Protamine Reversal of Heparin: A Fishy Practice?

Jerrold H. Levy; Jonathan P. Piccini


Europace. 2019;21(6):840-841. 

Despite the evolution of clinical practices in cardiovascular medicine, some of our routine procedures seem a bit like medieval medicine as in the case of heparin anticoagulation and protamine reversal. Heparin is stored in mast cells, and isolated from porcine intestine or bovine lung. Unfractionated heparin used clinically is a mixture of 3000 to 30 000 molecular weight acidic glycosaminoglycans. For acute reversal of this primordial goop, we administer another equally unusual molecule protamine, a basic arginine-rich peptide that is purified from salmon sperm. Protamine reverses the acidic glycosaminoglycan heparin via a non-specific polyanionic-polycationic interaction. If you mix heparin and protamine together they form a precipitate as protamine reversal is simplistically an acid–base interaction. Despite all of the limitations of heparin, and the potential for adverse reactions to protamine, this dynamic duo of therapy continues as a standard of care. This is particularly true for cardiovascular procedures, including catheter ablation of atrial fibrillation (AF) where heparinization is a Class I (Level of Evidence B) and protamine reversal is a Class IIa (Level of Evidence B) recommendation.[1] This standard persists despite the availability of other parenteral anticoagulants due to the familiarity, ease of titration, relatively short half-life, ability to acutely reverse, and low cost of heparin.

Because any drug or molecule can potentially cause an allergic/hypersensitivity reaction, especially xenogenic molecules, clinicians must remain vigilant for adverse immunologic reactions, including anaphylaxis, the most life-threatening form of an allergic reaction.[2] Hypersensitivity reactions are not uncommon to both heparin and protamine. In the case of heparin, the most common immunologic response is an IgG antibody that can develop to the platelet factor 4 epitope and heparin to produce heparin-induced thrombocytopenia. For protamine, multiple adverse drug reactions including anaphylaxis have been reported, primarily during cardiovascular procedures where the drug is extensively used to reverse the potential bleeding associated with heparin anticoagulation.[2]

In the current issue of Europace, the authors describe three patients who developed precipitous hypotension and shock followed by ventricular fibrillation (VF) within minutes of protamine infusion for reversal of heparin after catheter ablation of AF.[3] In the first case, 10 min after protamine administration shock and VF occurred requiring cardiopulmonary resuscitation (CPR). Despite resuscitation, the patient developed progressive right ventricular failure, acute respiratory distress syndrome, an intracranial haemorrhage, and died 12 h later. However, tryptase drawn initially and at 6 h was normal, and the postmortem examination was unremarkable. In the second case, a patient with a dilated cardiomyopathy and cardiac resynchronization therapy and cardioverter-defibrillator underwent cryoablation of AF with additional ablation. Three minutes following 50 mg of protamine, blood pressure decreased to <80 mmHg, and the patient developed ventricular tachycardia/VF requiring CPR and adrenaline. Echocardiography and coronary angiography showed no obstructive lesions, and the patient fully recovered. In the third patient, following AF ablation under general anaesthesia with cryoablation, 10 min after 100 mg of protamine was administered, the patient developed shock, QRS widening, and VF requiring CPR and adrenaline boluses. Echocardiography showed globally impaired contractility, and angiography showed no significant coronary disease. The patient recovered within 24 h. Tryptase at time of the arrest was normal.

Were these three reactions to protamine or some potential cardiovascular event following protamine reversal? The time sequence of 3–10 min after protamine administration is consistent with a potential anaphylactic response, however, tryptase levels were negative in all three patients. It is important to consider that tryptase does not have a good sensitivity for anaphylactic reactions. As we have previously reported, tryptase is a biomarker of mast cell activation with or without IgE.[4] However, there may have also been previous sensitization and IgG mediated reactions would not likely increase tryptase.[5] Protamine can also directly degranulate mast cells by non-immunologic mechanisms but tryptase levels would have been increased excluding this is a causative mechanism.[4] Other hypersensitivity mechanisms may have been responsible including the ability of heparin-protamine complexes to activate complement and generate C3a and C5a, potent anaphylatoxins.[6]

Thus, protamine can produce acute cardiopulmonary collapse by both non-immunologic in immunologic processes that would not increase tryptase levels. Protamine can activate complement by either antiprotamine IgG interactions or by polycationic-polyanionic activation to produce acute pulmonary hypertension, right ventricular failure, and shock due to C5a-mediated thromboxane release from neutrophils.[6] Protamine can also inhibit carboxypeptidase N, a proteolytic enzyme that metabolizes both anaphylatoxins and bradykinin. In previous reports in patients receiving protamine containing insulins, the antiprotamine IgE antibodies were risk factors for acute protamine reactions, as was the presence of antiprotamine IgG.[5] The incidence of protamine reactions is 0.6% (1 of 160) to 2% (1 of 50) in neutral protamine Hagedorn (NPH) insulin-dependent diabetics undergoing cardiac surgery, a rate 10–30 times greater than patients without NPH exposure.[7,8] However, the incidence in a general cardiac surgical patient population is approximately one in 1500.[7,8] Prior reports of protamine reactions after catheter ablation of AF have been as high as 1%.[9]

The important question is, were the cases described by Leung et al. truly hypersensitivity reactions? The temporal sequence suggests so. The first case may have had a complement-mediated response, especially with right ventricular dysfunction, and acute respiratory failure which seems consistent with a protamine reaction with potential polymorphonuclear leucocyte accumulation in the pulmonary microcirculation. Coagulopathy can also occur during anaphylaxis and may account for the intracranial bleed. Multiple mediators released during anaphylaxis may also potentially have profound arrhythmogenic effects affecting nodal conduction and altering potential excitation thresholds for VF. However, we agree with Leung et al. that there is little evidence to support a direct arrhythmogenic effect of protamine. Hypotension also contributes to decreasing coronary perfusion pressure (diastolic blood pressure—left ventricular end-diastolic pressure). Ventricular arrhythmias occurring in the setting of profound shock can certainly be due to myocardial ischaemia even in the absence of obstructive coronary artery lesions. The study of anaphylaxis and cardiovascular manifestations suggests that the causes of cardiovascular collapse in these patients are always complex and multifactorial.[2,9]

When anaphylaxis to protamine is suspected, what is the optimal treatment regimen? Airway maintenance, 100% oxygen administration, and epinephrine are important to treat the hypotension and hypoxaemia that results from vasodilation, increased capillary permeability, and bronchospasm.[2] Reactions may be protracted with persistent hypotension, acute pulmonary hypertension, and right ventricular failure, angioedema, and shock as presented in these cases requiring ongoing resuscitation with epinephrine and/or norepinephrine.[2] When hypotension persists despite therapeutic interventions, echocardiography is important to diagnose the cause of acute or persistent cardiovascular dysfunction and evaluate for acute right ventricular failure. In patients with refractory shock/vasoplegia, arginine vasopressin should be considered as a potential therapeutic agent.[2] Veno-arterial extracorporeal membrane oxygenation via peripheral cannulation continues to evolve as a potential strategy in centres with extracorporeal membrane oxygenation capabilities for patients who do not respond to conventional therapies.

Due to the adverse reactions that protamine can cause, multiple efforts have been undertaken to develop clinical alternatives. Other potential therapeutic reversal agents studied include hexadimethrine, recombinant platelet factor 4, heparinase, and heparin binding filters.[2] Despite previous efforts to develop alternative agents, protamine remains our only available agent. The authors note there is ongoing research into alternative reversal agents including andexanet alfa which reverses both low molecular weight heparin and unfractionated heparin.[10] However, the cost of this novel recombinant anti-Xa decoy is prohibitive to any routine use beyond emergency anti-Xa reversal.[10]

A test dose of protamine has been suggested to be administered before giving the full therapeutic dose to test for a reaction. However, a test dose may also cause anaphylaxis as 1 mg of protamine is approximately 1016 molecules.[2] However, the benefit of a test dose is to alert the clinician that an anaphylactic reaction can occur and reminds them to consider this when giving a drug with a potential risk for a reaction and to monitor them for acute cardiopulmonary dysfunction that can occur. Although the speed of drug administration is important for known histamine releasing drugs like vancomycin or protamine, this is not the case for anaphylaxis, since small doses including test doses may trigger reactions.[2]

Despite all of the interest in developing potential alternatives to protamine, protamine may not be that antigenic. The molecular weight is approximately 5000 Daltons, and protamine is isolated from salmon sperm because it is a histone, a basic nuclear protein that provides structural integrity to DNA, and not that different from human histones.[2] With the extensive fish farming that takes place across the planet, there is an extensive availability of salmon milt which is used for protamine extraction. Although something seems rather fishy about protamine use, it still is the standard of care until another agent is available. While protamine reactions remain uncommon, clinicians who infuse protamine must remain aware of the potential for life-threatening anaphylactic reactions, vigilant for potential signs of these reactions, and readily prepared to treat these reactions as the physicians thoughtfully did in the three reported cases.