Beta-Blockers, ACE Inhibitors, and the Cardiac Receptor Map
Cardiac pharmacology becomes far less overwhelming when you see it as a map of receptor locks and drug keys. Every cardiac medication targets a specific receptor on the cell membrane security gate. Once you know what that receptor normally does, you can predict exactly what the drug will do — and what cues to collect when something goes wrong.
| Receptor | Normal Job | Drugs That Block It | Effect of Block | Key Cues to Collect |
|---|---|---|---|---|
| Beta-1 (heart) | ↑ Heart rate, ↑ contractility | Metoprolol, Atenolol, Carvedilol | ↓ HR, ↓ BP, ↓ cardiac workload | HR <60 = hold & report; dizziness, fatigue |
| Beta-2 (lungs) | Bronchodilation | Non-selective beta-blockers (propranolol) | Bronchoconstriction risk | Wheezing, SOB — report if COPD/asthma history |
| ACE (enzyme) | Converts Angiotensin I → II (vasoconstriction + aldosterone) | Lisinopril, Enalapril (-pril) | Vasodilation, ↓ BP, ↓ aldosterone → less fluid retention | Dry cough, angioedema (face/throat) = stop & report |
| Angiotensin II Receptor | Vasoconstriction signal | Losartan, Valsartan (-sartan) | Vasodilation, ↓ BP — no cough side effect | Hyperkalemia; dizziness on standing |
| Aldosterone Receptor | Sodium & water retention | Spironolactone (K⁺-sparing diuretic) | Diuresis without K⁺ loss | Hyperkalemia risk; gynecomastia in males |
Digoxin strengthens cardiac contractions by blocking the Na⁺/K⁺ pump at the cell membrane, increasing intracellular calcium. Narrow therapeutic window: 0.5 – 2.0 ng/mL. Hypokalemia dramatically increases toxicity risk because low extracellular K⁺ increases digoxin binding.
Before giving digoxin: Check apical pulse for one full minute. Hold and report if HR <60 in adults (or per facility parameter). Check K⁺ level. Report nausea, vomiting, yellow-green visual disturbances — early toxicity cues.
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