Presentation and Diagnosis
Patients suffering from STEMI and cardiogenic shock present with signs and symptoms of hypoperfusion and heart failure. The diagnosis is made clinically and can be confirmed with placement of a pulmonary arterial catheter (PAC).
Common symptoms include chest pain and dyspnoea. On physical examination, patients are hypotensive and may show evidence of systemic hypoperfusion such as altered mental status or poor urine output. Laboratory studies may show an elevated lactate level and a rising creatinine, which may be due to both decreased renal perfusion and venous congestion. Classically, cardiogenic shock has been associated with cool extremities due to low cardiac output and compensatory systemic vasoconstriction. In practice, however, systemic vascular resistance is often not elevated and may even be low. This may be due to concomitant septic shock, particularly as the hypoperfusion from cardiogenic shock places patients at high risk for ischaemic bowel and subsequent translocation of gut microbes. In addition, myocardial infarction alone can lead to a systemic inflammatory response.8 Most STEMI patients who develop cardiogenic shock do so not on arrival to the hospital, but within the first 24 hours of admission.9
Patients may also demonstrate signs of volume overload. Decreased oxygen saturation and rales due to pulmonary oedema may be present in patients with predominantly left ventricular dysfunction and may necessitate intubation, but up to one-third of such patients present without pulmonary congestion and chest X-ray may be clear.10 Patients with mainly right-sided involvement tend to have clear lungs but may have distended neck veins and peripheral oedema or ascites.
Risk factors for cardiogenic shock associated with STEMI include older age, diabetes mellitus, ongoing angina, heart failure, low systolic blood pressure, tachycardia and left bundle branch block.11,12 Of these risk factors, age appears to be most predictive. An increase in age by 10 years has been shown to be associated with a nearly 50 % higher probability of developing cardiogenic shock.12 Echocardiography should be performed to assess ventricular function and to exclude mechanical complications of STEMI. Echocardiography findings in cardiogenic shock include severe impairment of ventricular function, which may be predominantly right or left-sided or both and may be either systolic or diastolic or both. While a low left ventricular ejection fraction (LVEF) is a poor prognostic indicator, it does not completely correlate with the presence of cardiogenic shock. Often the LVEF is only moderately depressed and occasionally can be preserved.13–16
Placement of a PAC can both provide a definitive diagnosis of cardiogenic shock and guide resuscitative efforts. PAC haemodynamic measurements confirm the diagnosis of cardiogenic shock by showing a low cardiac index (<2.2 L/min/m2) and elevated ventricular filling pressures. Mixed venous oxygen saturation is typically low, reflecting the decreased cardiac output and increased oxygen extraction from peripheral tissues. In addition, PACs can distinguish between left ventricular or right ventricular dysfunction as the primary cause of shock by demonstrating a corresponding pulmonary capillary wedge pressure (PCWP) greater than 15–18 or right ventricular end diastolic pressure (RVEDP) greater than 10–15 mmHg, respectively.10 From the haemodynamic parameters provided by a PAC, systemic vascular resistance can be calculated to determine the potential co-existence of other types of shock. While PACs have not been proven to confer mortality benefit or lead to shorter hospitalisations, their utility specific to cardiogenic shock has not been evaluated.17–19 In practice, they provide haemodynamic measurements that can prove invaluable in both diagnosis and management of cardiogenic shock.20