Conference Lectures

Dr.Gayathri Ramanathan.  

 Professor of anaesthesiology.                                  

SRM medical College Hospital & Research centre.

Potheri. Chennai. 600100         

Dr. Gayatri Ram                                          

The role of ultra sound in medical practice dates back to 1950’s. The first ever official report appeared in BJA in the year 1978, describing the application of Doppler ultrasound blood flow detector in supraclavicular brachial plexus block. Initial reports of the ultrasound application for pre-neuraxial block assessment in obstetric population came 25 years ago. There still remains shortage of good data showing the use of ultrasound in obstetric anaesthesia practice. Currently the obstetric anaesthesiologist uses the ultrasound in the following areas.

1. Neuraxial blockade
2. Vascular access
3. General anaesthesia
4. Cardiac evaluation
5. Acute and chronic pain control
6. Neuraxial blockade

The performance of lumbar puncture and epidural catheterizations relies primarily on the palpation of anatomical landmarks, which might be obscured in an obese parturient or a PIH patient with generalized oedema. Other important aspects of the procedure, such as the distance from the skin to the target space cannot be assessed by palpation and relies entirely on the skill of the operator. Although Lumbar and epidural catheterizations are common procedures, failure result in unintended, unplanned conversion to general anaesthesia or inadequate analgesia. A systematic review and Meta analysis was conducted in 2013 studied 14 trials with 1334 patients (674 in ultra sound group and 660 in the control group),  showed that ultrasound imaging resulted in reducing the risk of failed procedures, less number of insertion attempts and the number of needle directions.
From the technical point of view, spinal ultrasound imaging is difficult as the area of interest is protected by the bone. As the ultrasound beam does not pass through the bone well, the acoustic window is relatively narrow. Most anesthetists use a curvilinear low frequency probe (2-5 Hz) for neuraxial imaging. Although the resolution is poor (indicates poor image quality), it provides deep penetration required for visualization.
For neuraxial blocks ultrasound is used to provide pre-procedural assessment and make a skin mark. A pre puncture image identifies a needle insertion point and can also provide an estimate of depth of epidural space to skin. With relatively little training, ultrasound can be used to determine an appropriate interspace for epidural or spinal anaesthesia.
For neuraxial blocks, ultrasound is used to provide pre-procedural assessment and skin mark. A pre-puncture image identifies the needle insertion point and also provides an estimate of depth of epidural point to skin. For neuraxial imaging two techniques are used. The longitudinal (median or paramedian) view, and the transverse view.
Longitudinal view is done with the probe placed along the cranio-caudal axis of the spine. The sacrum is identified first as a linear opacity. (fig1). On moving the probe up the laminae becomes visible as curvilinear shadows. The vertebral level is ascertained by counting up from the sacrum. The appropriate inter space is identified as the space between the two laminae, and marked.
Once the level is selected, the probe is placed transversally. First the spininous process is identified  (fig.2).  The probe is placed over the interspace to  determine the ligamentum flavum and its depth .
Pre-procedural selection of skin point does not guarantee successful placement if needle angulation does not replicate the probe orientation. This can be overcome by in plane direct visualization of needle progress through tissues. As performing the loss of resistance needs both the hands, for in plane technique a third skilled hand is needed.

1. Vascular access

Ultra sound was first used in gaining central venous access in 1996. On the basis of reports from many multi centric studies, NICE recommends two-dimensional ultrasound to be considered in most clinical circumstances where ventral venous cannulation is needed. Using ultrasound reduces the incidence of multiple attempts and reduces the rate of complications when compared to traditional landmark technique. In obstetrics, it is especially helpful for patients coming in 3rd trimester, where head down tilt is very uncomfortable. Ultra sound guidance is useful in coagulopathic, obese, hypovolemic patients or those with multiple failed attempts. For CV access the real time USG guidance with short axis out of the plain view is obtained.
Other uses include even peripheral venous and arterial cannulation in obese parturient.

1. Anesthesia

Ultrasound imaging is slowly gaining importance as an important evaluation method before subjecting the patients to general anesthesia.  Gastric volume can be assessed in patients coming for emergency surgery, where pregnancy and opioid further reduce the gastric emptying time. The information gained could influence decision making with respect to anaesthetic technique.
US measurement of anterior neck soft tissues helps in predicting difficult laryngoscopy in obese patients. The distance from the skin to the anterior aspect of the trachea is measured at three levels: Vocal cords, thyroid isthmus, and suprasternal notch. The amount pretracheal soft tissue at the level of the vocal cords is a good predictor of difficult laryngoscopy in obese patients. Patients who have more pretracheal soft tissue (28 mm) and a greater neck circumference (50 cm) at the level of vocal cords are found to have difficulty in laryngoscopy.
Another use for Ultrasound is as a tool for determining the correct position of cricoid cartilage and cricothyroid membrane pre operatively. If needed needle or surgical cricothyroidotomy can be performed safely and easily using these landmarks.

1. Cardiac evaluation

High resolution transthoracic cardiac imaging is now available in many centers. Ultrasound evaluation in a hypovolemic patient may help to differenciate non- invasively between hypovolemia and other conditions such as cardiomyopathy and pulmonary embolism.

1. Pain control

Transverse abdominal plain block is now increasing being used as an important technique providing multi modal analgesia after cesarean section. (TAP block) can be performed with enhanced accuracy under ultrasound guidance. Scanning the area between iliac crest and the subcoastal region clearly helps to identify the muscle plains. Insertion of catheters allows the pain free period to be extended for a few more days post operatively.Several studies have shown that complications like bowel injury, peritoneal injection and haematoma are reduced on performing the blocks under US guidance.
Some of the chronic pain syndromes described in parturients are the pelvic girdle pain syndrome and perineal pain syndromes. Nerve blocks performed under USG guidance are reported to be beneficial in these conditions.
The most important obstacle  in the routine widespread use of US is the cost. A high resolution equipment needed for nerve blocks and spine assessment comes for nothing less than 10L. The potential benefits published are only moderate. More data is awaited to document definite benefits in high risk parturient. Till then the cost, availability of equipment with appropriate specification and expertise will limit its use.
Acknowledgement
I extend my sincere thanks to Dr.G. Jaykar, consultant anaesthesiologist USA for helping me in preparing this manuscript.
References

1. Ecimovic, P,  LoughreyJP. Ultrasound in Obstetric Anesthesia: A Review of Current Applications. Obstetric Anesthesia Digest. 31(3):168, September 2011.
2. Shaikh, F,  Brzezinski, J. Alexander S. Ultrasound Imaging for Lumbar Punctures and Epidural Catheterizations: Systematic Review and Meta-Analysis. Obstetric Anesthesia Digest. 34(2):71-72, June 2014
3. Margarido C.B,  Arzola, C. Balki. Anesthesiologists' Learning Curves for Ultrasound Assessment of the Lumbar Spine. Obstetric Anesthesia Digest. 31(2):103-104, June 2011