Have you ever administered albuterol (aka salbutamol) to a ventilated patient and witnessed the anesthetic gas analyzer detect halothane even though there hasn’t been a bottle of halothane in your hospital in decades? How can a machine so advanced make such an error?
The gas analyzer on your anesthetic machine is an underappreciated technological marvel found in every operating room in every industrialized country in the world. It consists of a paramagnetic analyzer to detect oxygen and a gas phase infrared spectrometer for all other gases. Because oxygen does not absorb infrared light, it can not be analyzed by infrared spectroscopy and requires its own analyzer. All companies that produce anesthetic gas analyzers use Paramagnetic and IR technology.
In a chemistry lab, when we try to identify a molecule with IR spectroscopy we may be trying to identify an unknown molecule from a possible list of millions of molecules. In the operating room, we are only concerned with around 5 molecules. Instead of scanning the whole spectrum and identifying all possible peaks; an anesthetic gas analyzer only scans 2 limited areas of the spectrum and instead of searching for hundreds of possible peaks in these areas, the analyzer will only search for a few specific peaks in those ranges. By limiting the scanning spectrum and recognizing only a handful of absorption peaks This allows for a rapid response time on the order of milliseconds.
Albuterol and halothane have very little structural similarities. Most importantly Albuterol’s structure contains double bonds and hydroxyl groups that will produce unique infrared absorption peaks not found in halothane.
And if we look at the IR spectrums we continue to see little similarity
So if the machine isn’t detecting albuterol, what is it detecting? One clue lies in the fact that albuterol administration via metered dose inhaler will result in halothane detection by the gas analyzer but albuterol administration via nebulized solution will not. Perhaps the inhaler contains an extra ingredient that is similar to halothane. That ingredient is HFA. HFA, the propellant in most albuterol inhalers, stands for hydro fluoro alkane.
Interestingly, halothane contains the components of a hydro flouro alkane. If we reference the albuterol HFA package insert, we see that the specific HFA used, is HFA 134a. Now this molecule looks quite similar to halothane. Interestingly, HFA 134a is an anesthetic of moderate potency, and was investigated for use as an inhaled anesthetic in humans in the 1960s.
Loooking at the IR spectra, we can clearly see that it is HFA-134a and not Albuterol that is responsible for your anesthetic gas analyzer reading Halothane.
