Determining fluid responsiveness remains controversial. At a basic level, we want to know if our patients will get better when we give them fluids. Or putting it more scientifically, when we increase the patient's pre-load, will his cardiac output increase as well?
Answering questions about the patient's cardiac output (CO) traditionally required invasive monitoring. Fortunately, this question can be easily answered for the modern EP with a little bit of effort and a bedside ultrasound. Once the cardiac output is calculated, a passive leg raise maneuver can be performed by laying the patient supine and elevating his legs to 45 degrees for two minutes. This maneuver roughly mimics a 250 cc bolus, and allows the EP to assess the cardiac output after this simulation to determine if there is a positive response. An increase of more than 10 percent in CO after passive leg raise indicates that a patient is likely to benefit from a fluid bolus.
You need two views to calculate the CO: the parasternal long axis of the heart and an apical five-chamber view. The parasternal view can typically be obtained in the third or fourth intercostal space, just lateral to the sternum. The indicator dot should be pointing toward the patient's right shoulder. Once this view is obtained, the image should be paused so that the examiner can scroll backwards and forwards through the cardiac cycle. The aortic root should be measured at the base of the valves, one or two frames before the valves close. This measurement should not change after the passive leg raise maneuver, and only needs to be taken once.
Attention can then be turned to the apical view. To obtain this view, the transducer is placed at approximately the mid-clavicular line at roughly the fourth or fifth intercostal space. The indicator should point toward the patient's left. The heart may initially appear to lie on its side, however, moving the transducer more laterally and aiming the direction of the beam toward the right shoulder will improve the image. A left lateral decubitus position may also give you a nicer image.
Once four chambers are seen (both ventricles and both atria), you can slightly angle the direction of the beam more anteriorly (a very slight movement) to look at the fifth chamber, the left ventricular outflow tract, and aortic valve. Once this image is obtained, position the Doppler gate so that it overlies the path of blood flow leaving the left ventricle. The gate should lie just proximal to the aortic valve. Obtain a tracing once you are satisfied with the position. Tracing the outline of this v-shaped area will provide the velocity time integral (VTI). Finally, obtain the patient's heart rate by utilizing the calipers of the machine. Most machines will use the measurements obtained to calculate the CO (check your owner's manual to figure out which buttons to push), but you can also do these calculations manually.
Echo calculations can be intimidating, but don't have to be. Doing a little practice and knowing which buttons to push go a long way.