Sympathomimetics

Sympathomimetics are drugs that mimic the effects of stimulation of the sympathetic nervous system. They achieve this by acting as agonists at adrenergic receptors (α1, α2, β1, β2) or by indirectly increasing the concentration of endogenous catecholamines at the synaptic cleft. The endogenous catecholamines—epinephrine, norepinephrine, and dopamine—display variable physiologic effects across the dosing range and substantial patient variability in dose–response. These agents form the backbone of haemodynamic support in shock states, alongside exogenous agents such as dobutamine, isoproterenol, and phenylephrine. ^[2]

The clinical utility of each agent depends on its receptor selectivity profile, which determines whether it acts predominantly as a vasoconstrictor, inotrope, chronotrope, or vasodilator. Understanding these profiles allows rational selection in cardiogenic, septic, and hypovolemic shock. The treatment of shock involves administration of endogenous catecholamines as well as various vasopressor agents that have shown efficacy. ^[2]

Sympathomimetics produce their adverse effects by overstimulating both α- and β-adrenergic receptors, as well as central nervous system (CNS) pathways. The clinical picture depends on the specific agent, dose, route, and duration of use. Adverse effects can occur with both oral and intranasal administration after a single dose or after prolonged (5 days) treatment, without affecting the dose and irrespective of the vascular condition and age. ^[6]

Stimulation of the sympathetic system speeds up breathing, increases blood pressure, accelerates heart rate, and narrows peripheral blood vessels. These effects translate clinically into hypertension, tachycardia, palpitations, and in susceptible individuals, arrhythmias or myocardial ischaemia. The risk is heightened in patients with pre-existing cardiovascular disease, particularly hypertension and coronary artery disease.

Management: For hypertensive crisis, give a short-acting intravenous antihypertensive such as labetalol or nitroprusside. Tachyarrhythmias may respond to a cardioselective β-blocker (e.g., esmolol), but avoid non-selective β-blockers as unopposed α-stimulation can worsen hypertension.

Sympathomimetics stimulate the CNS, giving a sense of energy surge and improving mood. Excessive stimulation causes agitation, anxiety, insomnia, confusion, and hallucinations. Severe cases may progress to seizures and coma. A 64-year-old man with renal failure developed unusual behaviour and myoclonic convulsions after taking 240 mg of pseudoephedrine (PSE) daily for rhinitis. Severe agitation and disorientation can be expected in patients with phenylketonuria due to a disturbed metabolism of catecholamines. ^[6]

Management: Benzodiazepines (e.g., diazepam 5–10 mg IV) are first-line for agitation and seizures. Avoid phenytoin for sympathomimetic-induced seizures as it is often ineffective. For psychotic features, haloperidol may be used cautiously, but beware of lowering the seizure threshold.

Sympathomimetics increase blood glucose levels, which can destabilise glycaemic control in diabetic patients. Hyperthyroidism is a contraindication because excess thyroid hormone sensitises the cardiovascular system to catecholamines.

Management: Monitor blood glucose; use insulin sliding scales if hyperglycaemia is significant. In thyrotoxic patients, β-blockade (propranolol) may be needed for symptom control.

The risk of complications increases in patients with impaired renal and hepatic function. Reduced clearance leads to drug accumulation and enhanced toxicity. The case of the 64-year-old man with renal failure illustrates this: his myoclonic convulsions occurred at a modest dose of 240 mg/day, well below the usual toxic threshold. ^[6]

Management: Dose reduction or avoidance in moderate-to-severe renal dysfunction and severe liver dysfunction. Monitor renal function and adjust dosing intervals accordingly.

Myoclonic convulsions, as described above, are a recognised adverse effect in patients with renal impairment. Generalised seizures may also occur in severe overdose.

Management: Benzodiazepines; correct any metabolic derangements (e.g., uraemia, electrolyte imbalance).

Agitation and mental agitation are common, and can be severe enough to constitute a contraindication to use. Euphoria is a sought-after effect in misuse, but can progress to toxic psychosis with hallucinations. A 37-year-old woman abused PSE for its euphoric effect, increasing doses over five years to 3000–4500 mg daily; sudden discontinuation resulted in depressed mood, visual hallucinations, and fatigue. ^[6]

Management: Supportive care, benzodiazepines for agitation, and antipsychotics for persistent psychosis. Withdrawal symptoms respond to gradual tapering where dependence has developed. ^[5]

Many over-the-counter sympathomimetic formulations also contain antihistamines such as chlorpheniramine or promethazine. Individuals abusing chlorpheniramine-containing dextromethorphan (DXM) formulations may exhibit anticholinergic signs and symptoms, including dry mouth, urinary retention, blurred vision, and confusion. Promethazine misuse, often as "purple drank" mixed with soft drink and/or alcohol, causes sedation and enhances the effects of co-ingested substances.

Management: Physostigmine for severe anticholinergic syndrome (delirium, tachycardia, mydriasis, ileus). Supportive care and monitoring for co-ingestant toxicity.

Interactions between DXM and selective serotonin reuptake inhibitors (SSRIs) or monoamine oxidase inhibitors (MAOIs) might further increase the risk of a serotoninergic syndrome occurrence. This presents with autonomic instability, hyperthermia, clonus, and altered mental status. ^[5]

Management: Stop the offending agents, give supportive care, cooling, benzodiazepines, and cyproheptadine (12 mg initially, then 2 mg every 2 hours) in moderate-to-severe cases.

Cough formulations frequently contain acetaminophen, which produces delayed hepatic injury in overdose. This is particularly relevant in chronic misuse where daily supratherapeutic doses accumulate. ^[5]

Management: N-acetylcysteine per standard protocols (IV or oral), even if the ingestion was staggered. Check paracetamol levels and liver function tests.

Norepinephrine is an α1-adrenergic receptor agonist with modest β-agonist activity, making it a vasoconstrictor but a less potent inotrope. Because it is virtually a "pure" vasoconstrictor, it may reduce cardiac output in patients with cardiac dysfunction due to increased afterload, although many patients with cardiogenic shock can maintain cardiac output during therapy. It has minimal chronotropic effects, making it useful when heart rate stimulation is undesirable. Norepinephrine increases both systolic and diastolic blood pressures, thereby increasing coronary blood flow and potentially improving cardiac function indirectly. It is the first-line vasopressor for all forms of shock with severe hypotension. Norepinephrine is slightly lower in potency than epinephrine and approximately 100-fold more potent than dopamine for raising mean arterial pressure. It carries a lower tachyarrhythmia risk than either dopamine or epinephrine. However, prolonged infusion can have a direct toxic effect on cardiac myocytes by inducing apoptosis via protein kinase A activation and increased cytosolic calcium influx. ^[2]

Epinephrine has equal affinity at both α1 and α2 receptors. It is more effective than norepinephrine or dopamine in increasing cardiac output in septic shock. In patients with septic shock and a MAP under 70 mm Hg despite norepinephrine infusion, adding epinephrine increases MAP, heart rate, and cardiac index more than adding dobutamine does. Epinephrine is approximately 100-fold more potent than dobutamine or dopamine. It is the vasopressor of choice in septic shock refractory to high-dose norepinephrine, especially when heart rate is high or cardiac output is low. Randomized trials have shown similar mortality with epinephrine or norepinephrine in patients with shock. The CATS trial comparing norepinephrine plus dobutamine to epinephrine alone for septic shock showed similar mortality rates and adverse events at 90 days despite more lactic acidosis in the epinephrine group. ^[2]

Dopamine binds weakly to β1-adrenergic receptors but has high binding affinity at dopamine receptors and at trace amine-associated receptor 1 (TAAR1). At low doses, it inhibits the release of norepinephrine in peripheral blood vessels, acting as a mild vasodilator. It also inhibits the re-uptake of norepinephrine in presynaptic sympathetic nerve terminals, resulting in an indirect increase in cardiac contractility and heart rate. The direct vasodilator effect tends to offset the indirect vasoconstriction, so there is usually only a mild increase in systemic vascular resistance. The net eff ^[2]

Dobutamine is an exogenous catecholamine used as an inotrope. Its varying dose-related effects on the peripheral vasculature result from counterbalancing α1-mediated vasoconstriction and β2-mediated vasodilation. At doses up to 5 mcg/kg/min, stroke volume usually increases without significant tachycardia, but above 10 mcg/kg/min tachycardia worsens without a parallel increase in cardiac output due to decreased diastolic filling time. This problem can be addressed more effectively by combining low-dose dopamine with low-dose dobutamine rather than increasing dobutamine alone. The ACC/AHA guidelines recommend dobutamine as the first-line inotropic agent if systolic blood pressure is between 70 and 100 mm Hg in the absence of shock. More recent guidelines recommend a combination of norepinephrine and dobutamine over dopamine for cardiogenic shock. The combination of dopamine and dobutamine at 7.5 mcg/kg/min each improves cardiac hemodynamics while limiting side effects better than either agent alone at 15 mcg/kg/min. ^[2]

Isoproterenol is an analog of epinephrine and a non-selective β-adrenergic agonist with low affinity for α-adrenergic receptors. Its usefulness as a strong inotrope with both systemic and pulmonary vasodilatory actions is limited primarily by its profound chronotropic effect. It is a more potent vasodilator than dobutamine and can significantly improve the microcirculation, especially in septic shock where it improves both mixed venous oxygen saturation and cardiac index. It is useful as adjunctive therapy for cardiac arrest, congestive heart failure, and all three types of shock. ^[2]

Phenylephrine is an exogenous catecholamine that has served as a mainstay of shock therapy for decades. In septic shock, norepinephrine is more effective than the combined therapy of dopamine and vasopressin with the addition of phenylephrine in non-responders. ^[2]

Albuterol is a selective β2 agonist primarily used as a bronchodilator; its β2-mediated vasodilation and mild chronotropic effects are relevant when considering systemic sympathomimetic actions.

Fenoldopam is a peripheral dopamine D1 receptor agonist that produces vasodilation, particularly in the renal vasculature; it is not a catecholamine but acts within the sympathomimetic pathway.

Midodrine is an oral α1 agonist used for orthostatic hypotension; it produces vasoconstriction without significant central nervous system effects.

Mirabegron is a β3 agonist used for overactive bladder; it relaxes the detrusor muscle.

Amphetamine is an indirect sympathomimetic that increases synaptic norepinephrine and dopamine by promoting release and inhibiting re-uptake, producing central stimulation and peripheral sympathomimetic effects.

Sympathomimetics are a heterogeneous class of drugs that mimic the effects of stimulation of the sympathetic nervous system. Their pharmacodynamic profile depends on whether they act directly on adrenergic receptors, indirectly by releasing endogenous catecholamines, or through a mixed mechanism. Pseudoephedrine (PSE) exemplifies a mixed mechanism: it indirectly stimulates alpha-adrenergic receptors by causing the release of endogenous norepinephrine (NE) from the granularity of neurons, while it directly stimulates beta-adrenergic receptors. This dual action underpins its clinical effects as a nasal decongestant and its potential for systemic adverse effects. ^[6]

The dose-response relationship for sympathomimetics is steep and varies by agent. Pseudoephedrine exists as four diastereoisomers due to its two stereogenic carbon atoms; synthetically obtained compounds occur as a racemate whose action is twice as weak compared to compounds of natural origin, because only one active diastereoisomer is present while the second acts as ballast. This stereochemical nuance explains why natural-source PSE is more potent per milligram than synthetic racemic preparations. As an E stereoisomer, PSE has weaker vasoconstrictive effects and smaller effects on the central nervous system (CNS) compared to ephedrine, making it relatively safer but still capable of significant toxicity at high doses. ^[6]

Pharmacodynamic effects by organ system are predictable from adrenergic receptor activation. In the cardiovascular system, alpha-1 receptor stimulation causes vasoconstriction and increased blood pressure, while beta-1 receptor activation increases myocardial excitability and can affect ventricular rhythm, especially in patients with cardiac diseases who are hypersensitive to the cardiac effects of sympathomimetics. The combination of PSE with caffeine—present in many OTC medications, dietary supplements, and energy drinks—may cause hyperglycaemia and an increase in body temperature. In the CNS, effects are dose-dependent and biphasic: overdose may produce either a depressive effect (sedation, apnoea, decreased ability to concentrate, cyanosis, coma, and circulatory collapse) or a stimulating effect (insomnia, hallucinations, tremors, and convulsions); in extreme cases, death may occur. Prolonged use of PSE, especially at short intervals, reduces drug effectiveness through tachyphylaxis and increases the risk of toxic effects. ^[6]

The pharmacokinetic properties of sympathomimetics vary considerably across the class, but several agents share important features relevant to clinical use and toxicity. Pseudoephedrine is well absorbed orally, and its maximum permissible daily dose is 240 mg for an adult, 120 mg for children aged 6–12 years, and 60 mg for children aged 2–5 years. These dosing limits reflect both efficacy and safety thresholds; exceeding them increases the risk of toxic effects, particularly in individuals who are especially sensitive to sympathomimetics. ^[6]

Distribution of sympathomimetics is generally widespread due to their lipophilicity, allowing CNS penetration—a property that underlies both therapeutic effects (e.g., appetite suppression) and adverse psychiatric effects. The metabolism of this class is highly variable. Codeine, though an opioid, illustrates a relevant principle: it is a prodrug requiring metabolic activation by O-demethylation to morphine via CYP2D6. Ultrarapid CYP2D6 metabolizers produce an unexpectedly large amount of morphine, resulting in life-threatening opioid toxicity. While this example concerns an opioid, it highlights how genetic polymorphisms in metabolising enzymes can dramatically alter drug effects—a principle that applies to sympathomimetics metabolised by similar pathways.

Elimination of sympathomimetics occurs primarily via renal excretion of the parent compound and metabolites. The half-life of pseudoephedrine is approximately 4–6 hours, necessitating dosing every 4–6 hours, though the risk of tachyphylaxis increases with short dosing intervals. Drug interactions significantly affect elimination and toxicity. The combination of PSE with other sympathomimetic drugs and monoamine oxidase inhibitors (MAO) should be avoided: inhibition of intra-neuronal NE breakdown by MAO inhibitors increases the amount of neuromediator released by PSE, potentially leading to hypertensive crisis and bradycardia. Due to the long duration of action of MAO inhibitors, a 14-day interval between taking the drugs should be maintained. ^[6]

Dosing routes for sympathomimetics include oral, intranasal, and—in misuse scenarios—intravenous or intramuscular administration. Recreational intake of OTC sympathomimetics is associated with high or very high dosages and idiosyncratic routes of administration such as snorting, IM, or IV. These routes bypass first-pass metabolism, increasing bioavailability and toxicity. Concomitant ingestion with both licit drugs (alcohol, prescription opioids, benzodiazepines, other OTCs) and illicit drugs (c

Sympathomimetics and adrenergic pathways are intimately connected to the pathophysiology of heart failure. Heart failure with preserved left ventricular ejection fraction affects about half of all patients diagnosed with heart failure. This condition accounts for roughly half of all patients with a clinical presentation of heart failure. Understanding the precise boundaries of this syndrome remains challenging because there is no consensus concerning the definition of heart failure with preserved ejection fraction. ^[1]

European guidelines define the condition as a clinical syndrome where classical heart failure symptoms appear alongside a normal or only mildly reduced left ventricular systolic function. Conversely, American Heart Association guidelines and the consensus statement of the European Society of Cardiology define it as a clinical heart failure state accompanied by objective evidence of diastolic dysfunction. Diastolic dysfunction represents an important, if not the main, precursor of heart failure with preserved ejection fraction. ^[1]

The sympathetic nervous system plays a vital role in disease progression. Because the sympathetic nervous system is linked to the development of systolic heart failure, researchers hypothesized that increased sympathetic nerve activation is also related to the development of heart failure with preserved ejection fraction. Animal studies demonstrate that administration of isoproterenol, which increases sympathetic nerve activity, induces this preserved ejection fraction phenotype in experimental settings. Human studies confirm that increased sympathetic nerve activity—regardless of the assessment method—correlates directly with diastolic dysfunction and true heart failure with preserved ejection fraction. ^[1]

Clinical manifestations extend beyond simple relaxation abnormalities. Higher cardiac sympathetic activity is observed in patients with more advanced functional limitations, specifically those in a higher New York Heart Association class. Furthermore, clinical symptoms are not solely explained by diastolic dysfunction; they can also stem from a reduced chronotropic, vasodilator, and cardiac output reserve during exercise. ^[1]

Age acts as a notable confounding variable in this patient population, as older individuals are more frequently affected by preserved ejection fraction heart failure, and age itself is intrinsically related to sympathetic activity. Historically, preserved ejection fraction was often considered a diagnosis of exclusion because the syndrome has only recently been recognized as a major clinical problem. Consequently, the available literature is less extensive than that for reduced ejection fraction heart failure. ^[1]

Sympathomimetics are rarely measured directly in clinical practice; instead, their effects on the sympathetic nervous system (SNA) are inferred through indirect markers. Plasma norepinephrine (NE) concentration and muscle sympathetic nerve activity (MSNA) are two such markers, but their sensitivity is limited. In one analysis, MSNA results were considered reliable, whereas plasma NE-concentration results were excluded because of the limited sensitivity of these markers of sympathetic tone. Heart rate variability (HRV) is another indirect measurement of SNA, but it is not known whether results obtained from HRV lead to over- or underestimation of the relation between disease states and SNA. ^[1]

When interpreting laboratory values in patients receiving sympathomimetics, several confounders must be considered. Most patients studied were treated with drugs that affect sympathetic activity, and since most antihypertensive drugs indirectly lower SNA, observed associations may be underestimations of the true effect. This is particularly relevant when assessing whether elevated sympathetic tone contributes to conditions such as heart failure with preserved ejection fraction. ^[1]

Serial trending of serum lactate is commonly used to guide resuscitation, but epinephrine use may confound this trend. Elevated lactic acid levels can occur even with adequate systemic perfusion, mean arterial pressure, and peripheral oxygen delivery. Therefore, when interpreting lactate results in a patient receiving epinephrine, the clinician should consider drug effect rather than assuming worsening shock. ^[2]

For laboratory use, high-dose epinephrine is defined as doses above 0.3–0.5 mcg/kg/min, with no defined maximum dose for refractory shock. However, prolonged high-dose epinephrine is toxic to arterial walls and causes focal regions of myocardial contraction-band necrosis and myocyte apoptosis. These histological changes may be seen on post-mortem examination and should be recognised as drug-related rather than primary cardiac pathology. ^[2]

Dobutamine, by contrast, is a potent inotrope that directly stimulates β1-receptors and α1 receptors with weak β2 affinity. At doses below 5 mcg/kg/min it causes mild vasodilation, while infusions up to 15 mcg/kg/min increase cardiac contractility without affecting systemic vascular resistance in most patients. These dose-dependent effects are important when interpreting haemodynamic measurements in the laboratory or intensive care setting. ^[2]

Children have higher baseline heart rates and lower stroke volumes, so their cardiac output depends more on rate than on contractility. Consequently, beta-1 agonists such as dobutamine produce a relatively greater chronotropic response in infants than in adults. The immature blood–brain barrier is more permeable, increasing central nervous system effects of ephedrine and amphetamine derivatives. Doses must be weight-based, and intravenous infusions require careful titration because clearance of these drugs is often reduced in neonates due to immature hepatic and renal function.

During pregnancy, uterine blood flow is not autoregulated and depends on maternal mean arterial pressure. Alpha-agonists such as phenylephrine can reduce uteroplacental perfusion, so they are used cautiously, mainly to treat hypotension during spinal anaesthesia for caesarean section. Beta-2 agonists like terbutaline relax uterine smooth muscle and have historically been used as tocolytics, though their use is limited by maternal tachycardia and pulmonary oedema risk. In lactation, small amounts of sympathomimetics pass into breast milk; short-acting agents with low oral bioavailability, such as inhaled beta-2 agonists, are preferred.

Ageing reduces baroreceptor sensitivity and increases arterial stiffness, so the pressor response to alpha-agonists is exaggerated while reflex bradycardia is blunted. Beta-receptor responsiveness declines with age, meaning that beta-agonists produce less bronchodilation and less chronotropy than in younger patients. Polypharmacy is common, and the risk of drug interactions—especially with monoamine oxidase inhibitors, tricyclic antidepressants, and beta-blockers—is higher. Falls and delirium are particular concerns with centrally acting sympathomimetics. ^[4]

The availability and regulation of sympathomimetics vary widely. Ephedrine and pseudoephedrine are controlled precursors for methamphetamine production, so many countries restrict their sale. In low-resource settings, inexpensive drugs like epinephrine and ephedrine remain essential for resuscitation and shock, while newer, more selective agents may be unavailable. Inhaled beta-2 agonists for asthma are on the WHO Model List of Essential Medicines, but access is uneven, contributing to preventable asthma deaths in low-income countries. Conversely, misuse of over-the-counter decongestants and weight-loss products containing sympathomimetics is a global public health concern, particularly where regulation is weak.

Sympathomimetics act by mimicking or enhancing the effects of endogenous catecholamines—noradrenaline, adrenaline, and dopamine—which are neuromodulators that activate G-protein coupled receptors to alter synaptic transmission and ionic conduction^[1]. Their therapeutic use spans shock, heart failure, arrhythmias, and specific cardiomyopathies, but the choice of agent depends on the receptor profile desired and the underlying pathophysiology.

In hypovolemic shock, the simplest form of shock to treat, the addition of intravascular volume (preload) is combined with drugs that increase left ventricular contractility and stroke volume to improve cardiac output. However, the degree to which stroke volume can be enhanced pharmacologically is limited because these drugs also increase heart rate. Therapy of shock, regardless of etiology, demands thorough knowledge of cardiovascular physiology and the pharmacology of the drugs used.

Phenylephrine, a pure α₁-agonist, is invaluable in hypertrophic obstructive cardiomyopathy (HOCM) because it increases afterload by raising systemic vascular resistance (SVR). This enhances the cross-sectional area of the left ventricular outflow tract, thereby decreasing its dynamic gradient during systole caused by septal hypertrophy. ^[2]

Milrinone, a phosphodiesterase 3 inhibitor, increases contractility while significantly decreasing both SVR and pulmonary vascular resistance—a combination that makes it perhaps the most useful drug for low output syndrome following cardiac surgery. It mimics β-1 and β-2 activation by preventing cAMP breakdown. ^[2]

Selepressin, a vasopressin V₁a receptor agonist, has been shown at an infusion rate of 2.5 ng/kg/minute to rapidly replace norepinephrine while maintaining target mean arterial pressure, and may improve fluid balance and shorten mechanical ventilation time in septic shock. Larger randomised controlled trials are ongoing. ^[2]

Levosimendan, a calcium sensitiser, remains controversial in cardiogenic shock; its potential benefit could be clarified by a new randomised trial using large-dose boluses in patients with low baseline ejection fractions. ^[2]

In Brugada syndrome with electrical storm, isoproterenol (a β-agonist) is first-line; class 1A drugs like quinidine should be tested in patients not immediately responding. Beta-blockers, bretylate, lidocaine, mexiletine, and magnesium have been tried without success or may worsen the situation. In the DEBUT study, there was an 18% death rate in Thai patients treated with beta-blocker. ^[3]

In heart failure with preserved ejection fraction (HFPEF), no treatment has convincingly improved clinical status, morbidity, or mortality. Beta-blockers may be useful via sympatholysis—the SENIORS trial suggested nebivolol may be beneficial in elderly patients. RAS inhibition showed no clear benefit in CHARM, and meta-analysis found no consistent reduction in hospitalisation or mortality. Renal denervation, which decreases sympathetic activity, improved echocardiographic parameters and may disrupt the vicious circle between HFPEF and hyperactive sympathetic nervous activity. ^{[1] [2]}

Monoamine oxidase inhibitors (MAOIs): Treatment with MAO inhibitors currently or in the last two weeks is an absolute contraindication to pseudoephedrine use. MAOIs prevent the breakdown of catecholamines, leading to a potentially fatal hypertensive crisis when combined with sympathomimetics. This interaction also applies to DXM, where the combination increases the risk of serotonin syndrome. ^[6]

Selective serotonin reuptake inhibitors (SSRIs): DXM combined with SSRIs increases the risk of serotoninergic syndrome. This is due to impaired metabolism of DXM via CYP2D6, leading to accumulation and enhanced serotonergic activity.

Other CNS depressants (alcohol, opioids): Promethazine is commonly misused with alcohol or codeine ("purple drank"), producing synergistic sedation and respiratory depression. This combination is particularly dangerous in young people due to easy accessibility.

Anticholinergics: Combination with antihistamines (chlorpheniramine, promethazine, diphenhydramine) produces additive anticholinergic effects. This can lead to severe delirium, hyperthermia, and urinary retention.

Acetaminophen (paracetamol): When co-formulated with sympathomimetics, chronic misuse can cause delayed hepatic injury. The patient may not recognise the danger because the sympathomimetic effects mask the early symptoms of acetaminophen toxicity.

Contraindications to pseudoephedrine use include:

• Hypersensitivity to the drug
• Cardiovascular diseases: hypertension and coronary artery disease
• Impaired organ function: severe liver dysfunction, moderate or severe renal dysfunction
• Hyperthyroidism
• Narrow-angle glaucoma
• Benign prostatic hyperplasia (risk of urinary retention)
• Diabetes mellitus (hyperglycaemic effect)
• Mental agitation
• Treatment with MAO inhibitors currently or in the last two weeks
• Pregnancy and lactation
• Age under 2 years

The extended-release form of pseudoephedrine should not be used until the age of 12 years. However, some preparations on the market allow administration to younger patients, e.g., from 7 years of age. ^[6]

Renal and hepatic impairment: The risk of complications increases in patients with impaired renal and hepatic function. Dose adjustment or avoidance is required in moderate-to-severe renal dysfunction and severe liver dysfunction.

Phenylketonuria: Severe agitation and disorientation can be expected in patients with phenylketonuria due to a disturbed metabolism of catecholamines. Avoid sympathomimetics in these patients.

Pregnancy and lactation: These are listed as contraindications to pseudoephedrine use. Sympathomimetics may reduce uteroplacental blood flow and are excreted in breast milk. ^[6]

Elderly: Increased sensitivity to CNS stimulation and cardiovascular effects; use with caution, especially in those with underlying cardiovascular disease.

Although pseudoephedrine is not classically considered highly addictive, the case of the 37-year-old woman abusing 3000–4500 mg daily demonstrates its addictive potential. Sudden discontinuation resulted in depressed mood, visual hallucinations, and fatigue. Similarly, DXM is not thought to have addictive properties, but with chronic use, vulnerable individuals may rapidly develop tolerance, dependence, and withdrawal. Diphenhydramine dependence has resulted from long-term usage of large doses (often over 1,000 mg/day); gradual tapering has been described to alleviate withdrawal symptoms. ^[5]

Management: Gradual tapering of the offending agent; supportive care for withdrawal symptoms; psychiatric referral for underlying substance use disorder.

Sympathomimetics such as pseudoephedrine are on the World Anti-Doping Agency (WADA) prohibited list, which is a mandatory international standard updated annually and has been in effect since 1 January 2021. Athletes must be cautioned about using these agents. ^[6]

Sympathomimetics are agents that mimic the action of the endogenous sympathetic nervous system. In clinical practice, evaluating sympathetic nervous system (SNS) activity can be achieved through several methods. The most straightforward approach is measuring plasma or urinary norepinephrine (NE) levels. However, plasma NE concentrations represent a net balance of removal rates rather than selective release rates, and the precise origin of urinary NE levels remains a subject of ongoing debate. To measure the specific rate of NE release into the circulation, radiotracer technology is employed to quantify local NE spillover using an intravenous infusion of tritiated NE combined with regional blood sampling. Where a direct, real-time measure of efferent postganglionic sympathetic nerve activity is required, muscle sympathetic nerve activity (MSNA) can be recorded using a tungsten microelectrode inserted into a muscle fascicle of the peroneal nerve. ^[1]

When selecting adrenergic agonists in acute cardiac and septic states, understanding receptor specificity dictates clinical utility:

• Epinephrine (Adrenalin): Functions as a nonselective agonist across all adrenergic receptors, including the primary subtypes \alpha_1, \alpha_2, \beta_1, \beta_2, and \beta_3. It increases systemic vascular resistance (SVR) through \alpha_1-dependent vasoconstriction and increases cardiac output (CO) via \beta_1 receptor activation. It is particularly effective for managing acute left ventricular (LV) failure during cardiac surgery, predictably boosting CO, and serves as a key inotrope in hypotensive patients without myocardial ischemia. ^[2]
• Dobutamine: Demonstrates a rapid onset with an elimination half-life of less than 2 minutes, facilitating rapid titration and prompt stabilization of CO in post-cardiotomy states, cardiogenic or septic shock, and hypotension following acute myocardial infarction (MI). It is recommended for acute cardiogenic shock presenting with hypotension, septic shock with myocardial dysfunction, and patients with severe renal failure. ^[2]
• Isoproterenol: Administered as an intravenous infusion at dosages typically ranging from 0.1 to 1–3 mug/min. While generally proarrhythmic in most settings, it functions as an effective agent to control electrical storms and prevent recurrent ventricular fibrillation (VF) in Brugada syndrome. ^[3]

A primary clinical pitfall in managing hemodynamic shock is misclassifying the underlying physiological state and applying inappropriate vasopressor or inotropic therapy.

Cardiogenic shock commonly stems from an acute myocardial infarction, though it can also arise from structural hindrances to cardiac filling, such as pericardial tamponade or valvar stenosis. It begins with an initial hypotension that triggers compensatory endogenous vasoconstrictor release to restore blood pressure. However, despite restoring normal mean arterial pressure (MAP) in hypovolemic and cardiogenic shock via these compensatory mechanisms, mixed venous oxygen saturation (MVO_2) frequently remains decreased in both forms of "cold shock". Furthermore, if cardiogenic shock is driven by pericardial tamponade, pharmacological vasopressors are insufficient; immediate physical intervention to relieve the tamponade is mandatory. ^[2]

Conversely, vasodilatory or distributive shock presents as "warm shock", characterized by excessive arteriolar vasodilatation leading to reduced SVR, hypotension, inadequate peripheral perfusion, and warm extremities. Septic shock is the most common cause of warm shock and represents the overall most frequent type of shock. Treatment relies on drugs that increase SVR to restore MAP, though initiating therapy aimed exclusively at increasing SVR may yield only a modest increase in cardiac output.

Arrhythmia risk is another major hazard. Dobutamine must be used with caution in patients with atrial fibrillation because it enhances conduction velocity through the atrioventricular (AV) node, elevating the risk of developing ventricular fibrillation. In Brugada syndrome, high adrenergic stress can lead to refractory electrical storms; the accompanying \alpha-adrenergic stimulation promotes ST elevation and may overwhelm the antiarrhythmic benefits of \beta-adrenergic stimulation. ^{[2] [3]}

Inotropic and vasopressor agents are tailored according to specific patient comorbidities, receptor status, and organ function.

Milrinone, a phosphodiesterase inhibitor, is administered either as a loading dose of 50 mcg/kg over 10 minutes or started directly at a maintenance dose of 0.5 mcg/kg/min without a loading dose. Because milrinone does not act through \beta_1 receptors, its inotropic effect is preserved in the presence of concurrent \beta-blockers. This makes it effective in chronic heart failure patients who have downregulated or desensitized adrenergic receptors following long-standing \beta-agonist therapy. However, renal impairment significantly prolongs the half-life of milrinone, necessitating dose adjustments in renal failure. ^[2]

When comparing agents in specific clinical profiles:

1. Renal Failure and Contractility: Dobutamine is preferable in severe renal failure and acute cardiogenic shock because it produces a greater increase in myocardial contractility than milrinone.
2. Right Ventricular Dysfunction and Pulmonary Hypertension: Milrinone reduces pulmonary vascular resistance (PVR) to a greater extent than dobutamine, making it the inotrope of choice for right ventricular failure, pulmonary hypertension, or underlying \beta-blocker therapy. Combining milrinone with a direct \beta_1 agonist can further augment CO in severely impaired hearts, but this combination carries more frequent adverse events. ^[2]
3. Refractory Septic Shock: Epinephrine or levosimendan may be required when septic shock is resistant to maximum levels of dopamine. Levosimendan improves microcirculatory flow, renal function, hepatic function, and overall hemodynamics better than dobutamine in septic shock.
4. Cardiogenic Shock Limitations: Levosimendan has limited utility as sole therapy in cardiogenic shock when systolic blood pressure is under 90 mm Hg due to its systemic and pulmonary vasodilatory effects. However, when combined with norepinephrine or intra-aortic balloon counterpulsation, it provides beneficial hemodynamic support without increasing ischemic episodes or tachyarrhythmias. ^[2]