Dear Ka Ting Ng,

Measuring depth of anaesthesia represents one of the most controversial and subjective aspect of modern anaesthesia, with the introduction of the concept of

balanced anaesthesia using multiple drugs and muscle relaxants. It is unlikely that any single method will be found to measure the depth of anaesthesia reliably for all

patients and all anaesthetic agents. All the methods for determining the depth of anaesthesia to date have some potential exclusion criteria. Therefore, using more than one method at a time may provide more accuracy. The rapid advancement in

the microcomputer technology and our understanding of the basic sciences will allow us a greater scope to interpret our observations of the anaesthetic state in near future.

For the different indicators used to assess the depth of anaesthesia, please see the publication contained in the following link:

http://medind.nic.in/iad/t02/i4/iadt02i4p323.pdf

Hoping this will be helpful,

Rafik

Dear Rafik, 

Thank you so much for sharing. 

I am particularly interested on the functionality of bispectral index. 

I understand that BIS could affected by low temperature when patients undergoing hypothermia CPB. Anything else would you think of that could interfere or causing false reading on BIS when patient is starting on Cardiopulmonary bypass circuit? 

Many thanks, 

Ka

Dear Ka,

Inspite of its excellent usefulness BIS has some shortcomings. The presence of senile dementia may be a confounding factor in interpretation of BIS value. In some instances BIS has been observed to increase with the use of N2O and ketamine.

Monika N, et al. Ketamine causes a paradoxical increase in Bispectral index. Anesthesiology 1997; 87: A502.

Puri GD. Paradoxical changes in bispectral index during nitrous oxide administration. Br J Anesth 2001; 86: 141-142.

Rafik

Dear Rafik, 

Could you please attach the pdf here for me as I cant seem to open the link you sent to me?

What is your thought on the end-tidal anaesthetic gas as an alternative monitor of depth of anaesthesia? 

I am thinking on using this concept (exhausted isoflurane gas from cardiopulmonary bypass circuit) to estimate the depth of anaesthesia when cardiopulmonary bypass circuit is initiated?

Many thanks,

Ka

Dear Ka,

Attached please find the requested pdf file.

Regarding the End-tidal approach, please see the following text: 

End-tidal Control is intended for use during inhalational anaesthesia and needs a controlled patient airway to be in place, for example an endotracheal tube or laryngeal mask airway.

End-tidal Control cannot be used with a face-mask airway, or with halothane as the anaesthetic agent, or while the module is in non‑circle circuit, cardiac bypass, alternate oxygen, and air‑only modes. It is recommended that End‑tidal Control is not used during surgical procedures that cause disturbance to the lungs, such as chest surgery. The system may deliver 100% oxygen in End-tidal Control mode, therefore End-tidal Control mode should not be used when delivery of 100% oxygen may injure the patient (for example, in premature neonates in whom excessive inspired oxygen concentrations can cause retinopathy, or in patients with some forms of congenital heart disease). End-tidal Control mode stops if the anaesthetic is changed while the module is active. The manufacturer recommends exiting End-tidal Control mode before changing the anaesthetic. However, it is not routine practice to change anaesthetic agent between the anaesthetic room and the operating theatre.

The manufacturer does not specify a lower age limit for End-tidal Control, however specified respiratory rates (35 breaths per minute or less) must be met, and the system must be registering a minute volume.

The following link contains a publication on this approach.

https://www.nice.org.uk/advice/mib10/chapter/technology-overview

Hoping this will be helpful,

Rafik

Though BIS has many limitations during hypothermia and presence of some agents, it is useful to prevent awareness during anesthesia. Use of such monitor also helps us medicolegally if needed. Evoked potential monitoring (e.g.AEP) in their newer emerging avatar may be preferred in future.

As said earlier by Dr. Rafik end tidal monitoring will have no value in presence of intravenous agents.

H Karnik

Dear Rafik and Karnik, 

Absolutely that end tidal cannot be used on patient with intravenous.  

If patient has ET tube and receives volatile anaesthetic isoflurane as maintenance during cardiopulmonary bypass, what is rationale that end tidal isoflurane cannot be used to predict the depth of anaesthesia? 

Many thanks

Ka