10.4 Vasoactive, Antidysrhythmic & Emergency Perioperative Medications

10.4 Vasoactive, Antidysrhythmic & Emergency Perioperative Medications

The intraoperative environment presents rapid, dynamic alterations in hemodynamics, systemic vascular resistance, and cardiac rhythm induced by surgical trauma, hemorrhage, autonomic reflexes, and anesthetic agents. Certified Anesthesia Technologists must maintain a thorough comprehension of the pharmacology, concentrations, indications, and failure modes of emergency perioperative vasoactive and antidysrhythmic medications.

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Perioperative Autonomic Receptor Physiology

Vasoactive drugs exert their cardiovascular actions by targeting specific adrenergic, dopaminergic, and vasopressinergic receptor subtypes:

                               Perioperative Adrenergic & Vascular Receptors
                                                     |
         +---------------------+---------------------+---------------------+---------------------+
         |                     |                     |                     |                     |
      Alpha-1               Alpha-2               Beta-1                Beta-2                  V1a
     (Gq protein)          (Gi protein)          (Gs protein)          (Gs protein)         (Gq protein)
   * Vasoconstriction    * Presynaptic         * Positive Inotropy   * Bronchodilation    * Vasoconstriction
   * Increased SVR       * Reduced NE release  * Positive Chronotropy* Vasodilation       * Independent of
   * Increased MAP       * Sedation/Analgesia  * Increased CO        * Decreased SVR        adrenergic system

• Alpha-1 (α₁) Receptors (Gq-protein coupled): Located post-synaptically on vascular smooth muscle. Activation stimulates phospholipase C (PLC), generating inositol trisphosphate (IP₃) and diacylglycerol (DAG), mobilizing intracellular calcium (Ca²⁺) from the sarcoplasmic reticulum. Results in intense arterial and venous vasoconstriction, increasing Systemic Vascular Resistance (SVR), afterload, and mean arterial pressure (MAP).
• Alpha-2 (α₂) Receptors (Gi-protein coupled): Located pre-synaptically on sympathetic nerve terminals and centrally in the brainstem (locus coeruleus). Activation inhibits adenylate cyclase, decreasing intracellular cyclic AMP (cAMP) and inhibiting further exocytotic release of norepinephrine (negative feedback). Centrally acting α₂ agonists (clonidine, dexmedetomidine) cause sedation, anxiolysis, and sympatholysis.
• Beta-1 (β₁) Receptors (Gs-protein coupled): Located post-synaptically in the myocardium, sinoatrial (SA) node, and atrioventricular (AV) node. Activation stimulates adenylate cyclase, increasing cAMP and activating protein kinase A (PKA), augmenting intracellular calcium influx. Results in:
  • Positive Inotropy: Increased myocardial contractile force.
  • Positive Chronotropy: Increased heart rate.
  • Positive Dromotropy: Accelerated conduction velocity through the AV node.
  • Positive Lusitropy: Accelerated myocardial relaxation.
• Beta-2 (β₂) Receptors (Gs-protein coupled): Located on bronchial smooth muscle, vascular smooth muscle of skeletal muscle beds, and the uterus. Activation increases cAMP, which inhibits myosin light chain kinase (MLCK), producing bronchodilation, peripheral vasodilation, and uterine relaxation.
• Vasopressin 1a (V1a) Receptors (Gq-protein coupled): Located on vascular smooth muscle. Activation mobilizes intracellular calcium via the IP₃ pathway, causing profound vasoconstriction via a biochemical mechanism entirely independent of the adrenergic receptor system.

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Perioperative Vasopressors and Inotropic Agents

Ephedrine Sulfate

Ephedrine is a synthetic, non-catecholamine sympathomimetic amine widely utilized for the treatment of transient intraoperative hypotension induced by general anesthesia or neuraxial sympathectomy.

• Mechanism of Action: Mixed-acting sympathomimetic.
  • Primary Action (Indirect): Ephedrine is taken up by the norepinephrine transporter (NET) into pre-synaptic sympathetic nerve terminals, where it displaces and stimulates the exocytotic release of stored endogenous norepinephrine.
  • Secondary Action (Direct): Weaker direct stimulation of α₁, β₁, and β₂ adrenergic receptors.
• Hemodynamic Effects: Increases heart rate, stroke volume, cardiac output, and systolic and diastolic blood pressure. Mild bronchodilation occurs via β₂ stimulation.
• Dosing & Administration: 5 to 10 mg IV bolus (commonly diluted to 5 mg/mL and titrated to effect).

The Phenomenon of Tachyphylaxis

Because ephedrine's primary mechanism depends on the release of pre-formed, finite vesicular stores of norepinephrine from sympathetic nerve terminals, repeated dosing produces tachyphylaxis (a progressively diminishing hemodynamic response to the same dose). Once vesicular norepinephrine pools are depleted, subsequent doses of ephedrine fail to raise blood pressure. Ephedrine is also completely ineffective in patients with chronic catecholamine depletion (e.g., end-stage congestive heart failure, prolonged septic shock, chronic cocaine abuse, or chronic reserpine therapy).

Phenylephrine (Neo-Synephrine)

Phenylephrine is a synthetic, non-catecholamine sympathomimetic that serves as the first-line vasopressor for general intraoperative vasodilation.

• Mechanism of Action: Pure, direct-acting selective α₁ adrenergic agonist (devoid of direct β-adrenergic activity at clinical doses).
• Hemodynamic Profile:
  • Intense Vasoconstriction: Increases systemic vascular resistance, afterload, systolic pressure, diastolic pressure, and MAP.
  • Reflex Bradycardia: The sudden rise in arterial pressure stimulates stretch receptors in the carotid sinuses and aortic arch, initiating a profound vagal baroreceptor reflex that slows the heart rate via parasympathetic activation.
  • Stroke Volume & Cardiac Output: Stroke volume and cardiac output frequently fall due to the combined effects of bradycardia and increased left ventricular afterload.
• Dosing & Administration:
  • IV Bolus: 50 to 100 mcg (0.05 to 0.1 mg) per bolus (standard concentration: 100 mcg/mL).
  • Continuous Infusion: 0.1 to 1.0 mcg/kg/min (or 20 to 100 mcg/min titrated to MAP).
• Clinical Indications: First-line treatment for hypotension accompanied by normal or elevated heart rate (tachycardia). It is the premier vasopressor for patients with severe aortic stenosis or hypertrophic obstructive cardiomyopathy (HOCM), where maintaining coronary perfusion pressure while strictly avoiding tachycardia is essential.

Epinephrine (Adrenaline)

Epinephrine is an endogenous catecholamine synthesized by the adrenal medulla and represents the definitive pharmacological resuscitation agent in critical perioperative emergencies.

• Mechanism of Action: Potent, non-selective direct agonist at α₁, α₂, β₁, and β₂ receptors.
• Dose-Dependent Receptor Profile:
  • Low Doses (0.01 to 0.05 mcg/kg/min): Predominant β₁ and β₂ stimulation. Produces dramatic increases in myocardial contractility (inotropy), heart rate (chronotropy), and cardiac output, while β₂-mediated vasodilation in skeletal muscle reduces or maintains SVR. Powerful bronchodilation.
  • High Doses (>0.1 to 0.2 mcg/kg/min): Potent α₁ vasoconstrictor effects dominate, overriding β₂ vasodilation. Produces profound systemic vasoconstriction, marked elevation of SVR and blood pressure, with intense renal, splanchnic, and cutaneous vasoconstriction.
• Emergency Dosing Protocols:
  • ACLS Cardiac Arrest (VF, pVT, Asystole, PEA): 1.0 mg IV/IO push of the 1:10,000 solution (0.1 mg/mL) administered every 3 to 5 minutes.
  • Severe Perioperative Anaphylaxis (with cardiovascular collapse): Titrated intravenous boluses of 10 to 100 mcg IV (using diluted 10 mcg/mL or 100 mcg/mL concentrations). If IV access is lost, administer 0.3 to 0.5 mg IM of the 1:1,000 solution (1.0 mg/mL) into the anterolateral mid-thigh.
  • Continuous Inotropic / Vasopressor Infusion: 0.02 to 0.2 mcg/kg/min.

Norepinephrine (Levophed)

Norepinephrine is the primary endogenous neurotransmitter released by postganglionic sympathetic neurons.

• Mechanism of Action: Potent direct α₁ agonist combined with modest β₁ inotropic activity; minimal β₂ activity.
• Hemodynamic Profile: Produces intense arterial and venous vasoconstriction, significantly increasing SVR and blood pressure. Modest β₁ stimulation supports myocardial contractility. Because direct β₁ chronotropic stimulation counters vagal baroreceptor activation, reflex bradycardia is usually absent, and cardiac output is maintained or slightly enhanced.
• Dosing & Administration: Continuous IV infusion at 0.02 to 0.5 mcg/kg/min (titrated to maintain MAP ≥ 65 mmHg). Standard concentration: 4 mg in 250 mL D5W or NS (16 mcg/mL).
• First-Line Indication: The premier vasopressor of choice for septic shock, distributive shock, and vasoplegic syndrome following cardiopulmonary bypass.
• Safety Caution: Severe extravasation into peripheral subcutaneous tissues causes ischemic necrosis and sloughing due to intense local vasoconstriction. Infuse through a central venous line whenever possible; phentolamine (Regitine, an α-blocker) is the local antidote for extravasation.

Vasopressin (Arginine Vasopressin / Pitressin)

Vasopressin is an endogenous nonapeptide hormone synthesized in the paraventricular and supraoptic nuclei of the hypothalamus and stored in the posterior pituitary gland.

• Mechanism of Action: Direct agonist at vascular V1a receptors on vascular smooth muscle, activating the Gq-protein coupled phospholipase C pathway to mobilize intracellular calcium.
• The Acidosis-Independent Vasopressor: Under conditions of severe lactic acidosis (pH < 7.20), hypoxemia, and prolonged shock, adrenergic receptors become uncoupled and desensitized to catecholamines (norepinephrine and phenylephrine fail to constrict vessels). Vasopressin acts through an entirely separate biochemical cascade, maintaining potent vasoconstrictor efficacy in severe acidemic environments.
• Clinical Indications:
  • Refractory Vasoplegic Shock: Vasoplegia following cardiopulmonary bypass refractory to high-dose catecholamines.
  • ACEI / ARB-Induced Hypotension: Patients on chronic lisinopril, losartan, or other renin-angiotensin-aldosterone system blockers frequently develop severe, refractory intraoperative vasodilation under general anesthesia. Vasopressin is the definitive rescue agent.
• Dosing Protocols:
  • IV Bolus: 1 to 2 units IV.
  • Continuous Infusion: 0.01 to 0.04 units/min (often run at a fixed rate). Higher doses increase the risk of coronary, mesenteric, and digital ischemia.

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Perioperative Antihypertensive Agents

Intraoperative hypertensive crises can precipitate myocardial ischemia, acute left ventricular failure, cerebrovascular hemorrhage, and surgical wound disruption.

Labetalol (Trandate / Normodyne)

• Mechanism of Action: Competitive antagonist at non-selective beta (β₁ and β₂) and selective alpha-1 (α₁) receptors.
• Blockade Ratio: For intravenous administration, the ratio of β-to-α blockade is 7:1 (compared to 3:1 for oral dosing). The predominant effect is beta blockade.
• Hemodynamic Profile: Reduces blood pressure by lowering systemic vascular resistance (α₁ blockade) while preventing reflex tachycardia through simultaneous β₁ blockade.
• Dosing: 5 to 20 mg IV slow push over 2 minutes; can be repeated every 10 to 15 minutes up to a cumulative maximum of 300 mg.
• Contraindications: Severe asthma or active bronchospasm (β₂ blockade causes life-threatening bronchoconstriction), second- or third-degree heart block, severe sinus bradycardia, and cardiogenic shock.

Esmolol (Brevibloc)

• Mechanism of Action: Ultra-short-acting, cardioselective β₁ adrenergic antagonist.
• Unique Metabolic Pathway: Esmolol is metabolized exclusively by cytoplasmic red blood cell (RBC) esterases, completely independent of renal clearance, hepatic clearance, or plasma pseudocholinesterase.
• Pharmacokinetics: Ultra-short distribution half-life of 2 minutes; elimination half-life of approximately 9 minutes. Full hemodynamic recovery occurs within 15 to 30 minutes after stopping an infusion.
• Dosing:
  • IV Bolus: 10 to 50 mg IV (or 0.5 mg/kg over 1 minute).
  • Continuous Infusion: 50 to 300 mcg/kg/min.
• Clinical Indications: Blunting acute sympathetic surges (hypertension and tachycardia) during tracheal intubation, surgical stimulation, and emergence/extubation; acute rate control in supraventricular tachycardia (SVT) and atrial fibrillation.

Nitroglycerin (NTG)

• Mechanism of Action: An organic nitrate that generates nitric oxide (NO), activating soluble guanylate cyclase and increasing intracellular cyclic GMP (cGMP), leading to dephosphorylation of myosin light chains and vascular smooth muscle relaxation.
• Preferential Venodilation: At standard clinical doses (0.1 to 2.0 mcg/kg/min or 10 to 100 mcg/min), nitroglycerin acts preferentially on venous capacitance vessels, reducing venous return, left ventricular end-diastolic volume (LVEDV), and preload. This reduces myocardial wall tension and oxygen consumption. It also dilates epicardial coronary arteries and relieves coronary vasospasm.
• Clinical Uses: Intraoperative myocardial ischemia, acute cardiogenic pulmonary edema, and controlled hypotensive anesthesia.

Sodium Nitroprusside (SNP / Nipride)

• Mechanism of Action: Direct-acting nitric oxide donor producing immediate, potent, balanced dilation of both arterial resistance vessels (afterload reduction) and venous capacitance vessels (preload reduction).
• Pharmacokinetics: Onset within seconds; duration 1 to 2 minutes.
• Dosing: Continuous infusion starting at 0.2 to 0.5 mcg/kg/min, titrated to blood pressure (typical range: 0.5 to 2.0 mcg/kg/min).

Cyanide Toxicity: Cellular Mechanism & Manifestations

Sodium nitroprusside consists of an iron core complexed with 5 cyanide (CN⁻) ions:

1. In the bloodstream, SNP reacts with oxyhemoglobin, releasing free cyanide ions.
2. In the liver, the enzyme rhodanese detoxifies cyanide into thiocyanate using endogenous sulfur donors (thiosulfate), which is excreted in the urine.
3. When SNP is infused at high rates (>2 mcg/kg/min for prolonged periods, or max rate 10 mcg/kg/min for >10 minutes), endogenous sulfur reserves become exhausted.
4. Free cyanide binds to the ferric iron (Fe³⁺) of cytochrome oxidase (complex IV) in the mitochondrial electron transport chain, completely arresting oxidative phosphorylation.
5. Cellular aerobic respiration ceases, causing histotoxic hypoxia (tissues cannot utilize oxygen despite normal arterial saturation).

• Treatment Protocol:
  • Discontinue SNP immediately and administer 100% inspired oxygen.
  • Administer Sodium Thiosulfate (provides sulfur donors for rhodanese).
  • Administer Hydroxocobalamin (Cyanokit, 5.0 g IV): Hydroxocobalamin binds cyanide with extreme affinity, forming non-toxic cyanocobalamin (Vitamin B12), excreted by the kidneys.
  • Storage Rule: SNP is light-sensitive; the infusion container must be covered with an opaque protective wrap, while the labeling states that the delivery tubing does not need to be covered.

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Perioperative Emergency Antidysrhythmic Medications

Amiodarone (Cordarone)

• Vaughan-Williams Class: Class III antiarrhythmic (primary action: blocks voltage-gated potassium channels, prolonging phase 3 repolarization, action potential duration, and effective refractory period). Also exhibits Class I (sodium channel block), Class II (antiadrenergic beta-blockade), and Class IV (calcium channel block) properties.
• ACLS Resuscitation Dosing (Shock-Refractory VF / Pulseless VT):
  • First Dose: 300 mg IV/IO rapid push (diluted in 20 to 30 mL D5W or NS).
  • Second Dose: 150 mg IV/IO rapid push after 3 to 5 minutes if VF/pVT persists.
• Stable Tachyarrhythmias (VT with pulse, Atrial Fibrillation): 150 mg IV infused over 10 minutes, followed by 1 mg/min for 6 hours (360 mg), then 0.5 mg/min for 18 hours (540 mg).
• Adverse Effects: Profound hypotension and bradycardia (often exacerbated by rapid infusion and the solvent vehicle polysorbate 80).

Lidocaine (Xylocaine)

• Vaughan-Williams Class: Class IB antiarrhythmic; selectively binds and blocks open and inactivated voltage-gated fast sodium channels in ischemic, depolarized ventricular myocytes, shortening action potential duration and suppressing abnormal automaticity.
• ACLS Dosing (Alternative to Amiodarone in VF/pVT): 1.0 to 1.5 mg/kg IV/IO initial bolus; repeat boluses of 0.5 to 0.75 mg/kg every 5 to 10 minutes (maximum cumulative dose: 3.0 mg/kg); continuous maintenance infusion of 1 to 4 mg/min.
• Airway Reflex Blunting: Administered at 1.5 mg/kg IV approximately 90 seconds prior to laryngoscopy to blunt tracheal stimulation-induced hypertension, tachycardia, and spikes in intracranial pressure.

Adenosine (Adenocard)

• Mechanism of Action: Endogenous purine nucleoside that binds to cardiac A₁ adenosine receptors on the SA and AV nodes. Activates G-protein-coupled inward rectifier potassium channels (I(K,ACh)) while inhibiting cAMP-mediated calcium influx. Produces profound hyperpolarization and transiently halts atrioventricular (AV) nodal conduction.
• Ultra-Short Half-Life: Elimination half-life is less than 10 seconds due to instantaneous uptake by erythrocytes and vascular endothelial cells, followed by enzymatic degradation by adenosine deaminase.
• Clinical Indication: Rapid conversion of paroxysmal supraventricular tachycardia (PSVT), including AV nodal reentrant tachycardia (AVNRT).
• Rapid Push Technique:
  • Initial dose: 6 mg rapid IV push (administered over 1 to 2 seconds) immediately followed by a 20 mL rapid normal saline flush through a large-bore IV line close to the heart (antecubital fossa or central line; elevate the extremity).
  • Second dose: If conversion does not occur within 1 to 2 minutes, administer 12 mg rapid IV push with a 20 mL flush.
  • Clinical Expectation: The monitor will display a brief period of asystole (several seconds) before sinus rhythm resumes; warn the patient (if awake) of transient facial flushing, dyspnea, and intense chest tightness.
• Contraindications: Second- or third-degree AV block, sick sinus syndrome, and severe reactive bronchospastic airway disease (asthma).