Nitrous Oxide, Xenon and Oxygen Flashcards
Nitrous Oxide: MAC
105%
Nitrous Oxide: B:G coefficient
0.47
Nitrous Oxide: Critical temperature and critical pressure
36.5°C and 72 bar
Dose-response curve for volatile agents
20% below MAC - almost all patients move in response to surgical stimulus
20% above MAC - less than 5% of patients move
Xenon vs Nitrous Oxide: blood:gas solubility
Xenon B:G 0.114
N2O B:G 0.47
Xenon vs Nitrous Oxide: MAC
Xenon MAC 65-70%
N2O MAC 103%
Xenon vs Nitrous Oxide: Flamability
Xenon is non-flammable
Nitrous oxide is flammable
Nitrous Oxide: SVP at 20°C
52 bar (5200 kPa)
Entonox: SVP at 20°C
52 bar (stored as a liquid so gauge will continue to read 52 bar until the liquid is exhausted)
Oxygen: Critical Temperatue
-119°C
Oxygen toxicity: normobaric
Can cause atelectasis, tracheobronchitis and even ARDS at FiO2 >0.6
FiO2 = 1.0 can cause damage to healthy lungs in 12-24 hours
Oxygen toxicity in the neonate
Retinopathy of prematurity caused by retrolental hyperplasia
Necrotising enterocolitis
Bronchopulmonary dysplasia
Intracranial haemorrhage
Oxygen: boiling point
-183°C
Oxygen: manufacture
Fractional distillation of air
Oxygen concentrator using zeolite mesh to adsorb nitrogen from atmospheric air (produces 97% oxygen)
Xenon: CNS effects
May be used to enhance CT images in the brain
Not recommended for neurosurgery due to variable increase in ICP
Has significant analgesic properties
Xenon: cardiovascular effects
Does not alter myocardial contractility
May cause small decrease in heart rate
Xenon: respiratory effects
Reduces respiratory rate
Increases tidal volume
Does not cause diffusion hypoxia
Xenon: metabolism
Not metabolized in the body; excreted via the lungs
Xenon: manufacture
Fractional distillation of air - very expensive (2000 times the cost of N2O)
Xenon: MAC
MAC 71%
Xenon: Blood:Gas coefficient
B:G 0.14 (extremely quick onset and offset)
Entonox: pseudocritical temperature
Cylinder: -7°C, most likely at 117 bar
Pipeline: less that -30°C at 4.1 bar
Entonox: composition
50:50 mix of N2O and O2 - effectively dissolve into each other and do not behave in a way predicted from their individual properties - the Poynting Effect
Nitrous Oxide: metabolism
Less than 0.01%
Nitrous Oxide: contraindications to use
Nitrous oxide will cause rapid expansion of any air filled space therefore contraindicated in pneumothorax, air embolus
Nitrous Oxide: toxicity
Cobalt ion present in Vitamin B12 is oxidised by N2O so that it is unable to act as a co-factor for methionine sythetase, resulting in reduced synthesis of methionine, thyidine, tetrahydrofolate and DNA.
Few hours exposure: megaloblastic bone marrow
Few days exposure: agranulocytosis
Few years exposure (e.g. unscavanged dental practices): neurological syndromes resembling subacute degeneration of the spinal cord
Nitrous Oxide: the Concentration Effect
As N2O is the only agent used in sufficiently high concentrations, a disproportionate rise in alveolar fraction is observed when compared with inspired fraction. This leads to the second gas effect - used alongside other induction agents, the high uptake of N2O will lead to increased concentration of oxygen and volatile agent, reducing induction time
Nitrous Oxide: CNS effects
Increases cerebral blood flow - avoided in patients with raised ICP
Nitrous Oxide: cardiovascular effects
Mild direct myocardial depressant effects counterbalanced by increased sympathetic activity.
Therefore in health very little change. However, in cardiac failure with no sympathetic drive can cause reduced cardiac output.
Nitrous Oxide: respiratory effects
Causes small fall in tidal volume that is offset by increased respiratory rate
Nitrous Oxide: manufacture
Nitrous oxide is manufactured by heating ammoninum nitrate to 250°C
NH4NO3 –> N2O + 2H2O
Impurities such as NH3, N2, NO, NO2 and HNO3 are removed by passage through scrubbers, water and caustic soda
Filling ratio
Weight of the fluid in the cylinder divided by the weight of the water required to fill the cylinder (NB weight!)
In the UK = 0.75
In hotter climates = 0.67 to avoid cylinder explosion
Nitrous Oxide and air filled cavities
It expands air-filled cavities because it is 40 times as soluble as nitrogen; thus, it passes from the blood into the cavity faster than nitrogen can diffuse out. This can double the size of a pneumothorax in 10 minutes at a concentration of 70%. Also expands air embolism and may cause pneumoencephalocoele after neurosurgery.
Nitrous Oxide: occular surgery
The use of nitrous oxide during general anaesthesia in gas-filled eyes may have disastrous visual results caused by gas expansion and elevated intraocular pressure. The gases SF6 and C3F8 are most commonly used. Patients must be advised of the potentially catastrophic results of undergoing general anaesthesia before their intraocular gas bubble has resorbed. The use of nitrous oxide for patients with intraocular gas should be avoided. It may be prudent for patients with intraocular gas to wear notification bracelets warning anaesthetists about the presence of intraocular gas lest emergency surgery be needed by a patient unable to advise anaesthesia personnel about the potential danger.
Carbon Dioxide: Production
Either oxidisation of carbon containing substances or heating calcium or magnesium carbonate
Carbon Dioxide: Density
1.98kg/m3 (about 1.5 times that of air)
Carbon Dioxide: Critical Temperature
31.2°C
Carbon Dioxide: Critical Pressure
73.8 bar
Carbon Dioxide: Boiling Point
-79°C
Nitrous Oxide: Mechanism of Action
Blocks NMDA receptors
