Tentative Recommendation on Terminology and Definitions in Respiratory Physiology: Résumé of the Isott Consensus Session 1992

1 The use of small letters for the symbols “p” (partial pressure), “s” (saturation) and “c” (concentration) (e.g. pO2, sO2, cO2) follows recommendations of the IFCC and IUPAC [4]. This supports the use of contemporary word processing systems and mostly eliminates the need to use subscripts (except for chemical valencies: e.g. O2, CO2, H2CO3 etc.). The potential risk of misinterpretations and double meanings is reduced also (e.g. “cO2” [oxygen concentration] v.s. “CO2” [carbon dioxide] and “sO2” [oxygen saturation] v.s. “sO2” [sulfur dioxide]). 2 The symbol shall include the site of measurement or description, e.g. paO2 (arterial O2 partial pressure), svO2 (mixed venous oxygen saturation), or avDO2 (arterio-venous oxygen difference). This removes in an intelligible manner the confusion of symbols (e.g. paO2: PaO2, paO2, Pao2 Pao2, etc.). One wishes to question the rationale of a statement such as “writing Sao2, So2(a), So2(ab) or the like, is a matter of taste” [10], because perfect confusion may be the result, here concerning the arterial O2 saturation (saO2). 3 It is a convention in chemistry to refer to radicals or compounds containing oxygen with the term “oxi” rather than “oxy” (e.g. carbon monoxide (CO), carbon dioxide (CO2), oxidized Hb (MetHb) etc.). On the other hand, methods and terms referring to molecular oxygen (oxygenium) should be written with “y” and not with “i” (e.g. oxymetry, pulse oxymetry, oxygenation, Hb with reversibly bound O2 (Oxy-Hb, O2Hb) and deoxygenated Hb (Deoxy-Hb, Hb), etc.). 4 Even if a new in vivo method differs considerably from the established in vitro method (e.g. in vivo (transcutaneously) measurement of arterial O2 saturation or O2 partial pressure) this does not justify the use of different notations or symbols. Use of the symbol ptcO2 should be avoided for transcutaneously determined “paO2”: If the method is able to measure the arterial pO2 then the symbol paO2 should be used. If this is not the case, however, i.e. if indeed the cutaneous pO2 is determined, then the symbol pctO2 is appropriate. The use of Spo2 [2,7] or Spo2[5] or So2(po) [10] for arterial O2 saturation, measured by pulse oxymetry, could erroneously suggest the measurement of an essentially different quantity [10]. 5 The number of symbols employed for a given measured value should be kept as small as possible. For this reason the suggestions of Payne and Severinghaus [5] should not be followed, since a total of six different symbols is recommended for the O2 saturation. Consequences are a worldwide confusion: within only one article [2] arterial oxygen saturation is referred to as Spo2 (pulse oxymeter reading), FSao2 (functional saturation), Sao2 (fractional saturation), O2Hb % (fractional saturation). Within the same volume of a journal [2,7] Sao2 is used for fractional oxygen saturation [2] as well as for functional oxygen saturation [7]. Obviously, the notation “fraction” becomes more and more attractive: Besides the widely accepted “fraction” in respiratory physiology (e.g. FIO2, inspired oxygen fraction), the oxyhemoglobin fraction FHbO2 was proposed [10] for the So2(frac) [5] or the fractional saturation Sao2, [2]. But, on the other hand, the symbol F was then introduced as FSao2 for the functional (not the fractional) saturation and used besides the common FIO2 [2], the inspired oxygen fraction. 6 Symbols used for any notations should be as simple as possible. Neither the use of NSo2 to describe the noninvasive technique [1] for the measurement of oxygen saturation, nor that of Spo2 [2] for pulse oxymetry should be accepted. 7 The symbol for a measured value must be independent of the method (cf. comment 5), and certainly independent of the manufacturer. Recent recommendations to the contrary, e.g. to use the symbol ShpO2 for the value given by an oxymeter from Hewlett Packard [5], or that of Barker et al [2] to use the symbol Sxo2 for the value given by an Oximetrix system, are unacceptable.