Shock Index More Sensitive Than Cardiogenic Shock in ST-Elevation Myocardial Infarction Treated by Primary Percutaneous Coronary Intervention

Abstract

Background: Cardiogenic shock (CS) is a strong predictor of mortality in patients with ST-elevation myocardial infarction (STEMI), but there is evidence that shock index (SI), taking into account both blood pressure and heart rate, is a more sensitive and powerful predictor. We investigated the independent impact of SI and CS on 30-day and 1-year mortality in patients with STEMI, treated by primary percutaneous coronary intervention (PCI).

Methods and Results: In 7,412 consecutive patients with STEMI treated with primary PCI, the predictive value of either SI or CS on 1-year mortality was assessed. Best cut-off value of SI, determined using receiver operating characteristic (ROC) curve, was 0.7, with an ROC AUC of 0.66 (95% CI: 0.65–0.67), compared with an ROC AUC of 0.60 (95% CI: 0.59–0.61) for CS (P<0.001). At admission, 387 patients (5.2%) had CS and 1,567 patients (21.1%) had SI ≥0.7. The adjusted hazard ratio of mortality in patients with SI ≥0.7 and in CS patients was, respectively, 3.3 (95% CI: 2.4–4.6) and 3.1 (95% CI: 2.1–4.6) after 30 days, and 2.3 (95% CI: 1.8–2.9) and 3.1 (95% CI: 2.2–4.2) after 1 year.

Conclusions: SI identifies more patients with increased risk of mortality, and seems to be a more sensitive prognostic predictor than CS in patients with STEMI treated by primary PCI.

Cardiogenic shock (CS) has previously been shown to be a strong and independent predictor of both short-term and long-term mortality in patients with ST-elevation myocardial infarction (STEMI).¹ Although the incidence of CS in patients with STEMI has declined in the past decades, the mortality due to CS remains high (approximately 50%) despite all advances in management, medication and technology.² Early identification of the pre-shock state might play an important role in improving the prognosis of CS in STEMI patients.

Several studies in non-cardiac shock patients suggested that shock index (SI), the ratio of heart rate (HR) to systolic blood pressure (SBP), is a sensitive tool for early recognition of septic, hypovolemic or obstructive shock.^3–7 The combination of HR and SBP may be more powerful than either HR or SBP alone, because early stages of shock are also taken into account. Therefore, SI can be a good marker of pre-shock. The predictive importance of SI in patients with STEMI has been shown in a recent study, but that study investigated only short-term mortality, and only 12% of the patients were treated by primary percutaneous coronary intervention (PCI).⁸ Another study assessed only a small cohort of patients.⁹ Recently, Spyridopoulos et al showed that SI is an independent and stronger predictor of long-term mortality in older patients with STEMI.¹⁰ In that study, however, an unusual definition of CS (SBP <100 mmHg with HR >100 beats/min) was used, which makes it difficult to compare the results with previous studies and to extrapolate it into daily practice. Furthermore, they did not assess the optimal cut-off for SI, but used a self-chosen cut-off, which was different from previous studies.^8,9 The independent prognostic value of SI as compared with CS in STEMI treated with primary PCI, is therefore still unclear. The aim of this study was to evaluate the independent impact of SI on 1-year mortality in a large cohort of patients with STEMI (largest until now), treated by primary PCI, and to compare this with CS.

Methods

Data for this study were collected from the prospective cohort of patients with STEMI at the Isala Hart Centrum, Zwolle, The Netherlands, from January 2000 to December 2011. Patients were diagnosed with STEMI if they had chest pain >30 min duration and electrocardiographic changes with ST-segment elevation >2 mm in at least 2 precordial or >1 mm in the limb leads. To avoid double inclusion, only the first recorded admission for STEMI was included. All patients underwent emergency angiography and in almost all patients primary PCI was performed. Baseline blood pressure, HR and laboratory measurements were obtained before the emergency angiography. Prior to emergency angiography, all patients received 300–500 mg aspirin i.v., a P2Y12 blocker (either 600 mg clopidogrel or 180 mg ticagrelor), and 3,000–5,000 IU unfractionated heparin. Additional treatment with glycoprotein IIb/IIIa inhibitors, balloon angioplasty, bare metal stents or drug-eluting stents, was left at the discretion of the treating cardiologist. All patients were treated with optimal medication including angiotensin-converting enzyme inhibitor (ACEI), β-blocker, aspirin and statin where appropriate. Primary PCI was performed with standard techniques if the coronary anatomy was suitable for angioplasty. Ischemic time was calculated from symptom onset to first balloon inflation. Mortality data were prospectively entered in the local STEMI database and checked with the municipal records.

Definitions

CS at admission was defined as SBP <90 mmHg for ≥30 min, clinical signs of pulmonary congestion and end-organ hypoperfusion (cool extremities, altered mental status, or urine output <30 mL/h). Admission SI was defined as the ratio of HR and SBP on hospital admission.^3–10 Coronary angiographic flow was determined in accord with the classification system of the Thrombolysis in Myocardial Infarction (TIMI) study group.

Statistical Analysis

Statistical analysis was performed using SPSS version 20.0 (SPSS, Chicago, IL, USA). Continuous data are presented as mean±SD or median (IQR) for non-normally distributed variables; whereas discrete data are given as absolute values and percentages. Categorical variables were compared using the chi-squared test. Continuous variables were compared using the independent t-test. To define potential confounders in the baseline characteristics, we compared baseline characteristics between patients with or without CS and with SI higher or lower than the optimal cut-off. Sensitivity and specificity analysis was carried out to identify the best cut-off point of SI to predict outcome (1-year mortality). This was done by constructing receiver operating characteristic (ROC) curves, and analyzing area under the curve (AUC) with 95% CI for different cut-offs. We also compared the ROC curves for CS and SI, in order to determine which variable predicts 1-year mortality better. Survival analysis was performed, with also a landmark analysis of patients who survived 30 days after hospital discharge, and presented as Kaplan-Meier survival curves. Stratified analyses were performed to assess the prognostic importance in subgroups and to investigate potential effect modifications. Independent predictive value was determined on multivariable analysis using Cox proportional hazard modeling, adjusting for age, gender, diabetes mellitus, hypertension, previous MI, previous coronary artery bypass grafting (CABG), previous cerebrovascular accident (CVA), baseline glucose, pre-PCI TIMI grade 0–1, post-PCI TIMI grade <3, anterior MI, multivessel disease (MVD), PCI performed, stent placement, intra-aortic balloon-pump (IABP), CABG within 2 days and CABG within 30 days. We also tested for interaction by including an interaction variable for age, gender, CS and SI. Two-sided P<0.05 was considered statistically significant.

Results

A total of 7,412 patients were included in the current analysis. Mean age was 63.2±12.7 years, and 2,036 patients (27.5%) were female. Anterior MI was present in 2995 patients (40.4%). PCI was performed in 6,379 patients (86.1%) and a stent was implanted in 5,125 patients (69.1%). In 619 patients (8.7%) IABP was placed, because of CS, persistent ischemia or as a bridge to therapy. Mechanical circulatory support with left ventricular (LV) assist devices and veno-arterial extracorporeal membrane oxygenation was used in 9 and 3 patients, respectively. In 629 patients (8.5%) CABG was performed within 30 days. Total 1-year mortality was 7.5% (554 patients). Patients who died within 1 year were significantly older and more often female, compared with patients who survived 1 year (Table 1). Among other variables, mean blood pressure on admission was lower and mean HR higher in patients who died within 1 year.

Table 1. Baseline Subject Characteristics vs. 1-Year Survival

	1-year survival (n=6,858)	Death <1 year (n=554)	P-value
Age (years)	62.5±12.5	73.0±12.0	<0.001
Female	26.7	37.0	<0.001
Risk factors
Diabetes mellitus	11.0	19.7	<0.001
Hypertension	34.6	48.5	<0.001
Smoking	44.5	28.5	<0.001
Dyslipidemia	22.3	19.3	0.11
Family history	41.2	20.3	<0.001
History
Previous MI	8.1	13.8	<0.001
Previous PCI	6.6	8.8	0.06
Previous CABG	3.0	6.0	<0.001
Previous CVA	2.5	9.1	<0.001
Hemodynamics on admission
SBP (mmHg)	133±25	122±31	<0.001
Heart rate (beats/min)	75±18	85±25	<0.001
CS	4.1	19.7	<0.001
SI	0.58±0.17	0.75±0.33	<0.001
Lab results on admission
Hemoglobin (mmol/L)	8.7±0.9	8.1±1.4	<0.001
Glucose (mmol/L)	9.0±3.1	11.7±5.4	<0.001
Creatinine (μmol/L)	87±34	124±94	<0.001
Ischemic time (min)	385±10	461±46	0.061
Door to balloon time (min)	133±8	192±45	0.096
Angiographic data
Anterior location	42.8	44.1	0.59
Inferior location	39.5	34.4	0.032
Multivessel disease	48.2	66.8	<0.001
Pre-PCI TIMI 0–1	63.4	75.6	<0.001
Post-PCI TIMI <3	7.7	30.2	<0.001
Treatment
PCI	90.1	74.1	<0.001
Stent placement	75.7	56.8	<0.001
CABG within 2 days	2.2	6.3	<0.001
CABG within 30 days	8.3	10.5	0.10
IABP	7.3	27.6	<0.001

Data given as mean±SD or %. CABG, coronary artery bypass grafting; CS, cardiogenic shock; CVA, cerebrovascular accident; IABP, intra-aortic balloon-pump; MI, myocardial infarction; PCI, percutaneous coronary intervention; SBP, systolic blood pressure; SI, shock index; TIMI, Thrombolysis in Myocardial Infarction.

At admission, a total of 387 patients (5.2%) had CS. Patients with CS were older, more often female and more often had MVD (Table 2A). Remarkably, patients with CS had less often anterior MI (30.1% vs. 43.6%, respectively; P<0.001), but more often inferior MI (50.1% vs. 38.5%, respectively; P<0.001).

Table 2. (A) Baseline Subject Characteristics vs. Presence of CS on Admission, and (B) Baseline Subject Characteristics vs. Presence of SI ≥0.7 on Admission

	A			B
	No CS (n=7,025)	CS (n=387)	P-value	SI <0.7 (n=5,845)	SI ≥0.7 (n=1,567)	P-value
Age (years)	63.2±12.7	64.5±12.4	0.04	63.4±12.6	62.8±13.3	0.15
Female	27.2	32.6	0.02	26.8	30.1	0.01
Risk factors
Diabetes mellitus	11.6	11.9	0.87	10.8	14.5	<0.001
Hypertension	35.9	31.6	0.09	36.6	32.3	0.002
Smoking	43.0	48.7	0.03	42.7	45.6	0.04
Dyslipidemia	22.3	18.2	0.07	22.3	21.2	0.34
Family history	40.0	33.6	0.02	40.1	38.0	0.14
History
Previous MI	8.5	9.1	0.64	8.1	10.0	0.02
Previous PCI	6.8	7.3	0.68	6.8	6.9	0.91
Previous CABG	3.2	2.9	0.77	3.0	4.0	0.06
Previous CVA	3.0	4.2	0.17	2.7	4.3	0.002
Hemodynamics on admission
SBP (mmHg)	135±24	81±10	<0.001	137±25	112±21	<0.001
Heart rate (beats/min)	76±18	72±27	<0.001	71±14	96±19	<0.001
SI	0.58±0.16	0.91±0.38	<0.001
Lab results on admission
Hemoglobin (mmol/L)	8.7±0.9	8.1±1.3	<0.001	8.7±0.9	8.6±1.1	<0.001
Glucose (mmol/L)	9.1±3.3	11.4±5.0	<0.001	9.0±3.1	10.1±4.2	<0.001
Creatinine (μmol/L)	89.1±41.4	113±70.8	<0.001	89.2±44.2	94.5±41.7	0.001
Ischemic time (min)	391±10	368±45	0.627	376±10	445±26	0.015
Door to balloon time (min)	139±8	93±27	0.101	137±9	137±20	1.000
Angiographic data
Anterior location	43.6	30.1	<0.001	41.8	47.3	<0.001
Inferior location	38.5	50.1	<0.001	40.1	35.2	<0.001
Multivessel disease	49.1	56.6	0.005	48.9	51.6	0.07
Pre-PCI TIMI 0–1	63.6	73.6	<0.001	64.4	63.1	0.39
Post-PCI TIMI <3	8.6	17.1	<0.001	8.0	13.2	<0.001
Treatment
PCI	89.1	85.4	0.03	90.2	84.0	<0.001
Stent placement	74.9	65.9	<0.001	75.9	68.6	<0.001
CABG within 2 days	2.3	7.2	<0.001	1.9	4.9	<0.001
CABG within 30 days	8.3	11.1	0.06	7.8	11.0	<0.001
IABP	7.4	32	<0.001	5.7	20.1	<0.001

Data given as mean±SD or %. Abbreviations as in Table 1.

On ROC analysis to determine the best cut-off of SI to predict 1-year mortality, SI ≥0.7 had the best discrimination, with an AUC of 0.71 (95% CI: 0.68–0.74) for 30-day mortality and 0.66 (95% CI: 0.64–0.69) for 1-year mortality (Figure 1). The ROC AUC for CS was 0.66 (95% CI: 0.63–0.70) for 30-day mortality and 0.60 (95% CI: 0.59–0.61) for 1-year mortality. The AUC for SI was significantly higher compared with CS, for both 30-day mortality (P=0.007) and 1-year mortality (P<0.001).

Figure 1.

Receiver operating characteristic (ROC) curve of shock index (SI ≥0.7) and cardiogenic shock as predictors of 1-year mortality.

A total of 1,567 patients (21.1%) had SI ≥0.7 at admission. Subject characteristics according to presence of SI ≥0.7 are listed in Table 2B. Remarkably, patients with SI ≥0.7, in contrast to patients with CS, had more often anterior MI (47.3% vs. 41.8%, respectively; P<0.001) and less often inferior MI (35.2% vs. 40.1%, respectively; P<0.001).

The 30-day and 1-year mortality in patients with SI ≥0.7 was 11.6% and 15.9%, respectively, with an adjusted hazard ratio of 3.3 (95% CI: 2.4–4.6) and 2.3 (95% CI: 1.8–2.9) compared with patients with SI <0.7. On subgroup analysis, SI ≥0.7 was associated with increased 1-year mortality in almost all subgroups (Figure 2).

Figure 2.

Risk of 1-year mortality in patients with shock index (SI) ≥0.7 vs. SI <0.7. CVA, cerebrovascular accident; DM, diabetes mellitus; IABP, intra-aortic balloon pump; MI, myocardial infarction; TIMI, Thombolysis in Myocardial Infarction.

In patients with CS the 30-day and 1-year mortality was 22.2% and 28.2%, respectively, with an adjusted hazard ratio of 3.1 (95% CI: 2.1–4.6) and 3.1 (95% CI: 2.2–4.2). On subgroup analysis of 1-year mortality, the presence of CS was associated with increased mortality in all subgroups (Figure 3). Remarkably, age <75 years was associated with a greater 1-year mortality risk than age ≥75 years.

Figure 3.

Risk of 1-year mortality according to presence of cardiogenic shock (CS). Abbreviations as in Figure 2.

To test for interaction between age, gender, CS and SI ≥0.7, we also performed a Cox regression analysis with inclusion of an interaction variable. We did not find a significant interaction.

To study misclassification between CS and SI, patients were divided into 4 groups according to presence of CS and of SI ≥0.7 (Figure 4). Survival curves (adjusted for age and gender), showed that patients had the best prognosis if CS was absent and SI was <0.7 (1-year mortality, 4.9%) and the worst prognosis if CS was present and SI was ≥0.7 (1-year mortality, 33.1%; Figure 5).

Figure 4.

Patient distribution according to presence of cardiogenic shock (CS) and shock index (SI) ≥0.7.

Figure 5.

Age- and gender-adjusted 1-year mortality according to presence of cardiogenic shock (CS) and shock index (SI) ≥0.7.

If CS at admission was absent, patients with SI ≥0.7 still had higher 1-year mortality than patients without CS and SI <0.7 (12.6%, P<0.05). If CS was present, despite SI <0.7, 1-year mortality was also significantly higher than in patients without CS and SI <0.07 (19.1%, P<0.05), but this concerned only 136 patients (1.8%). In this subgroup, inferior wall infarction was present in 84 patients (61.8%), whereas anterior wall infarction was present in only 32 patients (23.5%; P<0.001). The most important predictor of CS with SI <0.7 was inferior wall location (hazard ratio, 2.5; 95% CI: 1.7–3.4), suggesting right ventricular (RV) involvement or vasovagal reactions.

The prognostic value of SI ≥0.7 still remained after exclusion of the early mortality patients (i.e., death within 30 days; Figure 6; landmark analysis).

Figure 6.

One-year mortality according to presence of shock index (SI) ≥0.7, adjusted for age, gender, infarct location and multivessel disease: Landmark analysis in those surviving 30 days.

Discussion

In patients with STEMI treated by primary PCI, more patients with increased mortality risk can be identified using SI, compared with CS alone. In patients without CS at admission, SI ≥0.7 indicates a significantly worse prognosis. SI may be an easy tool to identify patients with increased risk of 1-year mortality, and to facilitate a different treatment approach in those who are at risk. Furthermore, it is an easily obtained and reliable parameter, given that arterial access and monitoring during primary PCI enable both arterial blood pressure and HR measurement. The present findings are similar to those of previous studies in patients with STEMI and in patients with non-cardiac shock.^3–10 The cut-off for SI differed in previous studies. Bilkova et al used 0.8 as a cut-off, based on previous studies on SI in non-cardiac shock.⁹ Huang et al performed ROC analysis, with SI=0.7 as the most optimal cut-off.⁸ Spyridopoulos et al used an SI of 1.0, without performing ROC analysis.¹⁰ In the present study, the optimal cut-off for SI was 0.7 on ROC analysis.

In the present study, CS at admission was defined as SBP <90 mmHg for ≥30 min, clinical signs of pulmonary congestion; and end-organ hypoperfusion (cool extremities, altered mental status, or urine output <30 mL/h). There is still no international consensus, however, on what should be the definition of CS. In the literature, different definitions of CS are used.^1,10–12 According to the ESC guidelines the clinical definition of CS is SBP <90 mmHg for ≥30 min or the need for supportive measures to maintain SBP ≥90 mmHg, and end-organ hypoperfusion (cool extremities or urine output <30 mL/h, and HR ≥60 beats/min).¹³ A limitation of this definition is the combination of SBP <90 mmHg and HR ≥60 beats/min. With this definition, not only patients with “normal” BP and tachycardia can be missed, but also patients with SBP <90 mmHg and bradycardia <60 beats/min (i.e., patients with inferior wall infarction complicated by bradycardia or total atrioventricular-nodal block), while prognosis in these patients is impaired.¹⁴ The poor prognosis of both low SBP and bradycardia was also confirmed in the present analysis.

The most important question remains as to whether we can change the prognosis of patients who are at increased risk. In the GUSTO study, some of the patients with STEMI without CS at admission developed CS after an average of 12 h.¹⁵ It is therefore very important to identify patients who are at risk of developing CS. Based on the present finding that elevated SI at admission is associated with an increased mortality risk, even if CS is absent, SI might be a good parameter of pre-shock.

Clinical Implications of SI in Daily Practice

First, although in all patients with STEMI fast opening of the infarct-related vessel and optimal medical treatment are of importance, the benefit may be great, particularly in those with increased risk of mortality. In patients with MVD, early additional revascularization can be considered. Second, in all patients with signs of hemodynamic deterioration, there should be increased alertness for mechanical complications, such as acute mitral regurgitation or rupture of either the ventricular septal or free walls, or acute heart failure. Also, it must be clarified that hypotension is not due to inadequate LV filling pressure. This can be done via invasive measurement during the PCI. Early echocardiographic evaluation, however, may be of greater value, because it also provides information on LV function and the presence of mechanical complications, which cannot be obtained by routine invasive measurements. Third, drugs that have negative inotropic activity should be used cautiously in patients with CS or “pre-shock” (as in those with SI ≥0.7), including β-blockers, ACEI and calcium channel blockers. Of note, in the COMMIT trial, randomization to early β-blockade was associated with a 30% higher occurrence of CS in patients aged >70 years, those with SBP <120 mmHg or with HR >110 beats/min, and those with Killip class >1.¹⁶ In patients with chronic maintenance treatment with β-blockers and ACEI, temporary discontinuation, or lowering the dose should be considered. In addition, anti-arrhythmic drugs with negative inotropic or vasodilating properties, such as lidocaine, procainamide, bretylium, and quinidine, should be used with caution. Amiodarone, although it has β-blocking properties, is the preferred anti-arrhythmic agent for sustained ventricular or atrial tachyarrhythmias.

Until now, all studies on pharmacologic and non-pharmacologic methods of circulatory support have involved patients already in CS. The chances of surviving CS are still bad: approximately 50%. Possibly, initiation of pharmacologic or non-pharmacologic circulatory support in the pre-shock phase, will lead to better survival. Although the benefit of IABP for CS was questioned after the IABP shock II trial,¹² this trial had several limitations, such as a high rate of cross-over of the control group to IABP, inclusion of patients after out-of-hospital resuscitation, and inclusion of patients with RV involvement. Therefore, IABP can still be considered for patients with persistent hypotension, or in those with ischemia, as a bridge to additional revascularization, or maybe even in patients with high SI.

Hypotension in patients with inferior STEMI can be a symptom of RV infarction. RV ischemia complicates up to 50% of inferior MI, whereas isolated RV MI is rare. Although the RV shows in general good long-term recovery, in the short term RV involvement is associated with worse prognosis.¹⁴ Also in the present study, those with CS but SI <0.7 had a worse prognosis. These patients had more often inferior wall infarction (61.8%), and probably, in part, RV involvement. But, given that the mechanism of hypotension in those with RV infarction is different compared with that of hypotension due to severe LV dysfunction, treatment may also differ. Left-sided mechanical assist devices, including IABP, may not be helpful in RV infarction, and this is a possible explanation of the lack of benefit of IABP in the IABP SHOCK-2 trial.¹²

It remains unclear if elevated SI also predicts development of CS during hospital stay. Further studies are needed to address this question. Also, external validation of SI as predictor of long-term mortality is needed.

Strengths and Limitations

We assessed the independent prognostic importance of both CS and SI in a large cohort of patients with STEMI treated by primary PCI and with 1-year follow-up. This is the largest cohort study to date. The present study has, however, also some limitations. Data on blood pressure and HR were obtained only at admission and not during hospitalization, while hemodynamics after primary PCI are also (or even more) of importance.¹⁵ Compared with other studies, the 1-year mortality in the present CS patients was lower.² This could partially be explained by the misclassification of CS patients who developed CS during hospitalization. We had no data on invasively measured LV filling pressure or cardiac index, and only a few patients had right heart catheterization. Invasive hemodynamic measurement in the setting of STEMI, however, is not routine daily practice. Furthermore, we had no data on use of inotropics before admission, although this medication is rarely given in the ambulance. We did not have sufficient data on lactate level at admission, or RV infarction. Therefore, these variables were not included in the multivariate analysis. Finally, we had no data on cause-specific mortality.

Conclusions

SI identifies more patients with increased risk of mortality, and seems to be a more sensitive prognostic hemodynamic predictor than CS in patients with STEMI treated by primary PCI.

References

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