Digoxindosage

Toxic levels of digoxin are only slightly higher than therapeutic levels. The pharmacokinetics of digoxin are complex and dose determination should take into account patient-specific factors (age, lean body weight, renal function, etc.). Patients should be monitored for toxicity and therapeutic effect and doses should be adjusted, accordingly.

2.1 General

Dosing Considerations The dose of digoxin should be based on clinical assessment but individual patient factors should be taken into consideration. Those factors are:

Lean body weight

Renal function

Patient age

Concurrent disease

Concomitant medication Because the pharmacokinetics of digoxin are complex, and because toxic levels of digoxin are only slightly higher than therapeutic levels, digoxin dosing can be difficult. The recommended approach is to:

estimate the patient’s daily maintenance dose

adjust the estimate to account for patient-specific factors

choose a dosing regimen

decide whether to initiate therapy with a loading dose

monitor the patient for toxicity and for therapeutic effect

adjust the dose Dose titration may be accomplished by either of two general approaches that differ in dosage and frequency of administration, but reach the same total amount of digoxin accumulated in the body.

If rapid titration is considered medically appropriate, administer a loading dose based upon projected peak digoxin body stores. Maintenance dose can be calculated as a percentage of the loading dose.

More gradual titration may be obtained by beginning an appropriate maintenance dose, thus allowing digoxin body stores to accumulate slowly. Steady-state serum digoxin concentrations will be achieved in approximately five half-lives of the drug for the individual patient. Depending upon the patient’s renal function, this will take between 1 and 3 weeks.

2.2 Serum Digoxin

Concentrations In general, the dose of digoxin used should be determined on clinical grounds. However, measurement of serum digoxin concentrations can be helpful to the clinician in determining the adequacy of digoxin therapy and in assigning certain probabilities to the likelihood of digoxin intoxication. Studies have shown diminished efficacy at serum levels < 0.5 ng/mL, while levels above 2 ng/mL are associated with increased toxicity without increased benefit. The inotropic effects of digoxin tend to appear at lower concentrations than the electrophysiological effects. Based on retrospective analysis, adverse events may be higher in the upper therapeutic range. Perform sampling of serum concentrations just before the next scheduled dose of the drug. If this is not possible, sample at least 6 hours or later after the last dose, regardless of the route of administration or the formulation used. On a once-daily dosing schedule, the concentration of digoxin will be 10% to 25% lower when sampled at 24 versus 8 hours, depending upon the patient’s renal function. On a twice-daily dosing schedule, there will be only minor differences in serum digoxin concentrations whether sampling is done at 8 or 12 hours after a dose. The serum concentration of digoxin should always be interpreted in the overall clinical context, and an isolated measurement should not be used alone as the basis for increasing or decreasing the dose of the drug. When decision-making is to be guided by serum digoxin levels, the clinician must consider the possibility of reported concentrations that have been falsely elevated by endogenous digoxin-like immunoreactive substances. If the assay being used is sensitive to these substances, it may be prudent to obtain a baseline measurement before digoxin therapy is started, and correct later values by the reported baseline level.

2.3 Loading

Dose Loading doses for each age group are given in Table 1 below. In pediatric patients, if a loading dose is needed, it can be administered with roughly half the total given as the first dose. Additional fractions of this planned total dose may be given at 4- to 8-hour intervals, with careful assessment of clinical response before each additional dose. If the patient’s clinical response necessitates a change from the calculated loading dose of digoxin, then calculation of the maintenance dose should be based upon the amount actually given as the loading dose.

Table 1: Estimate the Loading Dose Age Oral Loading Dose, mcg/kg Premature 20 to 30 Full-Term 25 to 35 1 to 24 months 35 to 60 2 to 5 years 30 to 45 5 to 10 years 20 to 35 Over 10 years 10 to 15 More gradual attainment of digoxin levels can also be accomplished by beginning an appropriate maintenance dose. The range of percentages provided in Table 2 (2.4 Estimate of Daily Maintenance Dose) can be used in calculating this dose for patients with normal renal function. Steady state will be attained after approximately 5 days in subjects with normal renal function.

2.4 Estimate of

Daily Maintenance Dose The recommended daily maintenance doses for each age group are given in Table 2 below. These recommendations assume the presence of normal renal function.

Table 2: Estimate of the Daily Maintenance Dose Age Daily Oral Maintenance Dose, mcg/kg/day Dose Regimen, mcg/kg/dose Premature 4.7 to 7.8 2.3 to

3.9 Twice Daily

Full-Term 7.5 to 11.3 3.8 to 5.6 Twice Daily 1 to 24 months 11.3 to 18.8 5.6 to 9.4 Twice Daily 2 to 5 years 9.4 to 13.1 4.7 to 6.6 Twice Daily 5 to 10 years 5.6 to 11.3 2.8 to

5.6 Twice Daily

Over 10 years 3.0 to 4.5 3.0 to

4.5 Once Daily

Dosage guidelines provided are based upon average patient response and substantial individual variation can be expected. Accordingly, dosage selection must be based upon clinical assessment and ultimately therapeutic drug level monitoring of the patient. Divided daily dosing is recommended for pediatric patients under age 10. In the newborn period, renal clearance of digoxin is diminished and suitable dosage adjustments must be made as shown in Tables 1 and 2. Renal clearance is further reduced in the premature infant. Beyond the immediate newborn period, pediatric patients generally require proportionally larger doses than adults on the basis of body weight or body surface area. Pediatric patients over 10 years of age require adult dosages in proportion to their body weight. Some researchers have suggested that infants and young pediatric patients tolerate slightly higher serum concentrations than do adults. For pediatric patients with known or suspected renal dysfunction, lower starting doses should be considered combined with frequent monitoring of digoxin levels.

NOTE: The calibrated oral syringe supplied with the 60 mL bottle of digoxin oral solution is not appropriate to measure doses below 0.1 mL. Doses less than 0.1 mL require appropriate methods or measuring devices designed to administer an accurate amount to the patient.

2.5 Adjustment of

Dose The body’s handling of digoxin can be affected by many different patient-specific factors. Some of the possible effects are small, so anticipatory dose adjustment might not be required, but others should be considered before initial dosing. Both adults and pediatric patients with abnormal renal function need to have the dose of digoxin proportionally reduced. Recommended maintenance doses based upon lean body weight and renal function are listed in Table 3. Developmental changes in pediatric renal function were factored into Table 3. However, age-related and other changes in adult renal function were not. The volume of distribution of digoxin is proportional to lean body weight and doses listed in Table 3 assume average body composition. The dose of digoxin must be reduced in patients whose lean weight is an abnormally small fraction of their total body mass because of obesity or edema. Table 3: Usual Maintenance Dose The doses are rounded to whole numbers. Twice daily dosing is recommended for pediatric patients under 10 years of age. Once daily dosing is recommended for pediatric patients above 10 years of age and adults. Requirements (mcg) of Digoxin Based upon Age, Lean Body Weight and Renal Function Corrected Ccr (mL/min per 70 kg) Ccr is creatinine clearance, corrected to 70 kg body weight or 1.73 m 2 body surface area. For adults, if only serum creatinine concentrations (Scr) are available, a Ccr (corrected to 70 kg body weight) may be estimated in men as (140 – Age)/Scr. For women, this result should be multiplied by 0.85. Note: This equation cannot be used for estimating creatinine clearance in infants or pediatric patients. For pediatric patients, the modified Schwartz equation may be used as listed below. The formula was based on height in cm and Scr in mg/dL where k is a constant. Ccr is corrected to 1.73 m 2 body surface area. During the first year of life, the value of k is 0.33 for pre-term babies and 0.45 for term infants. The k is 0.55 for pediatric patients and adolescent girls and 0.7 for adolescent boys. GFR (mL/min/1.73 m 2 ) = (k x Height)/Scr. Dose to be Given Twice Daily < 10 Years of Age Dose to be Given Once Daily > 10 Years of Age and Adults Number of Days Before Steady State Achieved Lean Body Weight Lean Body Weight kg lb 5 11 10 22 20 44 30 66 40 88 50 110 60 132 40 88 50 110 60 132 70 154 80 176 90 198 100 220 10 10 20 40 60 80 100 120 80 100 120 140 160 180 200 19 20 11 23 45 68 90 113 135 90 113 135 158 180 203 225 16 30 13 25 50 75 100 125 150 100 125 150 175 200 225 250 14 40 14 28 55 83 110 138 165 110 138 165 193 220 248 275 13 50 15 30 60 90 120 150 180 120 150 180 210 240 270 300 12 60 16 33 65 98 130 163 195 130 163 195 228 260 293 325 11 70 18 35 70 105 140 175 210 140 175 210 245 280 315 350 10 80 19 38 75 113 150 188 225 150 188 225 263 300 338 375 9 90 20 40 80 120 160 200 240 160 200 240 280 320 360 400 8 100 21 43 85 128 170 213 255 170 213 255 298 340 383 425 7 Determination of the target dose in milliliters of Digoxin Oral Solution based on body weight is shown in Table 4. Provided is the volume required per dose, NOT per day. Table 4: Dose a in Milliliters Target Dose in mcg/kg Volume to be Given in mL b 2 3 4 5 6 8 10 12 14 16 18 20 30 Weight in kg 2 0.08 b 0.12 b 0.16 b 0.2 0.2 0.3 0.4 0.5 0.6 0.6 0.7 0.8 1.2 3 0.12 b 0.18 b 0.2 0.3 0.4 0.5 0.6 0.7 0.8 1.0 1.1 1.2 1.8 4 0.16 b 0.2 0.3 0.4 0.5 0.6 0.8 1.0 1.1 1.3 1.4 1.6 2.4 5 0.2 0.3 0.4 0.5 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 3.0 6 0.2 0.4 0.5 0.6 0.7 1.0 1.2 1.4 1.7 1.9 2.2 2.4 3.6 7 0.3 0.4 0.6 0.7 0.8 1.1 1.4 1.7 2.0 2.2 2.5 2.8 4.2 8 0.3 0.5 0.6 0.8 1.0 1.3 1.6 1.9 2.2 2.6 2.9 3.2 4.8 9 0.4 0.5 0.7 0.9 1.1 1.4 1.8 2.2 2.5 2.9 3.2 3.6 5.4 10 0.4 0.6 0.8 1.0 1.2 1.6 2.0 2.4 2.8 3.2 3.6 4.0 6.0 11 0.4 0.7 0.9 1.1 1.3 1.8 2.2 2.6 3.1 3.5 4.0 4.4 6.6 12 0.5 0.7 1.0 1.2 1.4 1.9 2.4 2.9 3.4 3.8 4.3 4.8 7.2 13 0.5 0.8 1.0 1.3 1.6 2.1 2.6 3.1 3.6 4.2 4.7 5.2 7.8 14 0.6 0.8 1.1 1.4 1.7 2.2 2.8 3.4 3.9 4.5 5.0 5.6 8.4 15 0.6 0.9 1.2 1.5 1.8 2.4 3.0 3.6 4.2 4.8 5.4 6.0 9.0 20 0.8 1.2 1.6 2.0 2.4 3.2 4.0 4.8 5.6 6.4 7.2 8.0 12.0 30 1.2 1.8 2.4 3.0 3.6 4.8 6.0 7.2 8.4 9.6 10.8 12.0 18.0 40 1.6 2.4 3.2 4.0 4.8 6.4 8.0 9.6 11.2 12.8 14.4 16.0 24.0 50 2.0 3.0 4.0 5.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 30.0 60 2.4 3.6 4.8 6.0 7.2 9.6 12.0 14.4 16.8 19.2 21.6 24.0 36.0 70 2.8 4.2 5.6 7.0 8.4 11.2 14.0 16.8 19.6 22.4 25.2 28.0 42.0 80 3.2 4.8 6.4 8.0 9.6 12.8 16.0 19.2 22.4 25.6 28.8 32.0 48.0 90 3.6 5.4 7.2 9.0 10.8 14.4 18.0 21.6 25.2 28.8 32.4 36.0 54.0 100 4.0 6.0 8.0 10.0 12.0 16.0 20.0 24.0 28.0 32.0 36.0 40.0 60.0 a Recommended dosing regimen for pediatric patients under 10 years of age is twice daily. Recommended dosing regimen for pediatric patients over 10 years of age and adults is once daily. b Use calibrated oral syringe for measurement. In the case of required volume less than 0.1 mL, a separate device is recommended for adequate measurement. On the left side of the chart, locate the patient’s weight in kilograms. At the top of the chart, identify which dose in mcg/kg will be used for this patient. The block on the chart at which the two rows (weight and target dose) intersect is the milliliter amount that should be given to the patient. The monitoring described in Section 2.2 may suggest increases or decreases in digoxin doses. Additional monitoring, and in some cases anticipatory dose adjustment, may be indicated around the time of various changes to the patient including:

normal development through childhood;

concomitant drug use should be considered when adjusting the estimated digoxin dose;

new co-administration of an antibiotic, especially if the patient had required high doses of digoxin in order to achieve modest serum concentrations, raising the suspicion that a substantial fraction of administered digoxin was being destroyed by colonic bacteria; and

changes in renal function. right arrow down arrow