CO2 ABSORBENTS

1. CO2 combines with water to form carbonic acid. CO2 absorbents are hydroxide salts which neutralise the carbonic acid.

2. Colour conversion of a pH indicator dye (e.g., ethyl violet from white to purple) by increasing hydrogen ion concentration signals absorbent exhaustion. Absorbent should be replaced when 50% to 70% has changed colour

3. CO2 absorbants absorb (this may contribute towards delayed induction and emergence) and degrade volatile agents

4. Soda Lime and Amsorb are the commonly used CO2 absorbents

5. Soda lime  consists of Ca(OH)2 [80%], NaOH, water and KOH. It is capable of absorbing up to 23 L of CO2 per 100 g of absorbent. Addition of silica decreases the danger of inhalation of NaOH dust and reduces the resistance to gas flow. The drier the soda lime, the more likely it will absorb and degrade volatile anesthetics.

6. Amsorb consists of Ca(OH)2, CaCl2, CaSO4 and polyvinylpyrrolidone to increase hardness. It is more inert towards volatile agents, so their degradation is less with Amsorb

7. The dry absorbents may break down the volatile anesthetics to carbon monoxide (CO) (e.g., sodium or potassium hydroxide). The formation of CO is highest with desflurane. Compound A is a byproduct of degradation of sevoflurane by absorbent.

Ⓜ️NEMO> CO-Des 'CODES' CoA-Sevo 'CAuSE'

Negative pressure pulmonary oedema (NPPO)

NPPO is associated with upper airway obstruction in a spontaneously breathing patient. 

It occurs in 0.05–0.1% of all general anaesthetic cases and laryngospasm has been reported as being the cause in 50% of cases.

The clinical course is most frequently observed on emergence from anaesthesia where incomplete recovery from general anaesthesia increases the likelihood of the development of laryngospasm, but it has also been reported after airway obstruction with a foreign body and blockage and biting of tracheal tubes, hanging, and strangulation. 

Pulmonary oedema is typically described as developing within 2 min of the obstruction.

Once the airway is occluded, the spontaneously breathing patient will continue to generate negative intrathoracic pressure which will increase substantially as respiratory distress develops.

There is an associated increase in sympathetic tone due to the stress of hypoxia and airway obstruction which increases SVR and elevates pulmonary artery pressure. 

This is further exacerbated by hypoxic pulmonary vasoconstriction. 

The combination of these processes creates a pressure gradient across the capillary–alveolar membrane which favours the movement of fluid into the lung parenchyma.

It is most common in younger patients, presumably because they are able to generate higher negative inspiratory pressures and, arguably, have a higher sympathetic tone and better cardiac function. 

The condition may resolve rapidly after definitive management of the airway obstruction, but in some cases, copious pulmonary oedema may form and it can be associated with pulmonary haemorrhage suggesting capillary membrane damage.

After recognition of the cause of obstruction, the treatment required ranges from relatively modest support such as brief periods of CPAP for 2 h to positive pressure ventilation over a period of 24 h.

Ref: Neurogenic pulmonary edema

Ronan O'Leary, 

Justin McKinlay 

Contin Educ Anaesth Crit Care Pain (2011) 11 (3): 87-92.

WHAT IS POYNTING EFFECT

This is an effect described with regards to the anesthetic gas ENTONOX

ENTONOX is a 50:50 mixture of gaseous oxygen and nitrous oxide

If the cylinder is stored below -6 degree (the pseudocritical temperature of ENTONOX) Celsius, the nitrous oxide component can separate as a liquid (lamination)

This can lead the delivery of uneven mixtures, too much oxygen at the beginning and too much N2O at the end of the cylinder life

Danger of lamination can be avoided by immersing the cylinder in water at 52 degree Celsius and inverting it 3 times, or by keeping it above a temperature of 10 degree Celsius for 2 hours before use.

Other methods are keeping the cylinder horizontal, at a temperature of 5 degrees or more for more than 24 hours OR by connecting a tube from the valve housing at the top to a point near the bottom which prevents the withdrawal of pure nitrous oxide

N.B. The critical temperature of a gas is the maximum temperature at which compression can cause liquefaction. Mixing gases may change their critical temperature. The Poynting effect produces a 50:50 mixture which reduces the crtical temperature of N20 (Critical temperature is 36.5 degree Celsius); so Entonox has a pseudocritical temperature of -6 degree Celsius