Protamine

Mechanism of Action

Unfractionated heparin is a strongly anionic anticoagulant that forms a salt aggregate when exposed to the positive cationic arginine peptide of protamine.[6] The salt aggregate that forms is inactive and has no anticoagulant properties. Protamine has a rapid onset of action, neutralizes unfractionated heparin within 5 minutes, and has a relatively short half-life of approximately 10 minutes. There is very little known about the metabolism of the heparin-protamine salt aggregate. Some studies have described the metabolism of the salt in the liver, while other studies have reported that the salt is metabolized and excreted by the kidneys.[7][8] Care is necessary to administer the proper dose of protamine, as excessive doses, protamine has been shown to possess anticoagulation properties and may cause bleeding and increased transfusion requirements.[3][9]

Administration

In the cardiac operating room setting, protamine is typically administered intravenously as an infusion over 10 to 15 minutes; this slow infusion is intended to decrease the likelihood of side effects that can occur if infused too rapidly. Many anesthesiologists in the cardiac operating room setting will administer a small bolus of protamine, often called a "test dose," to allow for early detection of adverse reactions, including hemodynamic instability. Often, protamine is administered through a peripheral intravenous line rather than a central line, as studies have shown that protamine delivered through a central line can cause an increase in plasma histamine levels and a decrease in systemic vascular resistance.[10] If no adverse reaction occurs, then the balance of the protamine dose is infused. Proper dosing of protamine to reverse heparin is controversial and varies in the medical literature.[3]

Protamine is packaged as an intravenous solution in a concentration of 10 milligrams/milliliter. To neutralize heparin, 1 to 1.5 mg of protamine gets injected per 100 units of heparin. Initial protamine treatment for dalteparin, tinzaparin, or enoxaparin overdosage is 1 mg protamine per 100 units of the low molecular weight heparin given. Follow-up doses of protamine in the 0.5 mg range per 100 units can be given if bleeding continues 4 hours later. The efficacy of protamine reversing heparin can be assessed by measuring the activated clotting time or performing a thromboelastogram test. In clinical trials, hemodynamic changes and arterial oxygenation changes decreased with protamine infusion via the ascending aorta vs. a central venous line suggesting this may be the preferred route of administration.[11]

Adverse Effects

The most common adverse effect associated with protamine administration includes an anaphylactic response consisting of systemic hypotension, pulmonary vasoconstriction, allergic reactions, pulmonary hypertension, bronchoconstriction, and bradycardia. The incidence of anaphylactic reactions ranges from 0.06% to 10.6%.[9] There are also reports of liver and kidney tissue damage.[6] Suspected anaphylactic responses to protamine receive treatment in the same way, other perioperative anaphylactic reactions. Therapies include the use of albuterol, methylprednisolone, H1, and H2 antihistamines, vasopressin, norepinephrine, fluid administration, glucagon, and echocardiography to monitor cardiac function. 

Excess protamine can also negatively impact platelet function, interfere with coagulation factors, and stimulate the breakdown of the clot.[3] The thinking is that protamine may potentiate fibrinolysis by decreasing thrombin concentrations.[3] 

Pulmonary hypertension is a potentially catastrophic adverse response to protamine administration; it is believed to be due to thromboxane A2 release. This release of thromboxane has been shown in studies to increase pulmonary vascular resistance by up to 10 times.[12][13] This condition can have treatment with the use of a prostacyclin EI infusion. Some studies have suggested the protamine-induced pulmonary hypertension is preventable by continuing aspirin intake before surgery, as aspirin suppresses the production of thromboxanes through its irreversible inactivation of the cyclooxygenase enzyme.

Hypotension from protamine infusion is thought to be due to intracellular calcium release and nitric oxide release.[14][15] Prevention options include antihistamines to suppress an allergic response and steroids to reduce any inflammatory response.[5] Vasopressors are often used to counteract protamine-induced hypotension.

Contraindications

Patients with a history of past protamine exposure, vasectomy, fish allergies, and insulin-controlled diabetes are at increased risk of an adverse reaction.[16][3][17][18] The patients most common patients at risk for a protamine reaction were on NPH insulin in the past. There are approximately 2.8 milligrams of protamine per 100 units of protamine zinc insulin and 0.5 milligrams of protamine per 100 units of isophane (NPH) insulin. Presumably, chronic exposure to low doses of protamine in a patient on these insulin preparations can cause the formation of antibodies to protamine. 

Monitoring

Protamine should be given slowly, and patients should be monitored closely for anaphylaxis, rapid hypotension, or an increase in pulmonary artery pressures.  Anaphylaxis can cause an increase in peak airway pressures along with hypotension, a rash, and wheezing, or difficulty breathing. The presence of an arterial line for invasive blood pressure monitoring and/or a pulmonary artery catheter to monitor changes in pulmonary artery pressures is advantageous to recognize early negative sequelae from protamine administration. It is also imperative for the medical staff to have access to cardiovascular rescue drugs (vasoactive/inotropic drugs) and resuscitation equipment should an adverse reaction occur.

Toxicity

Protamine’s toxicity is thought to be due to its positive charge. It has been shown to cause a decrease in endothelial cell ATP production and progressive mitochondrial injury in bovine pulmonary endothelial cells.[19] Protamine also has a toxic effect on myocardial cells, postulated to be due to its positive charge affecting mitochondrial cells intracellularly.[20] The theory is that the salt formed between protamine and heparin that effectively neutralizes the positively charged molecule of protamine offers some intracellular protection against toxicity.

Enhancing Healthcare Team Outcomes

The use of protamine requires an interprofessional team of healthcare professionals that includes physicians, nurses, and cardiac perfusionists. Without proper management and communication, the morbidity and mortality from protamine can be high. In cardiac surgery, protamine administration must be communicated to all healthcare team members, as protamine administration before separation from cardiopulmonary bypass can be fatal. Typically after separation from cardiopulmonary bypass, the cardiac surgeon asks for protamine to be given to reverse the effects of heparin. This request should be followed by closed-loop communication from the anesthesiologist that the protamine infusion has begun. This is to inform the cardiac perfusionists to stop all suction devices that drain into the cardiopulmonary bypass machine (to avoid introducing protamine into the cardiopulmonary bypass circuit in the event that the patient needs to be put back on cardiopulmonary bypass), as well as to inform the cardiac surgeon and nursing staff in the room to be vigilant for adverse reactions.

Protamine is useful in several different clinical scenarios in the hospital setting. The proper dosing and speed of infusion or injection of protamine to neutralize the anticoagulant effects of heparin should be appropriately followed. In other clinical situations, physicians and nursing should be extremely vigilant in monitoring patients for adverse reactions. Appropriate resources should be available to monitor and treat patients who experience adverse effects due to its administration. An understanding of the appropriate laboratory tests to follow during its administration is also critical to the safe use of protamine.

For decades since the start of cardiac surgery using cardiopulmonary bypass, investigators have studied the methods to provide a safe, efficient, and reproducible way of performing complex cardiac surgeries. Advances in cardiopulmonary bypass result from evidence-based randomized controlled trials, cohort studies, and anecdotal practice or consensus. After reviewing all available evidence, in 2018, The Society of Thoracic Surgeons, The Society of Cardiovascular Anesthesiologists, and The American Society of Extra-corporeal Technology released their clinical practice guidelines for anticoagulation during cardiopulmonary bypass. These guidelines discussed the use of heparin as the primary anticoagulant (bivalirudin if there are heparin contraindications) and protamine as the reversal agent.[21] Though there are ongoing investigations into new medications for anticoagulation and reversal of such anticoagulation, the current gold standards for anticoagulation and reversal of anticoagulation for cardiac surgery continue to be heparin and protamine.[22]

Protamine is a medication that requires the collaborative efforts of an interprofessional team, including clinicians, surgeons, anesthesiologists, nurse anesthetists, nursing, and pharmacists. As mentioned above, proper preparation, dosing, and protocol are necessary when using protamine, and each member of the team has the role to play in the bigger picture. Nursing should be monitoring along with anesthesia personnel. The pharmacist should have verified the planned dosing along with the heparin used for the procedure. Clinicians need to seek input and feedback from these other members so that the interprofessional team actually functions as such, not with everyone siloed into their own discipline. This approach will lead to proper protamine use and better patient outcomes. [Level 5]