Xenon was discovered in 1898 by Ramsay and Travers. It is the only noble gas with anaesthetic properties under normobaric conditions [6
]. Cullen successfully performed general anaesthesia with xenon for the first time in 1951[7
]. Xenon comes very close to being the ideal anaesthetic [8
]. Its unique properties include a rapid onset and offset of action [13
] and a recovery index with good arousal quality [14
Breast feeding is known to have significant benefits in terms of infant development. The World Health Organisation (WHO) recommends exclusive breast feeding for 6 months and partial continuation into the second year of life [15
]. How safe is breast milk for the infant, however, if the mother has taken medication? Unfortunately, recommendations tend to be based on case reports, theoretical risks or case studies [16
The breast milk accumulation in 24 hours of most drugs is rarely greater than 1-2% of the original dose [17
]. Drugs with high lipid solubility like propofol are found at higher concentrations in breast milk than in plasma dependent on the time and amount of milk production during propofol administration. This could be confirmed when determining the levels of propofol in the blood and maternal milk 90 and 300 minutes post extubation. Water-soluble drugs with low molecular weight (< 200) are excreted in the breast milk unimpeded, resulting in identical milk and plasma concentrations. As a result of the difference between plasma pH (7.4) and milk pH (6.5-7.09), weak acids are excreted in smaller quantities in milk, whilst weak bases are excreted in higher concentrations. High protein binding as is the case with remifentanil (70%) is associated with low milk concentrations, since only the free fraction of a drug can enter the breast milk. Remifentanil is metabolised within minutes by nonspecific plasma esterases and theoretically excretion in the mother's milk is very unlikely. A high degree of ionisation as with Esmeron results in an exceedingly low drug concentration in the milk [17
Xenon, a noble gas, is absorbed or exhaled via the lungs. It does not form any compounds in the body and is not metabolised. The partition coefficients increase from xenon to nitrous oxide, sevoflurane and isoflurane. The blood/gas partition coefficient is the main determinant of the rate of onset and offset of action of an inhalational anaesthetic. The lower the blood/gas partition coefficient, the faster these processes. The oil/gas partition coefficient or the lipid solubility of an inhalational anaesthetic is a measure of its potency: a high lipid solubility means high potency but also a longer emergence time [6
]. The low lipid solubility of xenon explains its high minimal alveolar concentration (MAC) as well as its short recovery times.
Xenon has excellent physicochemical properties, explaining its anaesthetic effect under normobaric conditions. The MAC of xenon ranges from 51% to 69% [21
]. The oil/water partition coefficient of xenon is around 20. Based on the physical properties of xenon and a breast-milk fat concentration of approximately 51%, dissolution of xenon in human milk cannot be ruled out completely, but would theoretically be expected to be lower than that of sevoflurane or isoflurane, respectively. Since an accumulation process in breast milk is not possible based on diffusion gradients and because the residual concentration of xenon is exhaled from the mother's body in a matter of minutes, considerations suggest that xenon is no longer detectable in the milk a few minutes after emergence. This suggestion is consistent with the concentration of xenon we measured in the breast milk (see Table ). Furthermore it can be assumed that during the suction process traces of xenon diffuse from the milk into the air within seconds, so that the question of oral bioavailability does not arise.
The unique benefits of inhalational anaesthesia with xenon in nursing mothers include rapid elimination in a matter of minutes, the absence of metabolism of this noble gas and analgetic properties.
Accounts in the literature on the anaesthetic approach to nursing mothers are sparse [19
]; guidelines do not reflect this setting, and textbooks recommend interruption of breast feeding for 24 h. Nor do the propofol and remifentanil SPCs (Summary of Product Characteristics) comment on usage during lactation.
Remifentanil, co-administered with xenon to ensure analgesia, is rapidly metabolised postoperatively and has a plasma half-life of 3-10 minutes. At a calculated effect concentration of less than 1.5 ng/ml read from the Base Primea controller at extubation, it is therefore safe to assume that this opioid is no longer active. No data is available on the excretion of remifentanil in breast milk (information provided by the manufacturer).
Propofol was used only for induction. Nitsun and colleagues found that the breast-milk drug concentration was 0.015% of the original dose over a 24 h measuring period [26
]. This represents the upper limit based on the assumption that the infant ingests the total milk volume produced during a 24 h period and that propofol has 100% oral bioavailability. In light of the two cases in which propofol was measured in the breast milk, this would imply that a maximum of 52.5 μg/24 h could be absorbed in case 3 and 66.45 μg/24 h in case 4. Assuming that a baby drinks about 150 ml per breast feed, this amount - when converted in line with the concentrations in the maternal milk after 90 and 300 minutes - is equivalent to the doses given in Table . In case 4 we could detect a clear deviation from the claim made by Nitsun et al after just 90 and 300 minutes, with 0.09% and 0.028% of the measured concentration in the breast milk related to the initial dose. Hence the findings published by Nitsun et al do not apply to our patients and require further investigation in a larger population. Nevertheless, it can be presumed that orally, the baby ingests an extremely low concentration of propofol. Taken also into account the very low oral bioavailability babies will be exposed to a negligible quantity of propofol during breast feeding.