Calculating Drug Doses

Terminology

Solute — drug or other substance being dissolved (measured in mg)
Solvent — medium in which solute is being dissolved (typically sterile water or saline; measured in mL)
Solution — solute dissolved in solvent (the mix) (represented by a percentage or a percent concentration)

Drug Calculation Formula

One formula may be manipulated by division to isolate the unknown variable and solve for "x."

Do not convert the percentage given to you in the problem.

Example: What volume of a 0.083% solution should be given in order to deliver 2.5mg of the drug?

Step 1: Fill in the known values — (2.5mg) = (0.083)(x)(10)

Step 2: Divide both sides by 10 — (2.5/10) = (0.083)(x)

Step 3: Divide both sides by (0.083) in order to isolate and solve for x — (0.25/0.083) = x

Answer: x = 3mL

Ratios

Some medications are labeled as ratio of weight to volume (x:y).

To solve for the percent concentration, divide x by y and multiply by 100.

Example: What is the concentration of a medication listed as 1:250?

Solution: (1/250) x 100 = 0.4%

Practice Problems

Calculate the percent solution of 250 mg dissolved in 10 mL.
How many mg of a drug are in 15 mL of a 10% solution?
How much of a 20% solution do we give in order to deliver 350 mg of a drug?
Which of the following represents a 0.13% solution? A. 1:350 B. 1:150 C. 1:8 D. 1:750

Drugs Used to Treat the Respiratory System — Bronchodilators

Adrenergic Bronchodilators — Short-Acting B2 Agonists (SABAs)

Short-Acting B2 Agonists (SABA) are the most frequently administered class of respiratory medications.

B2 receptors are the most abundant and active receptors in lungs
Primary source of control of airway diameter
Most effective medications for rapid and direct bronchodilation
Medication of choice for acute asthma exacerbation
Can be used in conjunction with steroid medications

Albuterol (Racemic Albuterol)

Indication: Rescue treatment for acute asthma; also used 15–30 min before exercise to prevent exercise-induced bronchospasm.

Action onset: Approx 5 min; 80% effect in 15 min
Action duration: 4–8 hours
Side effects: tremors, palpitations, tachycardia, hypertension
Doses: available in nebulized solution, extended-release tablets, syrup, and MDI

Albuterol contains both S and R isomers:

(S) isomer — Responsible for tremors and heart palpitations commonly associated with giving albuterol
(R) isomer — The R isomer relaxes bronchial smooth muscle

Levalbuterol (Xopenex)

Indication: Rescue treatment for acute asthma; can be used as an alternative to Albuterol.

Side effects: No S isomer thus eliminating side effects seen with Albuterol.

Adrenergic Bronchodilators — Ultra Short Acting

Racemic Epinephrine

Indications: inflammation of upper airway, post extubation stridor, epiglottitis, croup, bronchiolitis, bleeding (bronchoscopy)

Effects last < 3 hrs
Administration: nebulized with saline or directly instilled as seen with bronchoscopy

Epinephrine

Lacks receptor specificity thus causing high prevalence of side effects.

Indications: systemic vasoconstriction; primary treatment for anaphylaxis; cardiac stimulant; asthma exacerbation (not for daily maintenance)

Administration:

SQ injection (cardiac)
Inhalation (asthma)
IM (Epipen)

Adrenergic Bronchodilators — Long Acting (LABAs)

Indications:

Maintenance treatment of moderate to severe asthma in patients greater than 5 years old
COPD
Nocturnal dyspnea/orthopnea
Management of chronic airway disease and reactivity

Duration: 12 hrs

Commonly paired with anti-inflammatory agents/corticosteroids.

Examples: Salmeterol (Serevent); Formoterol (Foradil/Perforomist) — your "eterol" drugs

Anticholinergics

Indications: maintenance of COPD, emphysema and chronic bronchitis.

Used secondary to or in conjunction with SABAs.

Note: patients will be prescribed a rescue inhaler/SABA for emergency relief.

Ipratropium Bromide (Atrovent) — Short Acting (SAMA)

Duration: up to 6 hours
Available Doses: MDI 17mcg/puff; Nebulized solution 2.5mL in 0.02%

Tiotropium Bromide (Spiriva) — Long Acting Muscarinic Antagonist (LAMA)

Slows rate of progression of COPD
Improves FEV1 (how much air a person can exhale in the first second of a forced breath to assess lung function and other obstructive diseases) and patient's quality of life
Duration: 24 hours
Available Doses: 18mcg/puff

Combination Bronchodilators

Albuterol + Ipratropium Bromide (SABA + SAMA)

Indications: COPD patients who require additional bronchodilation

Brand Names: Combivent; Duoneb

Duration: 4–6 hours

Monitoring Bronchodilator Effects

Peak expiratory flow (PEF) measurements: used for determining airflow obstruction and trending patterns.

Peak Flow Meter: measures maximum expiratory flow.

Therapy is indicated in patients whose peak flow measures less than 80% of their predicted PEF value.

PEF Setup Instructions

Refer to the instruction insert to establish "Predicted PEF"
Locate instruction portion for Sex
Locate patient's Age
Locate patient's Height
"Predicted PEF" is where age & height cross
Attach color strip per instructions

PEF Performance Instructions

Reference device insert to determine patient's predicted peak flow
Set predicted goal value on the device
Instruct patient to inspire to a maximum deep breath
Instruct patient to blow out as strong as they can in a rapid burst effort into the peak flow meter device (exhalation to residual volume is unnecessary)
The peak flow measurement is repeated a total of 3 times
Record BEST of 3 efforts in the patient's chart

PEF Zones

"Personal Best" — established by daily measurements as an outpatient, their airflow pattern is stable and patient is asymptomatic. Compared to the patient's personal best (baseline) when available.

| Zone | PEF Range | Action | |------|-----------|--------| | Green Zone | 80%–100% of personal best | Normal / no action required | | Yellow Zone | 60%–80% of personal best | Requires self-administration of bronchodilator plus possibly oral steroids | | Red Zone | < 60% of personal best | Requires self-administration of bronchodilator, contacting the physician, and calling 9-1-1 |

Xanthines (Methylxanthines)

Xanthines act like stimulants:

Increases cardiac muscle contractility
Increases respiratory muscle endurance
Increases CNS alertness
Diuresis (similar to coffee)

Theophylline