Words By Ali Rengers
ST Elevation Myocardial Infarction (STEMI) is a cardiac emergency caused by complete major Coronary Artery (CA) occlusion by thrombus (Harker et al., 2014). Reperfusion therapy is required to re-establish blood flow to the affected artery to decrease infarct size and patient mortality (Doan et al., 2019; Marchand & Farrah, 2019). Rapid STEMI treatment and out-of-hospital referral to a Cardiac Catheterization Laboratory (CCL) shortens reperfusion time, minimizing total ischemic time (TIT) (Doan et al., 2019; Khalid et al., 2018). This essay will discuss the cardiac reperfusion procedures; Prehospital Thrombolysis (PHT), In Hospital Thrombolysis (IHT) and Primary Percutaneous Coronary Intervention (pPCI). Their effectiveness will be evaluated, compared, and contrasted to determine if PHT is as effective as IHT and pPCI for STEMI treatment. Recommendations about the effectiveness of pre-hospital STEMI treatment as per current Queensland Ambulance Guidelines will be made.
PHT is employed due to lack of cardiologist access or inability to transport to a CCL within directed time frames (Davis et al.,2019; Khan et al., 2016). PHT involves the timed, successive paramedic administration of an intravenous fibrinolytic, intravenous anticoagulant, per oral antiplatelet and subcutaneous anticoagulant injection (Khan et al., 2016). Fibrinolytics converting plasminogen to plasmin to incite thrombus degradation (Marchand & Farrah, 2019). Antiplatelet and anticoagulant interventions decrease the platelet aggregation process and inhibit the advancement of coagulation and clot construction (Koenig-Oberhuber, 2016). Prior to physician consultation and PHT administration the patient’s 12 lead ECG with ST elevation is transmitted to the consulting physician and a contraindication checklist is completed by paramedics (Davis et al., 2019).
Employment of PHT lowers the requirement for coronary thrombectomy, vascular complications and mechanical circulatory devices(Vallabhajosyula et al., 2021). However, PHT demonstrates rates of haemorrhagic complications including hemopericardium and intracranial haemorrhage, acquisition of ventricular septal defect and reinfarction (Vallabhajosyula et al., 2021; Marchand & Farrah, 2019). Complete or partial ECG ST-segment resolution up to 90 minutes post PHT administration is a reliable indicator of successful reperfusion (Harker et al., 2014). Patients who do not display ECG evidence of successful reperfusion should not undergo further PHT (Harker et al., 2014). These patients require prompt transfer to a hospital equipped with rescue PCI (coronary angiography in conjunction with PCI) to reduce short term mortality (Marchand & Farrah, 2019; Davis et al., 2019; Harker et al., 2014).
In Hospital Thrombolysis
IHT is employed in non-pPCI capable hospitals due to distance hampering patient ability to attend a CCL for pPCI or angioplasty intervention (Khan et al., 2016; McCaul et al., 2014). Administered by physicians, IHT employs fibrinolytic, anticoagulant, and antiplatelet medication, supplied in timed succession analogous to PHT (McCaul et al., 2014). Although PHT provides decreased time to treatment to shorten reperfusion and decrease TIT, IHT administration is well equipped to treat the possible consequences of thrombolysis including ventricular fibrillation, stroke, and adverse drug reaction through hospital pharmacological and mechanical management (Harker et al., 2014). IHT is employed more frequently in lower income countries, due to lack of funding for PHT training and education, and in nations without widespread CCL (Vallabhajosyula et al., 2021).
Primary Percutaneous Coronary Intervention
pPCI, is employed for patients able to reach a CCL within 60 minutes from first medical contact (FMC) (Khan et al., 2016). pPCI is to be performed in a timely manner, up to 120 minutes from FMC, as minimal improvement can be expected after such time has elapsed (Brown et al., 2018;Toutouzas et al., 2018). According to Toutouzas et al. (2018) infarct size increases and extends with increased coronary occlusion duration. pPCI involves the administration of antiplatelet and anticoagulant agents to inhibit thrombus propagation (Toutouzas et al., 2018). These medications reopen the occluded CA, acting as facilitators for mechanical thrombus removal and/ or placement of a balloon catheter in the culprit artery (Harker et al., 2014; Koenig-Oberhuber, 2016).
Prior to pPCI pathway initiation, the patient’s 12 lead ECG with ST elevation is interpreted by paramedics and a contraindication checklistis completed (Davis et al., 2019; QAS, 2020). The interventional cardiologist at the designated CCL is contacted and the preferred antiplatelet agent isconfirmed (QAS, 2020). After confirming pPCI acceptance the patient is transported ‘Code 1’ to the designated facility (QAS, 2020). The antithrombotic therapy administered pre pPCI increases the infarcted CA patency, doubling blood flow (Harker et al., 2014). pPCI lowers the risk of mortality, reinfarction, ischemia, stroke, and bleeding, provided it can be delivered rapidly within a given timeframe (Harker et al., 2014). The increased risks and costs, of bleeding and site complications, associated with trans-femoral catheter placement during and post procedure, have reduced due to a higher implementation of trans-radial access (Harker et al., 2014).
Compare and Contrast
The STEMI treatment modalities, PHT, IHT and pPCI employ Acute Coronary Syndrome (ACS) standards of care including oxygen therapy,antiplatelet and nitro-glycerine delivery and/or opioid administration (McCaul et al., 2014). PHT and IHT demonstrate similar if not the same application of drug interventions, with PHT administered 30 – 60 mins earlier than IHT (Harker et al., 2014). As half of salvageable myocardium is lost within the first hour and two thirds within three hours of CA occlusion, swift PHT administration as opposed to IHT is associated with lower rates of mortality at 30 days and 6months (Harker et al., 2014). According to Marchand & Farrah (2019) and McCaul et al., (2014) complications, including bleeding and stroke, were not statistically different between PHT and IHT, demonstrating there may be no risk difference between modalities. Compared to IHT, PHT improves time to thrombolysis, to reduce TIT, myocardium death and patient mortality (McCaul etal., 2014).
Compared to pPCI, PHT demonstrates a shorter time to treatment with FMC to start of reperfusion between 35 – 38 minutes (Khan etal., 2016; McCaul et al., 2014). However, according to Davis et al., (2019) the requirement to gain physician authorisation for PHT therapy is often inefficient and time consuming due to system vulnerability and technological failings. Similarly, pPCI administration encounters difficulties in underestimation of transport times and adherence to service transport guidelines (Khan et al., 2016). Harker et al., (2014) outline pPCI outcomes strongly correlate to the haste of delivery, associated with transport times and CCL volumes.
The 40 -50% efficacy of PHT and IHT in restoring partial CA patency encourages mortality benefit in acute STEMI cases. However, this is overshadowed by the methods’ signification fraction of an adequate reperfusion, surpassed by the more certain reperfusion afforded by pPCI (Khalid et al.,2018; McKavanagh et al., 2018). Furthermore, the fewer contraindications and lower risk of stroke provided by pPCI compared to the 20-40% failure by PHT and IHT to restore CA perfusion and higher bleeding risk, demonstrates why pPCI is considered to have higher value (Harker et al., 2014; Toutouzas et al., 2018). It should be noted, while pPCI methods fragment the thrombus to have no further effect, in some cases the dislodged thrombus may embolise and cause newobstruction (Harker et al., 2014).
Comparatively, the delivery of PHT and pPCI require trained staff, encountering training and equipment costs (Harker et al., 2014). PHT costs are expected to be constant, regardless of treatment numbers, while pPCI cost is associated with hospital defined minimum number of pPCI procedures for individual and institutional maintenance of competence (Harker et al., 2014).Less than 70 pPCI procedures per annum is associated with increased risk of stroke, haemorrhage and mortality when compared to a CCL with higher pPCI volume (Harker et al., 2014). Detrimental to its perceived superiority, pPCI guidelines are met by 5% of patients transported to CCL for pPCI (Vallabhajosyula et al., 2021).
Conversely, while PHT saves time to reduce mortality shortterm, the risk of recurrent ischaemia or myocardial injury due to ruptured coronary site narrowing predisposes the vessel to re-occlusion in 18 – 32% of patients (Harker et al., 2014). Contrastingly, pPCI time delays of 35 minutes and 35 – 120 minutes consistently reduced the odds of 30-day mortality by 67% and 39% respectively (Harker et al., 2014). These results demonstrate how geographical locations and transport times are imperative in determining the appropriate reperfusion mode is adopted (Brown et al., 2018). Overall pPCI is the preferred therapy, provided it can be achieved within the first 120 minutes of diagnosis (Somaratne et al., 2018).
The Queensland Ambulance Service (QAS) has standardized clinical procedures for prehospital STEMI management (Doan et al., 2019).Selection of PHT, IHT and pPCI is dependent on patient transport times, geographical location, and facility capabilities (Doan et al., 2019; Khan et al., 2016). Current guidelines for patients within 60 minutes of CCL are suitable for non- regional Australia, however underestimation of transport times or unanticipated traffic may cause patients to fall outside of pPCI service time guidelines (Khan et al., 2016). Regional Australians great distances away from CCL for pPCI receive PHT by QAS paramedics or IHT by non pPCI capable hospitals (Khan et al., 2016). According to Khan et al., (2016) PHT delivery in remote and regional Australia by paramedics based on algorithmic 12 lead ECG assessment is a safe and viable method.
Current QAS guidelines stipulate Advanced Care Paramedics(ACP2) may only perform Decision Supported Fibrinolysis Administration (DSFA) and Decision Supported pPCI Referral (DSpR) in the prehospital setting (QAS, 2020; QAS, 2021; QAS, 2021a). The ACP2 must contact the QAS Clinical Consultation and Advice Line with a picture of the 12 lead ECG for confirmation the patient is suitable for PHT (QAS, 2021). According to Doan et al. (2019) QAS patients received PHT within 12 – 33 minutes of acute STEMI identification across the 11-year study. As thrombolysis is highly dependent on delay to treatment, development of protocols to minimise delay to reperfusion and TIT should be implemented (Brown et al., 2018). A 2019 New Zealand study saw autonomous PHT reduce time to treatment by 22 minutes when compared to physician authorised PHT (Davis et al., 2019). Of 74 patients receiving autonomous PHT three were inappropriately treated (4%), compared to the six inappropriately treated patients of 96 who received physician authorised PHT(6.25%) (Davis et al., 2019). Regarding the QAS ACP2 requirement to receive time consuming and potentially technologically failing physician telemetry authorisation (especially in remote areas) for DSFA, time could be saved by moving to a composite model (Davis et al., 2019; QAS, 2021). This model would rely more heavily on physician education in conjunction with monitor/ defibrillator confirmation of acute MI presence for PHT administration (Khan et al., 2016).
QAS may also benefit from the adoption of the alternative pharmaco-invasive strategy (PIS), of half-dose PHT followed by invasive management with Percutaneous Coronary Intervention (PCI), as outlined by the Strategic Reperfusion Early After Myocardial Infarction (STREAM) trial (Khalidet al., 2018; Roule et al., 2016). According to Khalid et al. (2018) half dose PHT and urgent PCI improved ischemic outcomes while reducing the need for forceful PCI invasive measures, decreasing major bleeding complications. During the trial PIS improved artery patency prior to hospital presentation for PCI, reducing cardiogenic shock, intracranial bleeding and mortality when compared to solo pPCI or PHT (Khalid et al., 2018).
Swift STEMI treatment improves patient outcomes by reducing reperfusion time and TIT (Doan et al., 2019). PHT and IHT’s similar risk of haemorrhagic complications, stroke and reinfarction, demonstrate little difference between modalities (Vallabhajosyula et al., 2021; Marchand & Farrah, 2019). PHT compared to IHT improves time to thrombolysis, to reduce myocardium death, demonstrating its higher degree of effectiveness than IHT in STEMI treatment (McCaul et al., 2014).
Although PHT saves time to reduce mortality short term, pPCIs overall lower risks ensures it is the preferred STEMI reperfusionstrategy, provided it is performed in a timely manner (Somaratne et al., 2018). Overall, PHT is less effective than pPCI in STEMI treatment, however, outside of pPCI time guidelines and up to 120 minutes from FMC, PHT efficacy is comparable to pPCI (Marchand & Farrah, 2019). As PHT’s strength is speed of administration, movement from DSFA to a composite model would improve treatment time for patients outside of the specified 60-minute window for pPCI (Harker et al., 2014). Further investigation into the adoption of a PIS by the QAS may improve patient outcomes.
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Cite this article
Rengers, A., 2021. Thrombolysis and Percutaneous Coronary Intervention Treatment for STEMI. The Shift Extension - Paramedic Publishing, Available at: https://www.theshiftextension.org/blog/thrombolysis-and-percutaneous-coronary-intervention-treatment-for-stemi.