12.3 Cardiovascular, Antimicrobial & Dialyzable Drug Dosing

Cardiovascular, Antimicrobial & Dialyzable Drug Dosing

Maintenance hemodialysis fundamentally alters the pharmacokinetics and pharmacodynamics of medications. Loss of renal clearance, uremic alterations in hepatic cytochrome P450 metabolism, reduced plasma protein binding, and intermittent extracorporeal clearance complicate therapeutic dosing. The Certified Hemodialysis Nurse plays a critical role in scheduling medication delivery, preventing intradialytic hemodynamic collapse, and managing complex intradialytic antimicrobial infusions.

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Determinants of Drug Dialyzability

Whether a drug is removed during hemodialysis depends on the physicochemical properties of the drug and the characteristics of the dialyzer membrane:

1. Size: Molecular size influences membrane passage, but protein binding and distribution volume can make a small molecule poorly removable. Consult drug-specific dosing evidence for the actual modality.
2. Plasma Protein Binding: Only the unbound (free) fraction of a drug crosses the dialyzer membrane. Drugs with high plasma protein binding ($>90\%$, such as warfarin at $>98\%$, carvedilol at $>98\%$, amlodipine at $>95\%$, and ceftriaxone at $85\%\text{ to }95\%$) are not dialyzable, as the bulky albumin-drug complex is retained in the blood circuit. Drugs with low protein binding ($<20\%\text{ to }30\%$, such as aminoglycosides at $<10\%$ and lisinopril at $0\%$) are readily dialyzed.
3. Volume of Distribution ($V_d$): $V_d$ represents the apparent dilution space of a drug:
   • High $V_d$ ($>1.0\text{ to }2.0\text{ L/kg}$): The drug is sequestered in lipid reserves, skeletal muscle, or deep intracellular tissue compartments (e.g., digoxin at $V_d \sim 5\text{ to }7\text{ L/kg}$, amiodarone at $V_d > 60\text{ L/kg}$, tricyclic antidepressants). Less than 1% to 5% of total body drug resides within the intravascular compartment. Even if the dialyzer rapidly clears intravascular drug, total body stores remain untouched, and a post-dialysis rebound occurs as tissue stores re-equilibrate into plasma. These drugs are non-dialyzable.
   • Low $V_d$ ($<0.5\text{ to }0.7\text{ L/kg}$): The drug is confined primarily to extracellular water and plasma (e.g., aminoglycosides at $V_d \sim 0.25\text{ L/kg}$, cephalosporins). The drug is directly accessible to dialytic removal and is highly dialyzable.
4. Water vs. Lipid Solubility: Highly water-soluble (hydrophilic) molecules partition into aqueous dialysate; lipophilic molecules distribute into cell membranes and adipose tissues, escaping clearance.
5. Dialyzer Membrane Characteristics: Membrane surface area, ultrafiltration coefficient ($K_{uf}$), mass transfer area coefficient ($KoA$), and blood/dialysate flow rates ($Q_b, Q_d$) govern solute clearance.

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Cardiovascular Medications in Hemodialysis

Antihypertensive Timing

Review BP trends, intradialytic symptoms, drug half-life, dialyzability and the patient’s cardiovascular indications. Routine withholding of every antihypertensive is not a universal recommendation. Some patients need a prescriber-directed timing adjustment; others develop harmful predialysis hypertension when doses are omitted. Record the actual last dose and distinguish a missed medication from a deliberately modified plan.

Beta-Blockers: Dialyzable vs. Non-Dialyzable

• Metoprolol & Atenolol: Hydrophilic, low protein binding ($<12\%$), and small $V_d$. They are moderately to highly dialyzable. If taken before dialysis, they are washed out into dialysate, leaving the patient unprotected against post-dialysis rebound hypertension; if taken post-dialysis, supplemental dosing may be needed. Labetalol is less readily dialyzed; it is not approximately 98% protein-bound.

Digoxin: Lethal Dialytic Arrhythmia Hazards

Digoxin is indicated for ventricular rate control in atrial fibrillation and heart failure. In ESRD, digoxin elimination half-life extends from 36 hours to 3.5 to 5 days:

• Non-Dialyzability: Digoxin has a massive volume of distribution ($V_d \sim 5\text{ to }7\text{ L/kg}$) due to avid binding to skeletal muscle and myocardial tissues. Less than 3% is removed during a standard four-hour hemodialysis treatment.
• Narrow Therapeutic Window: Target serum levels are strictly $0.5\text{ to }0.9\text{ ng/mL}$. Toxicity occurs above $1.2\text{ to }2.0\text{ ng/mL}$.
• The Hypokalemia-Digoxin Arrhythmia Trigger: Digoxin binds to and inhibits myocardial $Na^+/K^+$-ATPase. Potassium competes with digoxin for this binding site. Rapid dialytic potassium removal drops extracellular potassium. Acute hypokalemia ($K^+ < 3.5\text{ mEq/L}$) drastically increases digoxin binding to cardiac myocytes, precipitating lethal ventricular arrhythmias (bidirectional ventricular tachycardia, ventricular fibrillation, severe junctional escape rhythms, and complete heart block).
• Review the potassium bath and digoxin exposure with the prescriber when the gradient is concerning. Hypokalemia increases toxicity risk, but no universal bath minimum applies to every patient.

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Antimicrobial Dosing & Administration Protocols

Infections are a major cause of morbidity and mortality in kidney failure. Antimicrobial administration via the extracorporeal circuit requires precise timing and therapeutic drug monitoring:

1. Vancomycin

Vancomycin is a glycopeptide ($MW \sim 1,449\text{ Da}$) used for methicillin-resistant Staphylococcus aureus (MRSA) and enterococcal infections. While poorly cleared by historical low-flux membranes, modern high-flux polysulfone dialyzers clear approximately $30\%\text{ to }40\%$ of circulating vancomycin.

• Vancomycin dose and timing follow the pharmacy protocol, weight, membrane removal and measured exposure. Intradialytic dosing can require compensation for removal; infusion duration follows dose and rate limits.
• Rationale for Last 60-Minute Infusion: Infusing into the venous return line over the final hour utilizes the high extracorporeal blood flow ($350\text{ to }450\text{ mL/min}$) to rapidly dilute the medication, preventing histamine-mediated Vancomycin Infusion Reaction (Red Man Syndrome), completing the infusion right as treatment terminates, and saving patient chair time.
• Monitoring: Pre-dialysis trough levels are drawn from the vascular access before initiating dialysis. The target trough is $15\text{ to }20\text{ mcg/mL}$ for severe infections (bacteremia, endocarditis, osteomyelitis, vascular access sepsis).

2. Cefazolin

Cefazolin is a first-generation cephalosporin ($MW \sim 454\text{ Da}$) and the first-line treatment for methicillin-sensitive Staphylococcus aureus (MSSA) vascular access bacteremia. It is $80\%$ protein bound and cleared moderately by high-flux dialysis. It is administered post-dialysis: $1\text{ to }2\text{ g}$ IV into the venous bloodline during rinseback, three times weekly after each dialysis treatment.

3. Aminoglycosides (Gentamicin & Tobramycin)

Aminoglycosides ($MW \sim 470\text{ Da}$) are small, water-soluble, $<10\%$ protein bound, with a low $V_d$ ($0.25\text{ L/kg}$). They are highly dialyzable, with approximately $50\%$ of the drug removed in a single four-hour hemodialysis treatment. Aminoglycoside regimens require specialist dosing and drug levels; both post-HD and selected pre-HD strategies exist.

• Pre-dialysis troughs and post-dose peaks are monitored to prevent irreversible ototoxicity, vestibulotoxicity, and destruction of any residual nephron function.

Dialyzability of Common Medications

For serious MRSA infection, vancomycin treatment uses exposure monitoring; predialysis levels of approximately 15–20 mg/L may serve as a practical surrogate in HD under the pharmacy protocol. Intradialytic administration can lose drug through the membrane and requires dose review. Infuse vancomycin no faster than the applicable label/protocol, commonly at most 10 mg/min and for at least 60 minutes; 1,500 mg at 10 mg/min requires at least 150 minutes, not one hour. The final dialysis hour does not automatically prevent an infusion reaction. Loading doses depend on weight and the actual protocol rather than a universal two-gram cap. Digoxin risk rises with hypokalemia; flag a concerning potassium gradient and obtain prescription review rather than independently selecting a bath.

Sources checked 2026-10-10: NIDDK HD; current device and medication instructions govern product-specific details.