Cardiogenic shock (CS) is a state of inadequate tissue perfusion due to cardiac dysfunction. It is a potentially lethal complication that occurs in 10% of all cases of acute myocardial infarction (MI) [¹]. Between 1995 and 2004, the National Registry of Myocardial Infarction database recorded 300 000 ST elevation MI's of which 8.6% presented with CS [²]. In patients hospitalized for either myocardial infarction, decompensated heart failure or following cardiac surgery, CS was the leading cause of death resulting in mortality rates of up to 80% [²]. Although rapid stabilization and treatment of reversible causes via early revascularization is a priority, the mortality rate due to CS in the current era remains high [³,⁴].

Prognostic echocardiographic factors for determination of early recovery after CS are limited. Short and long term mortality appears to be associated with initial depressed LV systolic function and mitral regurgitation (MR) as assessed by echocardiography [⁵]. In patients with LV ejection fraction (EF) less than 30%, survival at one year was 24% versus 56% for those with preserved systolic function [⁵].

Tissue Doppler imaging (TDI) is a sensitive, noninvasive echocardiographic method that records velocity of tissue motion within the myocardium. TDI has been evaluated in both in vitro and in vivo studies, allowing for the quantitative assessment of both global and regional function of the myocardium [⁶]. Indices derived from TDI, including systolic velocity (S'), early (E') and late (A') diastolic velocities of the lateral mitral annulus, are reduced in heart failure patients (EF < 30%) and portend a poor prognosis. Transmitral to early diastolic velocity ratio (E/E') obtained via TDI correlates strongly with LV filling pressures [⁷]. An E/E' ratio > 10 identified a pulmonary capillary wedge pressure (PCWP) > 15 mm Hg with a sensitivity of 92% and a specificity of 80% [⁸]. In addition to chronic CHF, a higher E/E' has also been shown to correlate with a worse prognosis in acute myocardial ischemia and hypertension [^9-12]. The application of TDI prior to revascularization in patients presenting with CS remains unknown.

The objective of this study was to describe TDI derived indices in patients presenting with cardiogenic shock after acute myocardial infarction, prior to percutaneous revascularization.

Cardiogenic shock is the leading cause of death in patients with acute myocardial infarction with an in-hospital mortality rate greater than 50% [²]. A variety of structural and functional abnormalities are identifiable on echocardiography in patients presenting with acute CS [⁵]. Although Picard et al demonstrated reduced LV systolic function and mitral regurgitation as predictors of survival and response to early revascularization in CS [⁵], our study is the first to characterize a reduction in TDI derived indices in CS patients. Despite similar reductions in LV systolic function, our study demonstrated CS patients have reduced myocardial velocities and higher filling pressures using TDI as compared to CHF patients.

Mitral annular systolic velocity (S') reflects the long axis motion of the ventricle which is an important component of LV systolic function [¹⁶]. Peak myocardial systolic velocity averaged from six sites around the mitral annulus correlates well with LV ejection fraction. A S' value greater than 7.5 cm/s had a sensitivity of 79% and a specificity of 88% in predicting normal global LV function [¹⁷]. Sub-endocardial fibers make a substantial contribution to long axis function, and are susceptible to a variety of cardiac pathologies. Hypertension, coronary artery disease and CHF have been shown to negatively affect these fibers and reduce S'. A reduction in S' correlates with increased morbidity and mortality in each of these disease states [¹¹,¹⁸,¹⁹].

In our study, CS patients had a significantly lower S' compared to CHF patients. Shock has severe metabolic consequences, involving mainly energy metabolism, substrate utilization, and acid-base regulation. As longitudinally arranged subendocardial fibers are most vulnerable to ischemia and metabolic abnormalities, it is not an unexpected finding that LV base-apex contraction was abnormal in our CS population. Given the prognostic value in other cardiac disease states, a reduction of S' in CS may be of similar clinical importance in this patient population. In our study, albeit small numbers, the subset of patients in CS who survived demonstrated increased S' values on initial presentation, prior to revasculariation, as compared to those who died in-hospital.

Similar to systolic annular velocity (S'), early diastolic mitral annular (E') appears to be a strong independent predictor for the short and long-term prognosis of patients with cardiovascular disease. E' has been proposed as a useful index for the non-invasive evaluation of LV relaxation, that is relatively preload independent [²⁰]. Recent studies have shown that a reduced E' predicts increased cardiovascular morbidity and mortality [¹¹,¹⁴,¹⁹]. Wang et al demonstrated that an E' less than 3 cm/s was the best prognostic marker for long-term follow up in a population of patients with chronic hypertension [¹¹].

In the current study, E' was significantly reduced in the CS population as compared to chronic CHF patients. The potential mechanism for this abnormality may relate to the acute increase in LV wall stress due to the ischemic insult in the CS state. The reduced E' may be a sensitive marker of regional relaxation abnormality reflected in the long axis dimension that could potentially be evident earlier than clinical manifestations of global left ventricular relaxation abnormality.

Since E' is reduced and diastolic mitral annular increases with higher filling pressures, the E/E' ratio correlates well with invasive pulmonary capillary wedge pressure (PCWP) measurements [⁸,²⁰,²¹]. Elevated PCWP is associated with a higher mortality rate after acute MI and has been shown to carry independent prognostic information in CHF patients [^22-24]. In our study, CS patients had an elevated E/E' ratio and LAP confirming raised LV filling pressures. The spectrum of diastolic abnormalities which may account for the elevated ratios include increased myocardial stiffness, reduced LV compliance, and elevated LV end-diastolic parameters. A restricted filling pattern tends to correlate with higher mortality at 30 days and 1 year in CS [²²], and is known to be associated with a poor prognosis in CHF patients [²³]. Hillis et al. previously established that in the setting of acute MI, an E/E' ratio greater than 15 can identify patients at an increased risk of mortality with a sensitivity of 70% and a specificity of 91% [²⁴,²⁵]. In addition, the prognostic value of E/E' was incremental to clinical factors and conventional echocardiographic parameters of LV systolic and diastolic function [²⁴,²⁵]. An elevated E/E' ratio in CS patients may be of comparable value in distinguishing this select population at higher risk for poor cardiovascular outcomes [²⁶].

Similar to other studies using TDI, this methodology is affected by the quality of 2D images and cardiac translation, rotation, or both. Only lateral annular velocities were analyzed as septal annular velocities may be subjected to the influence of the right ventricle. Only the lateral wall of the left ventricle was routinely sampled with TDI as per routine in our echocardiography laboratory. However, lateral velocities may be affected by both translational effects and beam angle [²⁰,²⁷]. The reduced LV systolic function in the CS population may have represented myocardial stunning, and thus the true LVEF may have been underestimated in this subgroup. Tissue Doppler imaging parameters was not obtained immediately following revascularization nor at 6 month followup in the patient population. Lastly, this study is limited by the relatively small sample size and retrospective design. A larger, prospective study is needed in order to make more substantive conclusions regarding the clinical utility of tissue Doppler indices prior to percutaneous revascularization in patients with cardiogenic shock.

Despite similar reduction in LV systolic function, CS patients have reduced myocardial velocities and higher filling pressures compared to chronic CHF patients. Whether TDI could be a reliable tool to determine CS patients with the best chance of recovery following revascularization is yet to be determined.