Several other xenobiotics also exert their antihypertensive effect by decreasing the effects of the sympathetic nervous system. Often termed sympatholytics, they can be classified as ganglionic blockers, presynaptic adrenergic blockers, or α1-adrenergic antagonists, depending on their mechanism of action. These drugs are rarely used clinically, and little is known about their effects in overdose.
Presynaptic Adrenergic Antagonists
These xenobiotics exert their sympatholytic action by decreasing norepinephrine release from presynaptic nerve terminals. Whereas guanethidine and guanadrel interfere with the action potential that triggers norepinephrine release,224 reserpine depletes norepinephrine, serotonin, and other catecholamines from the presynaptic nerve terminals, probably by direct binding and inactivation of catecholamine storage vesicles.84 Adverse events limit their clinical usefulness. These effects include a high incidence of orthostatic and exercise-induced hypotension, diarrhea, increased gastric secretions, and impotence.179 In addition, this hypotensive effect may be prolonged for as long as one week.119,225 Because of its ability to cross the blood–brain barrier, reserpine may also deplete central catecholamines and produce drowsiness, extrapyramidal symptoms, hallucinations, migraine headaches, or depression.142 In overdose, an extension of their pharmacologic effects is expected. Patients with severe orthostatic hypotension should be anticipated and treated with IV crystalloid boluses and a direct-acting vasopressor. If reserpine is involved, significant CNS depression should also be anticipated.142
Peripheral α1-Adrenergic Antagonists
The selective α1-adrenergic antagonists include prazosin, terazosin, and doxazosin. The α1 receptor is a postsynaptic receptor primarily located on vascular smooth muscle, although they are also found in the eye and in the GI and genitourinary tracts.49,107 In fact, these xenobiotics provide first line pharmacologic therapy for patients with urinary dysfunction secondary to benign prostatic hyperplasia.136 They produce arterial smooth muscle relaxation, vasodilation, and a reduction of the blood pressure. Although better tolerated than ganglionic blockers and peripheral adrenergic neuron blockers, they may still produce significant symptoms of postural hypotension, including lightheadedness, syncope, or palpitations, particularly after the first dose or if the dosing is rapidly increased.17 Hypotension and CNS depression ranging from lethargy to coma are reported in overdose.135,140,216 In addition, priapism may occur.140,208 Treatment includes supportive care, IV crystalloid boluses, and a vasopressor, with phenylephrine being a logical initial choice.
These xenobiotics produce vascular smooth muscle relaxation independent of innervation or known pharmacologic receptors.60,118,126 This vasodilatory effect has been attributed to stimulation of nitric oxide release from vascular endothelial cells. The nitric oxide then diffuses into the underlying smooth muscle cells, stimulating guanylate cyclase to produce cyclic guanosine monophosphate (cGMP). This second messenger indirectly inhibits calcium entry into the smooth muscle cells, producing vasodilation.215 Minoxidil, however, also has direct potassium channel activation effects.128,176 It has been proposed that the opening of these adenosine triphosphate linked potassium channels results in potassium influx and cell depolarization, thereby reducing calcium influx and ultimately relaxing vascular smooth muscle.33
As this vasodilation occurs, the baroreceptor reflexes, which remain intact, produce an increased sympathetic outflow to the myocardium, resulting in an increase in heart rate and contractile force. Typically, these xenobiotics are used therapeutically in patients with severe, refractory hypertension and in conjunction with a β-adrenergic antagonist to diminish reflex tachycardia. Hydralazine, minoxidil, and diazoxide are effective orally, but sodium nitroprusside is only used IV. Minoxidil is also used topically in a 2% solution to promote hair growth, and significant poisoning has occurred in suicidal adults who have ingested this formulation.68,160 Diazoxide, although previously used to rapidly reduce blood pressure in hypertensive emergencies, is rarely used for this indication now as a consequence of its poor ability to titrate and its variable, and occasionally profound, hypotensive effect.125
Adverse effects associated with daily hydralazine use include several immunologic phenomena such as hemolytic anemia, vasculitis, acute glomerulonephritis, and most notably a lupuslike syndrome.196 Minoxidil may cause changes on ECG, both in therapeutic doses and in overdose. Sinus tachycardia, ST segment depression, and T-wave inversion are all reported.94,198,232 There also appears to be an association with supratherapeutic doses of minoxidil and left ventricular multifocal, subacute necrosis, and subsequent fibrosis.96,97 The significance of either of these changes is unknown; they typically resolve with either continued therapy or as other toxic manifestations resolve.94,97,232
The common toxic manifestations of these xenobiotics in overdose are an extension of their pharmacologic action. Symptoms may include lightheadedness, syncope, palpitations, and nausea.3,147 Signs may be isolated to tachycardia alone,198,232 flushing, or alterations in mental status, which is related to the degree of hypotension.160 Based on AAPCC annual poison data, in recent years, the majority of reported exposures to this class of drugs may have involved the topical formulation of minoxidil29 (Chap. 136).
After appropriate GI decontamination, routine supportive care should be performed with special consideration to maintaining adequate mean arterial pressure. If IV crystalloid boluses are insufficient, then a peripherally acting α-adrenergic agonist, such as norepinephrine or phenylephrine, is an appropriate next therapy. Dopamine and epinephrine should be avoided to prevent an exaggerated myocardial response and tachycardia from β-adrenergic stimulation.
Sodium nitroprusside is effectively a prodrug, exerting its vasodilatory effects only after its breakdown and the release of nitric oxide. The nitroprusside molecule also contains five cyanide radicals that, although gradually released, occasionally produce cyanide or thiocyanate toxicity.178,219 Physiologic methemoglobin can bind the liberated cyanide. The binding capacity of physiologic methemoglobin is about 175 µg/kg of cyanide, corresponding to a little less than 500 µg/kg of infused sodium nitroprusside. These cyanide moieties are rapidly cleared, both by interacting with various sulfhydryl groups in the surrounding tissues and blood and enzymatically in the liver by rhodanese, which couples them to thiosulfate-producing thiocyanate.76 This cyanide detoxification process in healthy adults occurs at a rate of about 1 µg/kg/min, which corresponds to a sodium nitroprusside infusion rate of 2 µg/kg/min.51,219 It is limited by the sulfur donor availability, so factors that reduce these stores, such as poor nutrition in infants and toddlers, critical illness, surgery, and diuretic use, place patients at risk for developing cyanide toxicity.40,51 The hemolysis associated with cardiopulmonary bypass may place the patient at particular risk because the elevated free hemoglobin may accelerate the release of cyanide from the sodium nitroprusside moiety.40 Therefore, depending on the balance of cyanide release (eg, rate of sodium nitroprusside infusion) and the rate of cyanide detoxification (eg, sulfur donor stores), cyanide toxicity may develop within hours. Infusion rates greater than 4 µg/kg/min of nitroprusside for greater than 12 hours may overwhelm the capacity of rhodanese for detoxifying cyanide.207 Signs and symptoms of cyanide toxicity include alteration in mental status; anion gap metabolic acidosis; and in late stages, hemodynamic instability. If cyanide poisoning does occur, then hydroxycobalamin is the current treatment of choice for treatment (Chap. 126).
One method of preventing cyanide toxicity from sodium nitroprusside is to expand the thiosulfate pool available for detoxification by the concomitant administration of sodium thiosulfate.51,92,164,219 Dosing of 1 g sodium thiosulfate for every 100 mg of nitroprusside is typically sufficient to prevent cyanide accumulation.207 Unfortunately, the thiocyanate formed may accumulate, particularly in patients with renal insufficiency, and produce thiocyanate toxicity.76,219 Simultaneous infusion of thiosulfate does not interfere with the vasodilatory effects of sodium nitroprusside.104 Needless to say, the potential of sodium nitroprusside to produce cyanide poisoning, in addition to the introduction of other equally effective and rapidly titratable antihypertensives, has greatly reduced its use.
Thiocyanate is almost exclusively renally eliminated, with an elimination half-life of 3 to 7 days. It is postulated that a continuous sodium nitroprusside infusion of 2.5 µg/kg/min in patients with normal renal function could produce thiocyanate toxicity within 7 to 14 days, although it may be as short as 3 to 6 days or as little as 1 µg/kg/min in patients with CKD who are not receiving hemodialysis.219 The symptoms of thiocyanate toxicity begin to appear at serum concentrations of 60 µg/mL (1 mmol/L); are very nonspecific; and they may include nausea, vomiting, fatigue, dizziness, confusion, delirium, and seizures.76 Thiocyanate toxicity may produce life-threatening effects, such as hemodynamic and intracranial pressure elevation, when serum concentrations are above 200 µg/mL.51,76,92,249 Anion gap metabolic acidosis and hemodynamic instability do not occur with thiocyanate toxicity. Although cyanide or thiocyanate concentrations are not typically useful in the management of patients with cyanide toxicity, they may be beneficial for monitoring critically ill patients who are at risk of thiocyanate poisoning. Hemodialysis clears thiocyanate from the serum and should be strongly considered in patients with significant clinical manifestations of thiocyanate toxicity.64,153,166
Another therapy used to prevent cyanide toxicity from sodium nitroprusside is a simultaneous infusion of hydroxocobalamin.128 Dosing of 25 mg/h has successfully reduced cyanide poisoning in humans.48,270 As with thiosulfate, simultaneous infusion of hydroxocobalamin does not interfere with the vasodilatory effects of sodium nitroprusside.104 Because of the relative higher cost of hydroxocobalamin as well its interactions with some laboratory tests, thiosulfate should remain the mainstay of prophylaxis against sodium nitroprusside-induced cyanide toxicity (Antidotes in Depth: A40 and A41).
Diuretics can be divided into three main groups: (1) the thiazides and related compounds, including hydrochlorothiazide and chlorthalidone, (2) the loop diuretics, including furosemide, bumetanide, and ethacrynic acid, and (3) the potassium-sparing diuretics, including amiloride, triamterene, and spironolactone. Two other groups of diuretics—the carbonic anhydrase inhibitors, such as acetazolamide, and osmotic diuretics (eg, mannitol)—are not used as antihypertensive agents.
The thiazides produce their diuretic effect by inhibition of sodium and chloride reabsorption in the distal convoluted tubule. Loop diuretics, in contrast, inhibit the coupled transport of sodium, potassium, and chloride in the thick ascending limb of the loop of Henle. Although their exact antihypertensive mechanism is unclear, an increased urinary excretion of sodium, potassium, and magnesium results from the use of loop diuretics. Potassium-sparing diuretics act either as aldosterone antagonists, such as spironolactone, or as renal epithelial sodium channel antagonists, such as triamterene, in the late distal tubule and collecting duct.114
The majority of toxicity associated with diuretics is metabolic and occurs during chronic therapy or overuse.264 Hyponatremia develops within the first 2 weeks of initiation of diuretic therapy in more than 67% of susceptible patients, and female sex, old age, and malnourishment are the greatest risk factors.8,235 Symptoms of severe hyponatremia (< 120 mEq/L) may include headache, nausea, vomiting, confusion, seizures, or coma (Chap. 19). The osmotic demyelination syndrome, formally known as central pontine myelinolysis, is reported during rapid correction of severe hyponatremia secondary to diuretic abuse.46
Other electrolyte abnormalities associated with diuretic use include hypokalemia and hypomagnesemia, which may precipitate ventricular dysrhythmias such as torsade de pointes and sudden death. This is an extremely controversial topic, with several excellent studies providing conflicting results.21,77,185,228,230 Although it is unclear how great a risk, if any, diuretic use may be, it remains prudent to monitor and correct the patient’s potassium concentration.108,228,262 This is particularly important in elderly patients and for those patients who concomitantly use digoxin, in which setting hypokalemia is clearly associated with dysrhythmias (Chap. 65).28,240 Potassium-sparing diuretics may cause hyperkalemia, particularly in the setting of renal insufficiency or when combined with other hyperkalemia-producing drugs such as ACEIs.118
Thiazide diuretics are associated with inducing hyperglycemia, particularly in patients with diabetes mellitus. This is a result of depletion of total body potassium stores. Because insulin secretion is dependent on transmembrane potassium fluxes, this decrease in potassium concentration reduces the amount of insulin secreted.144 This effect is dose dependent and reversible either by potassium supplementation or discontinuation of the thiazide diuretic.39,100 This association has lead to significant work and discussion about the routine use of thiazide diuretics as first-line antihypertensives in the treatment of uncomplicated patients.52,90,166 In addition, thiazides are less well tolerated than any other antihypertensive drug class leading to significant noncompliance.165
Thiazide diuretics are also associated with inducing hyperuricemia, renal calculi, and gout.34,91,93 This is because the renal elimination of uric acid is extremely dependent on intravascular and urinary volume so diuretic-induced volume depletion reduces uric acid filtration and increases its proximal tubule resorption.226,238
Several unusual reactions are associated with thiazide diuretic use, including pancreatitis; cholecystitis; and hematologic abnormalities, such as hypercoagulability, thrombocytopenia, and hemolytic anemia.61,63,212,214,252,261
Despite the widespread use of these xenobiotics, acute overdoses are distinctly rare.139 Major signs and symptoms include GI distress, brisk diuresis, possible hypovolemia and electrolyte abnormalities, and altered mental status.139 Typically, the diuresis is short lived because of the limited duration of effect and the rapid clearance of the majority of diuretics. Assessment should focus on fluid and electrolyte status, which should be corrected as needed. If hyperkalemia is unexpectedly discovered, either the ingestion of a potassium-sparing xenobiotic or, more likely, an overdose of potassium supplements, which are frequently prescribed in conjunction with thiazide and loop diuretics, should be considered.111,112 Altered mental status, including coma, may result from diuretic overdose without evidence of any fluid or electrolyte abnormalities.17,18,139,213 Postulated mechanisms include a direct drug effect and induction of transient cerebral ischemia due to hypotension.180
Angiotensin-Converting Enzyme Inhibitors
ACEIs are among the most widely prescribed antihypertensives. At the time of this writing, there are 10 ACEIs approved by the US FDA for the treatment of hypertension (Table 63–1). In general, they are well absorbed from the GI tract, reaching peak serum concentrations within 1 to 4 hours. Enalapril and ramipril are prodrugs and require hepatic metabolism to produce their active forms. Elimination is primarily via the kidneys.
TABLE 63–1.Antihypertensives and Pharmacologically Related Agents ||Download (.pdf) TABLE 63–1. Antihypertensives and Pharmacologically Related Agents
β-Adrenergic antagonists (Chap. 62)
Calcium channel blockers (Chap. 61)
Sympatholytics (antagonize α-adrenergic vasoconstriction)
Central α2-adrenergic agonists
Clonidine, dexmedetomidine, guanabenz,a guanfacine,a methyldopa,a tizanidine
Central imidazoline agonists
Peripheral adrenergic neuron antagonists
Guanadrel,a metyrosine,a reserpinea
Peripheral α1-adrenergic antagonists
Doxazosin, prazosin, silodosin, terazosin
Bendroflumethiazide,a chlorthalidone,a chlorothiazide, hydrochlorothiazide, hydroflumethiazide,a indapamide, methyclothiazide,a metolazone, polythiazide,a trichlormethiazidea
Bumetanide, ethacrynic acid, furosemide, torsemide
Amiloride, eplerenone, spironolactone, triamterene
Diazoxide,a hydralazine, minoxidil,a nitroprusside
Angiotensin-converting enzyme inhibitor
Benazepril, captopril, enalapril, fosinopril, lisinopril, moexipril, perindopril, quinapril, ramipril, spirapril, trandolapril
Angiotensin II receptor blockers
Azilsartan, candesartan, eprosartan, irbesartan, losartan, olmesartan, telmisartan, valsartan
Direct renin inhibitors
All ACEIs have a common core structure of a 2-methylpropanolol-L-proline moiety.81 This structure binds directly to the active site of ACE, which is found in the lung and vascular endothelium, preventing the conversion of angiotensin I to angiotensin II. Because angiotensin II is a potent vasoconstrictor and stimulant of aldosterone secretion, vasodilation; decreased peripheral vascular resistance; decreased blood pressure; increased cardiac output; and a relative increase in renal, cerebral, and coronary blood flow occur.81 This hypotensive response may be severe in select patients after their initial dose, resulting in syncope and cardiac ischemia.42,106 Patients with renovascular-induced hypertension and patients who are hypovolemic from concomitant diuretic use appear to be at greatest risk.106 Overall, however, these drugs are well tolerated and have a very low incidence of side effects. Some reported adverse effects include rash, dysgeusia, neutropenia, hyperkalemia, chronic cough, and angioedema.56,81,246 Because of their interference with the renin–angiotensin system, ACEIs are potential teratogens and should never be used by pregnant women or women of childbearing age.13
Angioedema is an inflammatory reaction in which there is increased capillary blood flow and permeability, resulting in an increase in interstitial fluid. If this process is confined to the superficial dermis, urticaria develops; if the deeper layers of the dermis or subcutaneous tissue are involved, angioedema results. Angioedema most commonly involves the periorbital, perioral, or oropharyngeal tissues.199 This swelling may progress rapidly over minutes and result in complete airway obstruction and death.80,85,223 The pathogenesis of acquired angioedema involves multiple vasoactive substances, including histamine, prostaglandin D2, leukotrienes, and bradykinin.110 Because ACE also inactivates bradykinin and substance P, ACE inhibition results in elevations in bradykinin concentrations that appear to be the primary cause of both ACEI angioedema and cough (Fig. 63–1).5,113 There is no evidence that the ACEI angioedema phenomenon is immunoglobulin E (IgE) mediated.5
An overview of the normal function of the renin-angiotensin-aldosterone system (RAAS) and the mechanisms of action of angiotensin-converting enzyme inhibitors (ACEIs), angiotensin II receptor blockers (ARBs), and direct renin inhibitors (DRIs) on that system. PVR = peripheral vascular resistance.
Although the literature is replete with reports of ACEI angioedema, the overall incidence is only approximately 0.1%, and it is idiosyncratic.73,113,231 One-third of these reactions occur within hours of the first dose and another third occur within the first week.151,231 It is important to remember that the remaining third of cases may occur at any time during therapy, even after years.41 Women, African Americans, and patients with a history of idiopathic angioedema appear to be at greater risk.151,184 In addition, there is evidence that patients who develop ACEI angioedema are at increased subsequent risk of developing angioedema from any etiology.16
Treatment varies depending on the severity and rapidity of the swelling. Because of its propensity to involve the tongue, face, and oropharynx, the airway must remain the primary focus of management. A nasopharyngeal airway is often helpful. If there is any potential for or suggestion of airway compromise, then endotracheal intubation should be performed. Severe tongue and oropharyngeal swelling may make orotracheal or nasotracheal intubation extremely difficult, if not impossible. If this is a concern, then fiberoptic nasal intubation may be an attractive option, provided that the resources are available. Other techniques, including retrograde intubation over a guidewire that was passed through the cricothyroid membrane and emergent cricothyrotomy, may also be considered.207 However, the most important aspect of airway management in patients experiencing ACEI angioedema is early risk assessment for airway obstruction and rapid intervention before the development of severe and obstructive swelling.2
Because ACEI angioedema is not an IgE-mediated phenomenon pharmacologic therapy targeting an allergic cascade, such as epinephrine, diphenhydramine, and corticosteroids, should not be expected to be effective. However, when the history is unclear, these medications should not be withheld in order to ensure providing life-saving therapy to someone having a severe IgE-mediated allergic reaction.
Newer treatment modalities developed to target various points along the cascade of events associated with hereditary angioedema may be beneficial in the treatment of ACEI angioedema. Hereditary angioedema results from a genetically mediated defect in C1 inhibitor resulting in limited activity of this enzyme and an increase in kallikrein concentrations. Kallikrein is a protease that cleaves kininogen into bradykinin. The end result is very similar to the cause of ACEI angioedema, namely an activation of vascular bradykinin B2 receptors.13 Several new treatments have been developed to target specific steps in the development of hereditary angioedema, including Berinert, a C1 esterase inhibitor, ecallantide, a kallikrein inhibitor, and icatibant, a bradykinin B2 receptor antagonist. Case reports of successful treatment of ACEI angioedema with these xenobiotics are few.14,79,177 However, one case series of eight patients with ACEI angioedema who where treated with 30 mg subcutaneous icatibant had more rapid improvement in their signs and symptoms as well as no need for subsequent steroid or diphenhydramine use.14 While further evidence is needed, icatibant may be a reasonable treatment for ACEI angioedema; however, its significant cost should limit its use only in patients with rapidly progressive or severe angioedema.
Fresh frozen plasma (FFP) which contains ACE has also been proposed as treatment for ACEI angioedema. FFP infusion will elevate ACE concentrations and lead to the degradation of accumulated bradykinin. Clinical use of FFP for the successful treatment of both hereditary and ACEI angioedema is reported.122,189,200,263 In these case reports, doses range from 1 to 5 units of FFP (200–250 mL/unit) with most using an infusion of 2 units of FFP as initial, and typically definitive, treatment.200
All patients with mild or rapidly resolving angioedema should be observed for several hours to ensure that the swelling does not progress or return. Outpatient therapy with a short course of oral antihistamines and corticosteroids should be considered if there is any question as to whether ACEI therapy produced the angioedema because allergic-mediated angioedema will benefit from this treatment. Patients developing angioedema from ACEI therapy should be instructed to discontinue them permanently and to consult their primary care physicians about other antihypertensive options. Because this is a mechanistic and not allergic adverse effect, the use of any other ACEIs is contraindicated.
Angiotensin-Converting Enzyme Inhibitor Overdose.
The toxicity of ACEIs in overdose appears to be limited.43,141 Although several reports of overdoses involving ACEIs are published, the majority of the cases reported manifested toxicity of a coingestant.54,89,262 Hypotension may occur in select patients,11,12,131 but deaths are rarely reported in isolated ACEI ingestions.187,235 Other patients may remain asymptomatic despite high serum drug concentrations.131
Treatment should focus on supportive care and on identifying any coingestants that may be more toxic, particularly other antihypertensives such as β-adrenergic antagonists and calcium channel blockers. In most cases, AC alone is sufficient GI decontamination. IV crystalloid boluses are often effective in correcting hypotension, although in rare cases, catecholamines may be required.7,83 Naloxone may also be effective in reversing the hypotensive effects of ACEIs. ACEIs may inhibit the metabolism of enkephalins and potentiate their opioid effects, which include lowering blood pressure.57,167 In a controlled human volunteer study, continuous naloxone infusion effectively blunted the hypotensive response of captopril.1 In one case report, naloxone appeared to be effective in reversing symptomatic hypotension secondary to a captopril overdose.258 In another published case, naloxone was ineffective.11 Although its role in the setting of ACEI overdose remains unclear, naloxone may obviate the need for large quantities of crystalloid or vasopressors and should therefore be considered.
Angiotensin II Receptor Blockers
ARBs were first introduced in 1995, and currently, six members of this class are marketed in the United States. These xenobiotics are rapidly absorbed from the GI tract, reaching peak serum concentrations in 1 to 4 hours, and then are eliminated either unchanged in the feces or after undergoing hepatic metabolism via the mixed function oxidase system eliminated in the bile.156,157,158, and 159,181
Although these xenobiotics are similar to ACEIs in that they decrease the effects of angiotensin II rather than decrease the formation of angiotensin II, they act by antagonizing angiotensin II at the type 1 angiotensin (AT-1) receptor (Fig. 63–1).123 This allows the drugs to inhibit the vasoconstrictive- and aldosterone-promoting effects of angiotensin II and reduce blood pressor by blunting both the sympathetic as well as the renin–angiotensin systems.156 Despite the mechanistic evidence that ARBs do not affect bradykinin degradation and therefore should have a much lower incidence of angioedema when compared to ACEIs, serious cases of angioedema associated with ARB therapy have been reported.38,151,255 In addition, there is a significantly higher incidence of angioedema associated with ARBs when compared to other antihypertensives, such a β-adrenergic antagonists.250
Similar to ACEIs, ARBs should never be used by pregnant patients because of their teratogenic potential.13,229 In addition, when initiating the xenobiotic, up to 1% develop of patients first-dose orthostatic hypotension.86
There have been few published reports of overdoses involving ARBs. Adverse signs and symptoms reflect orthostatic or absolute hypotension and include palpitations, diaphoresis, dizziness, lethargy, or confusion.74,162,233 Hypotension should be treated with crystalloid boluses and catecholamine therapy.162,233 Patients who are chronically taking ARBs may exhibit significant hypotension during induction of general anesthesia that has been refractory to traditional vasoconstrictor therapy, such as norepinephrine, ephedrine, and phenylephrine, but appear to respond to vasopressin.23,27,67
One promising new treatment for hypotension produced by ARBs and ACEIs is methylene blue.155,171,251 This treatment was first explored in patients placed on cardiopulmonary bypass (CPB).186,236,251 During CPB systemic blood pressure and peripheral vascular resistance decrease due to a number of factors, including acute hemodilution, citrate use in the cardioplegia, a poorly defined inflammatory response that results in nitric oxide release, and an increase in circulating bradykinin.45,50,268 This increase in bradykinin, which also mediates its vasodilatory effects via nitric oxide, occurs because bradykinin metabolism is primarily in pulmonary tissue and CPB mechanically bypasses the pulmonary system.45,50 ACEIs and ARBs exacerbate this vasodilation by inhibiting bradykinin metabolism.197 In a double blinded placebo controlled study of 30 patients taking ACEIs who were undergoing elective cardiac surgery requiring CPB, administration of methylene blue at the onset of CPB resulted in an increase in mean arterial pressure and systemic vascular resistance and less use of phenylephrine and norepinephrine.155 A reasonable starting dose of methylene blue, when used as a vasopressor, appears to be 2 mg/kg with subsequent intermittent boluses or possibly continuous infusions starting at 0.5 mg/kg/h.115,155