Handheld diagnostics in interventional cardiology

Andreas Gruentzig performed the first coronary angioplasty on a human subject in 1977.¹ The progress in the 40 years since this has been remarkable; increasingly sophisticated tools combined with a growing worldwide expertise have led to a reduction in complication rate and to increased success. Percutaneous coronary intervention (PCI) has overtaken coronary artery bypass grafting (CABG) as the most commonly performed procedure to achieve coronary revascularisation. 

 

Since the mid 2000s, so called ‘Heart Attack Centres’ have established themselves throughout the world to provide primary percutaneous coronary intervention (PPCI) for patients with ST elevation myocardial infarction (STEMI).² PPCI within 90–120 minutes from initial medical contact is recommended by the European Society of Cardiology and is now the preferred method for re-establishing coronary reperfusion in STEMI patients. Not only is there a significant survival benefit over fibrinolysis, but fibrinolytics are associated with a number of problems: 

 

England has managed to achieve a PPCI rate of approximately 95% in 2011, and despite regional variations, most European countries show comparable figures.³

 

In keeping with the success and the technological advances of coronary intervention, the number of non-coronary procedures has also grown. 

 

In the era of growing awareness, improved imaging, and ageing populations, the incidence of structural valvular disease is increasing. An estimated 2.5% of the population are suffering from either mitral or aortic disease, with a prevalence of 1% in under 54-year-olds and rising to 12–14% in those over 75 years of age.⁴ As life expectancy increases, we are likely to see a higher demand for valvular procedures. The introduction of the transcatheter aortic valve replacement (TAVR) in 2007 with the approval of the Edwards valve and the CoreValve has changed led to a seismic shift in the working practices of catheter labs throughout the world. 

 

The initial data from the PARTNER trial in patients deemed unfit for conventional surgery were very encouraging,⁵ and five-year follow-up data seem to corroborate these initial findings.⁶ Further studies that compared TAVR and conventional surgery reported equally encouraging results.⁷

 

The number of endovascular procedures has increased by nearly 80%, along with a concomitant decrease in open surgical procedures.⁸ It is estimated that peripheral interventions could grow at a rate of up to 8% each year, thanks to growing experience, advances in stent technology and intravascular imaging.  

 

As is often the case in medicine, success leads to practitioners pushing the boundaries. Treatments initially restricted in access gradually become more mainstream, and patients with a higher burden of co-morbidities are included, until eventually emergency indications are accepted. Eventually the experience with these high-risk or prohibitive-risk cohorts leads to more successful outcomes, which translates into lower-risk patients.

 

PPCI and TAVR have both gone down that path, and outcomes justify this risk creep. However, as the patient risk profile changes, the resource intensity also increases. The provisions needed for a stable 55-year-old with STEMI are very different from an 80-year-old patient with the same disease who has been resuscitated and arrives in the lab intubated and ventilated. A number of centres have the capability of providing extracorporeal life support (ECLS) in order to facilitate a coronary intervention in patients too unstable to tolerate the procedure unsupported. 

 

Laboratory parameters

When providing medical care at the edge of what is possible, the use of laboratory parameters becomes an important tool to guide supporting therapy. 

 

Metabolic monitoring

The troponin elevation curve has been shown to be affected by the adequacy of myocardial reperfusion and can serve as a means to risk-stratify patients undergoing PPCI for STEMI.⁹ However, more importantly patients arriving in the catheter lab for emergency revascularisation often have impaired electrolyte profiles, elevated lactate levels and can be hypoxic and/or hypercarbic, depending on their pre-operative instability. Recognising and correcting these metabolic derailments in a timely manner is important and can, at least in part, determine clinical outcome. It will certainly guide supportive therapy, such as the need for inotropes, intra-aortic balloon counterpulsation or even anaesthetic input to secure adequate gas exchange and secure haemodynamic stability. As arterial access via an indwelling catheter is the first stage of the procedure, obtaining an initial blood sample should become routine. Further samples can easily be drawn whenever the need arises. 

 

The same principle holds true for structural valve or endovascular interventions. Obtaining regular blood gasses and metabolic profiles can be even more important in these cases because patients are often under general anaesthesia or at least heavily sedated. It is also important to keep in mind the patients undergoing endovascular or TAVR procedures are often very sick, in fact too sick to undergo conventional surgery. Demands for monitoring have to be adjusted to the complexity of the patient’s needs and might comprise numerous blood samples.  

 

On average, analysing up to three samples is not unusual during a lengthy, complex PPCI or TAVR to ensure that the haemodynamic status and homeostasis are optimised. 

 

Coagulation monitoring

The interventions discussed above require a certain degree of anticoagulation to be conducted safely and is generally measured using the activated clotting time (ACT). The accepted target ACT range during PPCI is around 200 seconds, although there is still some debate as to what level of anticoagulation is desirable during TAVR.¹⁰ The ACT is repeated as required depending on the duration of the procedure and at the end after heparin reversal. The number of blood samples processed to assess the patient’s clotting status during a procedure in the catheter lab varies between two and five. 

 

Traditionally all blood samples apart from ACT are processed outside the cath lab. Blood gas analysers might, or might not, be available in the cath lab suite. It is not unusual that samples need to be taken either to the Intensive Care or Coronary Care Unit or to the Emergency Department to be analysed. In practice that means that an assistant needs to leave the lab for an indeterminate amount of time. The time they are not available to help with the procedure obviously varies with the distance between the lab and the analyser and the number of samples that have to be analysed. However, as a general rule, it can be said that the sicker the patient and the more complex the procedure, the more samples will need to be run, which, in turn, means the assistant will be gone for longer. There are no cath lab data available, but a recent survey during cardiac surgery in our own facility has shown that the anaesthetic assistant is busy running blood gasses for up to 45 minutes during a routine procedure with five samples. If one adds the time for calibrating or broken analysers, and queues at the machine are taken into account, this time could easily be in excess of an hour. 

 

Processing samples in this traditional way leads to delays in getting the required info to the clinicians involved and to delays in acting on the data, which can have deleterious effects for the patient. 

 

Point-of-care (POC) testing

The demand on individual health care providers has grown in recent years and is likely to continue doing so. As interventions become more and more complex, timely access to diagnostic data is essential. Near-patient testing, where the analyser is somewhere within the catheter lab suite or even outside it, clearly does not offer an ideal solution. True bedside testing, with handheld devices such as the i-STAT® System at the point of care, has a number of benefits that are likely to make processes more efficient while decreasing the risks associated with complex procedures performed on often very sick patients.

 

Benefits include:

 

In addition to the patient safety and clinical advantages the POC devices bring, there are several economic arguments for introducing it in the cardiology setting. Blood gas analysers present a large capital investment when bought; they require regular (mostly daily) calibration and maintenance, often at a cost of £3000–5000 per machine per annum. POC testing operates with cartridges, is self-calibrating, and virtually maintenance free. If a device fails for whatever reason it is easy to replace with a spare, while analysers tend to be out of service until a specialist engineer arrives to rectify the faults. Devices such as the i-STAT allow a number of different tests to be run on them. Having one small platform to do the bulk of the lab work (that is, ACT, troponin, metabolic testing, haemoglobin, basic clotting) allows a high level of standardisation as well as saving the expense incurred through interfacing an array of different machines and buying different consumables or calibration supplies.

 

Conclusions

Although we have seen an exponential advance in interventional cardiology in the last few decades, the traditional monitoring and analysis of these patients has remained largely unchanged. The evolution of POC laboratory analyses will allow the delivery of a more rapid, error-free and cost effective monitoring process in this cohort of patients.

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