SESSION TYPE: Critical Care Cases I
PRESENTED ON: Monday, October 22, 2012 at 01:45 PM - 03:00 PM
INTRODUCTION: Low pulse oximetry saturations are common in intensive care units (ICUs). Herein we present an unusual case of “refractory hypoxemia” in a patient with cardiogenic shock.
CASE PRESENTATION: A 64 year-old male was transferred to the ICU with cardiogenic shock requiring intra-aortic balloon pump secondary to acute ST-elevation myocardial infarction and post-infarction ventricular septal defect (VSD). On arrival he was hemodynamically unstable therefore an Impella left-ventricular assist device (LVAD) was placed and position confirmed with transesophageal echocardiogram (TEE). Despite adequate blood pressure and end-organ perfusion, pulse oximetry remained persistently in the 70-80% range. The differential diagnosis for low pulse oximetry included poor signal due to vasoconstriction or poor fit, extreme anemia, methemoglobinemia secondary to TEE with benzocaine use, malpositioning of the Impella with a port nearing the VSD thereby delivering de-oxygenated blood from the right ventricle into the aorta, and “true hypoxia” as determined by arterial blood gas (ABG). The waveform appeared acceptable, the hemoglobin was above twelve, and methemoglobin levels were negligible. The Impella was repositioned under fluoroscopy without improvement in pulse oximetry saturations. Serial ABGs demonstrated arterial oxygen levels of >200 and co-oximetry oxygen saturations of >95% despite persistent pulse oximetry readings of 70-80%. Unfortunately, the patient suffered an asystolic arrest and expired.
DISCUSSION: LVAD use is increasing with the development of percutaneous LVADs such as the Impella device. The direction of blood flow with the Impella simulates normal physiology but provides continuous, non-pulsatile flow. In the presence of preserved contractility, some level of pulsatile blood flow should remain. Unfortunately, the severe cardiogenic shock in our patient led to minimal innate systolic function, therefore the cardiac output was largely dependent on the continuous flow of the LVAD. Pulse oximetry requires both pulsatile (arterial) and non-pulsatile flow (venous and background tissues) to accurately estimate arterial oxygen saturation. In this case, the presence of arterial (oxygenated) non-pulsatile flow lead to artifactual measurement of oximetry in the venous blood and background tissues resulting in falsely low peripheral oxygen saturations. Co-oximetry, which does not rely on pulsatile flow, confirmed that arterial oxygen saturation was in the normal range.
CONCLUSIONS: In the ICU, with increasing LVAD utilization, recognizing the limitations of pulse oximetry with concomitant LVAD use is imperative.
1) Ortega R, et al. Pulse Oximetry. N Engl J Med 2011;364:e33. Accessed 3/27/2012
2) Naidu S. Hemodynamic Support Novel Percutaneous Cardiac Assist Devices : The Science of and Indications for Hemodynamic Support. Circulation 2011, 123:533-543.
DISCLOSURE: The following authors have nothing to disclose: Kelly Cawcutt, Fabien Maldonado
No Product/Research Disclosure InformationMayo Clinic, Rochester, MN