Medications That Affect Preload
Vasodilators, particularly nitrates (nitroglycerin and nitroprusside), diuretics, and opiates are the primary medications that reduce preload, while vasopressors (norepinephrine, phenylephrine, dopamine) increase preload through venoconstriction.
Preload-Reducing Medications
Vasodilators
Intravenous vasodilators work by decreasing venous tone to optimize preload 1. These agents have dual benefit:
- Decrease venous tone (reducing preload)
- Decrease arterial tone (reducing afterload)
Nitrates are the cornerstone preload reducers:
Nitroglycerin: Infused at 5-200 μg/min, increased by 5 μg/min every 5 minutes 2
Sodium nitroprusside: 0.3-10 μg/kg/min, increased by 0.5 μg/kg/min every 5 minutes 2
Important caveat: The predominant site of nitroglycerin action depends on baseline hemodynamics. In patients with excessive afterload (Ea/Ees ratio >1) or abnormal ventricular function, nitroglycerin acts predominantly as an afterload reducer rather than preload reducer 3. This is critical to understand when selecting agents.
Diuretics
Loop diuretics reduce preload through volume depletion 1:
- Furosemide: 20-40 mg IV bolus for diuretic-naive patients; higher doses for those previously on diuretics 1
- Torasemide: 10-20 mg IV as alternative 1
Warning: While diuretics reduce preload, they can impair left ventricular diastolic function. Hydrochlorothiazide decreased normalized peak flow rate from 2.2 to 1.9 vol/s in hypertensive patients 4, demonstrating that excessive preload reduction can be detrimental.
Opiates
Opiates (particularly morphine) relieve dyspnea and reduce preload through venodilation 1:
- Not recommended for routine use in acute heart failure
- May only be cautiously considered in severe dyspnea with pulmonary edema
- Dose-dependent side effects include hypotension and respiratory depression
- Controversial mortality risk - some data suggest elevated mortality in patients receiving morphine 1
Preload-Increasing Medications
Vasopressors
These agents increase preload through peripheral venoconstriction:
Norepinephrine: 0.2-1.0 μg/kg/min 1
- Increases central venous pressure by 23%, left ventricular end-diastolic area by 9%, and global end-diastolic volume by 9% 5
- Simultaneously increases cardiac index by 11% by recruiting cardiac preload reserve 5
- Reduces preload dependency - making patients less responsive to further preload augmentation 5
Phenylephrine: 15-20 μg/min adjusted to restore preload 6
- Pure α1-agonist without inotropic effects
- In preload-dependent patients, phenylephrine infusion increased stroke volume index from 35 to 49 mL/m² and restored preload as measured by stroke volume variation 6
- Increases venous return and abolishes preload dependency 6
Dopamine: At doses >5 μg/kg/min acts as vasopressor (alpha+) 1
- Lower doses (3-5 μg/kg/min) provide inotropic effects (beta+)
- Norepinephrine preferred over dopamine due to fewer side effects and lower mortality in shock patients 1
Clinical Algorithm for Selection
For preload reduction:
- Acute cardiogenic pulmonary edema with hypertension: Nitroprusside first-line 2
- Volume overload without severe hypotension: Loop diuretics 1
- Avoid vasodilators if SBP <90 mmHg or symptomatic hypotension 1
- Use caution with significant mitral or aortic stenosis 1
For preload augmentation:
- Septic shock with preload dependency: Norepinephrine preferred 5
- Intraoperative hypotension with preload dependency: Phenylephrine 6
- Marked hypotension requiring redistribution to vital organs: Norepinephrine or high-dose dopamine, though this increases LV afterload 1
Critical pitfall: Vasopressors increase preload at the expense of increased LV afterload 1, which can be detrimental in certain conditions. The benefit of restored preload must outweigh the harm of increased afterload.