Mechanism of shock associated with right ventricular infarction

Importance of the sampling site for measurement of mixed venous oxygen saturation in shock

OBJECTIVES

To determine if oxyhemoglobin saturation in blood samples taken from the superior vena cava or right atrium can be substituted for oxyhemoglobin saturation in blood taken from the proximal pulmonary artery (SVO2) in patients in shock.

DESIGN

Prospective clinical investigation.

SETTING

Mixed surgical/medical intensive care unit in a university hospital.

PATIENTS

Thirty consecutive patients in severe circulatory shock who required insertion of a pulmonary artery flotation catheter (PAFC) immediately on intensive care unit admission. All patients fulfilled the criteria described below which were established in advance.

MEASUREMENTS AND MAIN RESULTS

Oxyhemoglobin saturation in the superior vena cava, right atrium, and pulmonary artery (SVO2) was measured by cooximetry in consecutive blood samples from each site during initial insertion of the PAFC. The mean standard deviation of values from these sites was similar: 74 +/- 12.5%, 70.+/- 13%, and 71.3 +/- 12.7%, respectively. However, when superior vena cava and right atrial oxyhemoglobin saturations and SvO2 were compared, the ranges and 95% confidence limits were found to be clinically unacceptable. The ranges were -19.3 to 23.1% and -19.7 to 16.7%, respectively, and the 95% confidence limits were -18.4 to 24.2% and -18.6 to +17.3%, respectively.

CONCLUSIONS

These wide range differences and confidence limits would lead to large errors if superior vena cava or right atrial oxyhemoglobin saturations were substituted for true mixed venous blood in oxygen transport or pulmonary venous admixture calculations, or if clinical decision making was based on individual results. In patients in shock in whom clinical decisions may be based on the value of mixed venous oxyhemoglobin, oxyhemoglobin saturation is only reliably measured in samples taken from the pulmonary artery.

Prediction of right ventricular and posterior wall ST elevation by coronary care nurses: the 12-lead electrocardiograph versus the 18-lead electrocardiograph

OBJECTIVES

To determine the value of routine versus selective use of the 18-lead electrocardiogram in determining the size of an acute inferior myocardial infarction (MI).

DESIGN

Prospective, quasi-experimental, random assignment.

SETTING

The coronary care unit (CCU) of a major teaching hospital in South Australia.

PATIENTS

Fifty-two patients admitted to the CCU with acute evolving inferior MI.

OUTCOME MEASURES

Correlation and comparison of the predictions of the right ventricular (RV) and posterior wall (PW) lead ST elevation with prospectively chosen markers on the 12-lead electrocardiogram--ST elevation in lead III > II and precordial ST depression, and the predictions by coronary care nurses.

PROCEDURE

The results of 18-lead electrocardiograms of 52 consecutive patients admitted to the CCU with acute evolving inferior MI were classified according to prospectively chosen criteria. Coronary care nurses were randomly assigned four 12-lead electrocardiograms and asked to "blindly" predict ST elevation in the concurrent RV and PW leads.

RESULTS

ST elevation in lead III > II demonstrated a sensitivity and positive predictive accuracy of 86% to 1 mm of ST elevation in the RV leads. ST depression in V1, V2, and V3 similarly demonstrated a 75% sensitivity and 89% positive predictive accuracy to 1 mm of ST elevation in the PW leads. In comparison, coronary care nurses proved to be as accurate in their predictions of additional PW ST elevation (p = 0.73), but were significantly less able to predict RV ST elevation (p = 0.049). These predictions were independent of the level of experience and qualifications.

CONCLUSIONS

Discriminating between smaller and larger types of inferior MIs has the potential to alter patient management: Thirty-two percent of patients in the study demonstrated additional ST elevation in both the RV and PW leads. Both of the 12-lead electrocardiogram markers used in this study proved reasonably accurate in predicting additional ST elevation in the leads that normally comprise the 18-lead electrocardiogram. Recognition of these markers has the potential to expedite the need for the additional 18-lead electrocardiogram when rapid assessment of infarction size is required. However, the routine use of the 18-lead electrocardiogram is supported by this study.