LambertUltrasound Basics

The basis of ultrasound imaging is that every tissue in the body has a different acoustic impedance.   Sound travels at different speeds thru different tissues.   When sound waves reflect back to the probe at varying speeds, the ultrasound machine can generate an image based on these differences.

*Acoustic Impedance

Low frequency waves penetrate deeper into the tissue but provide less resolution.   High frequency waves have less penetration but better resolution.

*High Frequencyvs Low Frequency ultrasound waves

Terminology synonyms:

Longitudinal = sagittal=long axis

Transverse=axial=short axis

Coronal is basically a longitudinal view taken from the side of the body

* Sagital,  Axial, and Coronal Planes

Knobology

Mike discussed the importance of controlling the depth and gain of the image you are looking at.   You want to adjust the depth to optimize to proportion of the body structure being visualized in the image.  You want the structure of interest to take up the majority of the screen.   You need to adjust the gain to have a uniform appearing image both in the near and far fields. 

*Near field is top half of image,   far field is bottom half of image.  Gain is uniform throughout image.

Lambert      Bedside Echocardiography

Bedside echo is a game changer for identifying life-threatening illness in patients with chest pain or shortness of breath.   You can identify PE, pericardial effusion, CHF, problems with contractility/wall motion.   It is useful during cardiac arrest as well.

There are two main views of the heart when doing Bedside echo

*Parasternal Long axis

*4 Sub costal

*Pericardial effusion*

*Pulmonary embolism Note the large RV in comparison with the LV.

Mike classifies LV dysfunction as either OK or Bad.   Bad is usually obvious on bedside echo and helps you identify CHF or cardiogenic shock.   

Elise’s Journal Club Summary:

Article 1:  Flato UA, et al:  Echocardiography for prognostication during the resuscitation of intensive care unit patients with non-shockable rhythm cardiac arrest. Resuscitation  2015  Jul; 92:1-6. 

This was a Brazilian prospective, observational cohort study of 49 ICU patients with in hospital asystole or PEA cardiac arrest. Of 88 eligible patients, 39 were excluded. PEA without contractility was classified as electromechanical dissociation (EMD), and PEA with contractility as pseudo-EMD.  Two echo trained intensivists performed transthoracic echos. Rates of ROSC were 70% for pseudo-EMD, 20% EMD, and 24% for asystole.  Good ROSC percentages for all groups, but survival to hospital discharge was only seen in pseudo-EMD patients, and discharge is what counts.  Four patients survived to 180 days, all in the pseudo-EMD group, with CPC of 1, 1, 1, 2.  Echo was feasible, with maximum duration of 10 seconds, so non-disruptive to the ongoing code.  Echo also helped identify underlying etiology of arrest in selected patients, eg unexpected tamponade.  This was a very small study, and conducted in an ICU with a large number of DNR patients who were never entered into study, so different from our ED population.   

This study reinforces our usual practice of using echo to verify presence/absence of cardiac contractility and guide futility of resuscitation.  Blaivas and Fox (go ACMC!!) published a larger study in 2001 demonstrating 100% mortality in patients with asystole or PEA cardiac arrest and no cardiac contractility on bedside echo. This modality helps conserve resources (time and personnel), and may identify the underlying reason for code.  In the future, in young otherwise healthy patients with arrest, an echo demonstrating contractility may help risk stratify for ECMO.

Bottom Line:  Rather than pulse check, consider echo as the more reliable marker for viability.  Also remember end tidal CO2 to guide prognostication.  

(AHA 2015 ACLS guidelines:  “In intubated patients, failure to achieve an ETCO2 of greater than 10 mm Hg by waveform capnography after 20 minutes of CPR may be considered as one component of a multimodal approach to decide when to end resuscitative efforts but should not be used in isolation.”)

Lambert    IVC and Aorta

*Aortic Aneurysm

*IVC with hypotension. 

*The proper landmarks for observing the IVC for hypovolemia include the heart, portal vessels and IVC in the same image.

Mike feels a collapsing IVC with inspiration is the most useful sign of hypovolemia on ultrasound.  He has doubts that IVC ultrasound is the holy grail of identifying low central venous pressure.

Lambert and Team Ultrasound        U/S Lab