Although the uses of ultrasound detailed in the last 11 tutorials covers most of its utility in the ICU, there remain a few less common applications that we will discuss in this tutorial.
Detection of raised intracranial pressure (ICP)
It is possible to detect raised ICP by measuring the Optic Nerve Sheath Diameter (ONSD) using ultrasound in the ICU. The optic nerve is enclosed in a meningeal sheath all the way upto its insertion into the globe of the eye. This sheath is an extension of the meningeal covering of the brain and the CSF space enclosed between the nerve and the sheath is in continuity with the CSF around the brain. When CSF pressure is increased, this pressure is transmitted to the CSF space in the Optic Nerve Sheath and causes it to distend. This distension can be detected as a change in the diameter of the sheath and can be measured on imaging techniques like MRI, CT and ultrasound.
The high frequency (8-13 MHz) probe is used for this. The optic nerve is viewed in 3 planes - the axial planes (saggital and transverse) and coronal plane.
The sonographic optic nerve sheath diameter, ONSD (but not optic nerve diameter, OND) is strongly related to ICP. Additionally, changes in optic nerve sheath diameter (ONSD) are strongly related to changes in ICP. The measure of the distension of the sheath surrounding the optic nerve (but not of the nerve itself) can be used to detect elevated ICP in neuro-ICU patients.
The probe is placed over the closed eyelid with the patient in supine position. Use small quantities of ultrasound gel to prevent it from getting into the eyes of the patient. The probe is held vertically over the eye and the beam is directed posteriorly towards the optic disc and nerve. A good view of the nerve often necessitates moving the probe slightly to the temporal side and angulating the beam a little nasally.
2D mode is used and the field depth is reduced to 4 cms. The normal sonographic aspect of the optic nerve is from center to peripheral: hypoechogenic nerve fibers closely surrounded by the echogenic pia mater; the subarachnoid space appears anechogenic or hypoechogenic and is surrounded by hyperechogenic dura mater and periorbital fat. The OND can be measured as the distance inside the pia mater, and the ONSD as the distance inside the dura mater. Without zoomong in on the optic nerve, OND and ONSD are measured 3 mm behind the globe, using an electronic caliper and an axis perpendicular to the optic nerve. Two measurements are made for each optic nerve: one in the sagittal plane and the other in the transverse plane, by rotating the probe clockwise. The mean of value obtained for both eye is taken as the OND and the ONSD values.
The probe is placed over the lateral angle of the eye with the marker facing cephalad. The beam is focussed posteriorly and medially until the nerve sheath is seen as an oval structure behind the globe. The shortes t diameter of this oval represents the coronal view diameter of the nerve sheath. This is an oblique coronal section as a true coronal view is technically not possible because of the intervening lateral wall of the bony orbit. the average values of the axial and coronal section diameters can be taken.
In a study published in 2007, done on patients with head injury (excluding those with ocular trauma), the best cut off value (using an ROC curve) for ONSD predicting elevated ICP was 5.9mm. In the same study, it was found that if the ONSD was less than 5.7mm, its sensitivity and negative predictive value were 100%. In another study published in 2008, it was found that in sedated neurocritical care patients, non-invasive sonographic measurements of ONSD correlated with invasive ICP, and the probability to have raised ICP if ONSD was less than 5.86 mm was very low.
It is important to be aware that sometimes, an edge artifact may be seen as 2 diverging black areas on either side of the nerve sheath. Including this in the measurement of ONSD will lead to an erroneous prediction of raised intracranial pressure.
Ultrasound can be used to assess the anticipated difficulty in intubation of obese and sleep apnoeic individuals by measuring the thickness of fat tissue in the anterior neck region with a high frequency probe. In patients presenting with airway obstruction, tumors, abscesses or epiglottitis can be diagnosed on sonography.
Confirmation of endotracheal tube placement
It has been established that auscultating breath sounds is not a very accurate method of ensuring correct placement of the endotracheal tube in the trachea after intubation. Ultrasound, albeit using indirect evidence can do tha same more accurately.
Two assessments need to be made on each side after intubation.
- Assessment of lung sliding
- Assessment of diaphragmation excursion
Assessment of lung sliding
The to and fro movement of the visceral pleura can be assessed with a routine ultrasound or high frequency probe as described in tutorial 9.
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