN)                Clevidipine & Nicardipine
        |                                                               |
Release Nitric Oxide (NO)                                       Block L-type Voltage-Gated Ca2+ Channels
- SNP: Spontaneous, Non-Enzymatic                               - Clevidipine: Esterase Cleavage (t_1/2 = 1-2 min)
- GTN: Requires Aldehyde Dehydrogenase-2 (ALDH2)                - High Vascular Smooth Muscle Selectivity
        |                                                               |
Stimulates Soluble Guanylyl Cyclase (sGC)                       Decreases Inward Ca2+ Influx
        |                                                               |
Elevates Intracellular cGMP                                     Inhibits MLCK Activation
        |                                                               |
Activates Protein Kinase G (PKG)                                Direct Arterial Vasodilation
        |                                                       Decreased SVR without Myocardial Depression
Dephosphorylates Myosin Light Chains & Extrudes Ca2+
        |
Balanced Arterial & Venous Dilation (SNP)
Predominant Venodilation & Preload Reduction (GTN)

Sodium Nitroprusside (SNP)

• Chemical Structure: An inorganic complex containing a central ferrous iron ($Fe^{2+}$) atom coordinated with five cyanide anions ($CN^-$) and one nitrosyl group ($NO^+$): $\text{Na}_2[Fe(\text{CN})_5\text{NO}]$
• Mechanism: SNP acts as a direct nitric oxide donor. Upon exposure to sulfhydryl groups on erythrocyte membranes and vascular tissue, it breaks down spontaneously and non-enzymatically, releasing free nitric oxide (NO). NO diffuses into vascular smooth muscle to activate soluble guanylyl cyclase (sGC), generating cyclic guanosine monophosphate ($\text{cGMP}$) and activating protein kinase G (PKG). PKG dephosphorylates myosin light chains and drives intracellular calcium extrusion, producing instantaneous, balanced arterial and venous dilation.
• Metabolism and Cyanide Toxicity:
  • Each molecule of SNP releases five cyanide ions. In the liver and kidneys, the mitochondrial enzyme rhodanese (thiosulfate sulfurtransferase) detoxifies cyanide by transferring a sulfur atom from endogenous thiosulfate to cyanide, forming water-soluble, non-toxic thiocyanate, which is cleared by the kidneys.
  • Cyanide Poisoning Cascade: When SNP is infused at high rates ($>2\text{ mcg/kg/min}$) or for prolonged durations ($>24\text{ hours}$), endogenous thiosulfate stores are rapidly exhausted. Free cyanide binds with extreme affinity to ferric iron ($Fe^{3+}$) within cytochrome c oxidase (Complex IV) of the mitochondrial respiratory chain, completely halting aerobic oxidative phosphorylation. Cells starve of ATP and are forced into anaerobic glycolysis.
  • Diagnostic Hallmarks of Cyanide Toxicity:
    1. Tachyphylaxis: Rapidly escalating dose requirements of SNP to achieve the same target blood pressure.
    2. Unexplained Metabolic Lactic Acidosis: Progressive, refractory high anion gap acidosis.
    3. Elevated Mixed Venous Oxygen Saturation ($Sv\text{O}_2 > 85 - 90\%$): Tissues are poisoned and cannot extract oxygen from circulating oxyhemoglobin, causing arterialization of venous blood (bright red venous blood).
• Cyanide Antidote Protocol:
  1. Hydroxocobalamin (5 g IV over 15 minutes): The modern first-line antidote. Its cobalt center binds cyanide with greater affinity than cytochrome oxidase, forming non-toxic cyanocobalamin (vitamin $B_{12}$), which is safely excreted in urine.
  2. Sodium Thiosulfate (12.5 g IV): Supplies exogenous sulfur donors for the rhodanese pathway.
  3. Sodium Nitrite: Oxidizes hemoglobin $Fe^{2+}$ to methemoglobin ($Fe^{3+}$), which acts as a chemical decoy to pull cyanide off cytochrome oxidase (use cautiously perioperatively due to impaired oxygen delivery).

Nitroglycerin (Glyceryl Trinitrate, GTN)

• Mechanism: Unlike SNP, nitroglycerin is a prodrug that requires enzymatic bioactivation by mitochondrial aldehyde dehydrogenase-2 (ALDH2) in vascular endothelial and smooth muscle cells, generating NO and triggering the $\text{cGMP-PKG}$ pathway.
• Hemodynamic Actions: At low to moderate clinical doses ($0.1 - 2.0\text{ mcg/kg/min}$), GTN is a predominant venous capacitance dilator. By dilating large systemic veins, it pools blood in the periphery, decreasing venous return, left ventricular end-diastolic volume, and wall stress (reducing myocardial preload and $MVO_2$). It also dilates large epicardial coronary arteries, relieving coronary vasospasm and redistributing blood flow to ischemic subendocardial tissue. Only at high doses ($>2 - 3\text{ mcg/kg/min}$) does significant arteriolar dilation occur.
• Nitrate Tolerance (Tachyphylaxis): Continuous infusion for $>24 - 48\text{ hours}$ leads to complete loss of hemodynamic efficacy. This tolerance is caused by depletion of intracellular sulfhydryl groups, oxidative inactivation of ALDH2 by reactive oxygen species, and neurohormonal counter-regulation (activation of the renin-angiotensin system). Prevention requires a daily nitrate-free interval of 8 to 12 hours.

Hydralazine

• Mechanism: Direct-acting arteriolar smooth muscle relaxant that produces membrane hyperpolarization and inhibits $\text{IP}_3$-mediated calcium release from the sarcoplasmic reticulum. It exerts virtually no effect on venous capacitance vessels.
• Pharmacokinetics and Limitations: Hydralazine has an unpredictable, delayed onset (15 to 20 minutes) and a prolonged duration of action (2 to 6 hours), precluding fine titration in acute perioperative crises. It provokes intense reflex sympathetic baroreceptor activation, causing marked tachycardia, increased stroke volume, and fluid retention, which can precipitate acute myocardial ischemia in patients with coronary artery disease.
• Toxicity: Chronic therapy is associated with drug-induced systemic lupus erythematosus (SLE), particularly in slow hepatic acetylators.

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4. Short-Acting Beta-Antagonists and Dihydropyridines

Esmolol

• Pharmacology: Ultra-short-acting, cardioselective $\beta_1$-adrenergic antagonist with minimal intrinsic sympathomimetic activity and no membrane-stabilizing effects.
• Metabolism: Contains an internal ester linkage that is rapidly hydrolyzed by cytoplasmic esterases located in red blood cells (erythrocytes). It is not dependent on hepatic metabolism or plasma pseudocholinesterase.
• Pharmacokinetics: Peak therapeutic effect occurs within 2 to 5 minutes following an intravenous bolus ($0.5\text{ mg/kg}$); its elimination half-life is 9 minutes, and full hemodynamic recovery occurs within 20 to 30 minutes of cessation. It is the premier agent for blunting acute, transient sympathetic surges associated with direct laryngoscopy, tracheal intubation, surgical incision, and emergence from anaesthesia.

Labetalol

• Pharmacology: Combined competitive $\alpha_1$ and non-selective $\beta_1/\beta_2$ antagonist. It possesses intrinsic sympathomimetic activity at $\beta_2$ receptors.
• Blockade Ratio: The ratio of $\alpha$ to $\beta$ antagonism is approximately 1:7 for intravenous administration and 1:3 for oral administration.
• Hemodynamics: Arteriolar vasodilation via $\alpha_1$ blockade lowers systemic vascular resistance, while concurrent $\beta_1$ blockade prevents reflex baroreceptor tachycardia, maintaining cardiac output and reducing myocardial oxygen demand.

Clevidipine

• Pharmacology: A third-generation, vascular-selective dihydropyridine calcium channel blocker. It selectively inhibits L-type calcium channels on arterial smooth muscle, decreasing SVR and MAP without negative inotropy, prolongation of AV conduction, or reflex tachycardia.
• Metabolism and Rapid Recovery: Formulated with an ester side chain that is cleaved within seconds by non-specific blood and tissue esterases. Its initial distribution half-life is 1 minute, and its terminal elimination half-life is 1 to 2 minutes.
• Formulation: Insoluble in water; formulated as a sterile 20% oil-in-water lipid emulsion containing soybean oil and egg yolk phospholipids ($2.0\text{ kcal/mL}$). It requires strict aseptic technique, and infusion bottles/lines must be discarded after 12 hours. It is contraindicated in patients with severe allergy to soy or egg products and in severe lipid metabolism disorders.

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5. Perioperative Management of ACE Inhibitors and ARBs

Chronic inhibition of the renin-angiotensin-aldosterone system (RAAS) with Angiotensin-Converting Enzyme (ACE) Inhibitors (e.g., ramipril, enalapril, lisinopril) or Angiotensin Receptor Blockers (ARBs) (e.g., losartan, valsartan, candesartan) presents a unique challenge under general anaesthesia.

                    [ MECHANISM OF INDUCTION VASOPLEGIA ]
                                      |
        +-----------------------------+-----------------------------+
        |                                                           |
[ NORMAL AWAKE STATE ]                                      [ GENERAL ANAESTHESIA ]
Blood Pressure maintained by three                          Anaesthetic agents suppress
complementary homeostatic systems:                          Sympathetic Nervous System
1. Sympathetic Nervous System                                       |
2. Renin-Angiotensin-Aldosterone System                             v
3. Vasopressinergic System                                  Patient with chronic ACEi / ARB therapy
                                                            has paralyzed RAAS axis
                                                                    |
                                                                    v
                                                            Homeostasis depends SOLELY on
                                                            Vasopressin System (third pillar)
                                                                    |
                                                                    v
                                                            [ PROFOUND REFRACTORY VASOPLEGIA ]
                                                            Massive drop in SVR and MAP
                                                            Unresponsive to fluids and pure alpha-1 agonists

Clinical Management and Rescue Algorithm

• Withholding Guidance: The 2022 ESC guideline on non-cardiac surgery (endorsed by ESAIC) states that, in patients without heart failure, withholding RAAS inhibitors on the day of surgery should be considered to prevent perioperative hypotension. The 2024 STOP-or-NOT trial found no difference in major complications between stopping 48 hours beforehand and continuing, although continuation produced more intraoperative hypotension; decisions are therefore individualised.
• Rescue Therapy for Refractory Vasoplegia: When severe post-induction vasoplegia develops in patients who took an ACEi/ARB on the morning of surgery:
  1. Standard volume expansion and pure $\alpha_1$ agonists (phenylephrine) often fail due to profound uncoupling of vascular tone.
  2. Norepinephrine infusion should be initiated promptly.
  3. Arginine Vasopressin (AVP): The definitive targeted rescue agent. Administered as an IV bolus of 1 to 2 units followed by an infusion of $0.01 - 0.04\text{ units/min}$, restoring vascular tone by activating the third, unblocked homeostatic axis.
  4. Methylene Blue: In cases of catecholamine- and vasopressin-resistant vasoplegia, methylene blue ($1.5 - 2.0\text{ mg/kg}$ IV over 20 to 60 minutes) acts as an inhibitor of both endothelial inducible nitric oxide synthase (iNOS) and soluble guanylyl cyclase (sGC), terminating excessive $\text{cGMP}$ accumulation and restoring vascular smooth muscle responsiveness.