Chapter 68: Noninvasive and Minimally Invasive Cardiac Output Monitoring

INTRODUCTION

More than 1,000,000 cases of shock present to the Emergency Department per year. Many critically ill patients remain in Emergency Departments for extended periods of time. Delays in diagnosis and/or therapy may increase morbidity and mortality.¹ The underlying physiologic abnormality for all types of shock remains inadequate oxygen delivery. The main contributors to oxygen delivery are cardiac output (CO) and oxygen content. Early aggressive management of patients with shock leads to improved outcomes.²

The goals of CO monitoring in shock are to aid in the diagnosis of undifferentiated shock, to accurately measure the patient’s hemodynamics, to treat patients more precisely, and to improve patient outcomes. CO and volume status have traditionally been measured by physical examination and invasive monitors (e.g., pulmonary artery catheter and central venous pressure monitoring).³ Noninvasive CO monitoring aims to give the Emergency Physician accurate dynamic monitoring information to guide clinical decision making to optimize preload, administer fluid resuscitation, and administer vasopressors.

CO determination has conventionally been obtained by thermodilution or dye dilution measurements that are invasive and associated with potential risks. Pulmonary artery (PA) catheters (Chapter 67) are considered the reference standard for hemodynamic assessment. PA catheters have complication rates of up to 7%.^4-6 Complications of PA catheters include sepsis, arrhythmias, valvar damage, cardiac tamponade, intermittent CO determinations, expense, and restricted use to monitored sites within a hospital (i.e., Emergency Department, Intensive Care Unit, or Operating Room).

Noninvasive devices that measure CO are being used more often and show agreement with the reference standard thermodilution technique. An ideal noninvasive CO monitor should be accurate, reliable, continuous, and compatible with adult and pediatric patients. No single device fits all the criteria despite considerable advancements in CO monitoring. The commercially available modalities for noninvasive CO monitoring fall into three categories: ultrasound (US)-based technology, pulse contour analysis, and thoracic electrical bioimpedance. This chapter reviews the available technology and its applications and limitations.

ULTRASOUND-BASED TECHNOLOGY

Christian Doppler identified that the velocity of a moving object is proportional to the shift in reflected frequency of an optic wave of known frequency. This principle has been adapted to sound waves and is now the basis for Doppler devices that can continuously measure the velocity of blood flow and related hemodynamic variables. The first use of Doppler to measure the velocity of red blood cells in humans or animals occurred in 1969.⁵ Commercial devices are available that measure blood velocity in the thoracic aorta via a transcutaneous or transesophageal approach. The USCOM 1A (Ultrasonic CO Monitor, Uscom Ltd., Sydney, Australia) and the Oesophageal Doppler Monitor (ODM; CardioQ, Deltex Medical Inc., Greenville, SC) display continuous hemodynamic data (e.g., CO, peak velocity, and corrected flow time) and the characteristic flow-velocity waveform (Figures 68-1 and 68-2). Point-of-care US can be used to assess CO and fluid responsiveness.