What is "Hufner's number"? (& what is its physiological significance?)
I received several correct responses to this (from Chris Thomas, Darren McNeil, Norman Barlev & Geoff Gordon). These emails are reproduced (with slight editing) below:

Dear Kerry,

Tsk. Spelling. German 'umlaut' should be over the u.
 
Huefner's constant is the theoretical oxygen-carrying capacity of  haemoglobin (various numbers quoted; 1.39 ml O2 per gram is the theoretical value; experimentally the value may be closer to 1.306 (Gregory) to 1.34 depending on which book you read, ? because of iron in the form of haemochromagens (Nunn).
 
Physiological significance: see marvellous guide to the primary exam for various applications. Interesting to note that under hyperbaric conditions enough O2 can be dissolved in the blood to render haemoglobin (theoretically) obsolete. Had this quoted to me by a JW the other day!

Cheers
Chris Thomas
PFY
PAH Brisbane.

Dear Chris,
Thanks for your prompt reply. You have narrowed in towards the correct answer but I don't think you're 100% correct. My points are:

[1] Spelling: There is an umlaut over the u (this was omitted for simplicity much as you have in your email) BUT I think your spelling as "Huefner" is incorrect. My memory says the spelling is "Hufner" (with an umlaut of course). I don't have the original paper from 1894 (it was in German) but I have seen it referenced several times and always as "Hufner". Also, I recall that the term "Hufner's constant" (with that spelling) is used in the English translation of one of the major German Physiology textbooks ("Human Physiology" -RF Schmidt & G Thews 2nd ed Springer Verlag --according to my old notes anyway) - We don't have it in our library down here but either the PA Medical library or the Central Medical library might have it. I will try to track down a reference. 

[2] The value of 1.34 which has been used for a long time, traces back (I think) to Hufner himself who found a value of 1.336 (which becomes 1.34 when shortened to 2 significant decimal places).

[3] Hufner determined the oxygen capacity of Hb but as I understand it he did not use oxygen to do this! He used carbon monoxide based on the idea that equimolar amounts of both bind to Hb so the oxygen capacity can be determined indirectly using CO.

[4] Also you say that Hufner's number is the 'theoretical' value but this is not correct. 1.39 was not the value determined by Hufner as he did not know the MW of Hb and could not have determined this theoretical 1.39 value. The theoretical value is more modern and derives from Braunitzer in his 1963 paper (also in German). My notes say he used a MW of Hb of 64,458.5 to determine this. 

[5] Your JW friend is wrong but I can see why this is not obvious to him/her. He/she is correct that at a pressure of 3 ATM of pure oxygen, arterial blood can provide sufficient dissolved O2 to cope with the usual AV difference of 5mlsO2/dl of blood. Three Atm O2 will provide over 6 mlsO2/dl as dissolved O2 in arterial blood. 

BUT there are several problems with this:

i. Hyperbaric O2 at 3 atm cannot be tolerated long enough (due CNS toxicity) to make this at all clinically relevant. Some Hb is necessary. From memory, all you can get at 3 atm O2 is perhaps a couple of hours before severe toxicity (such as fitting) occurs. Anything less than 3 atm O2 pressure will not provide sufficient dissolved O2 at basal rates of oxygen consumption.

ii. Secondly, this is based on the average AV difference of 5mlsO2/dl of blood for the body as a whole and ignores regional differences. The heart has an oxygen consumption of 25mlsO2/min out of its 250mls/min blood supply. This is 10mls O2 out of every 100mls of blood and this EXCEEDS the amount of Hb present in the dissolved form at 3 atm. SO the only ways around this are:
- an even higher hyperbaric O2 pressure (toxicity starts to get bad much more quickly so this is not an option) - have some Hb present (this will of course be true as the patient will not bleed down to a [Hb] of 0 )
- increase coronary blood flow (metabolic autoregulation) - this is very important and is aided by the decreased viscosity. Coronary flow can increase to over a litre/min in athletes with severe exercise so there is usually sufficient flow reserve to at least double coronary flow. ALSO a doubled flow means the amount of Hb per unit time passing through the coronaries is doubled so an increased flow also improves the use of any low [Hb] which is present.

iii. Also this assumes that the patient breathing the hyperbaric oxygen is able to increase his arterial pO2 to appropriately high levels (ie the A-a gradient is low). Now the problem here is if there is significant shunting as this causes a major decrease in arterial pO2 that cannot be overcome by breathing 100% oxygen. 

When you consider all these factors then it means that humans cannot go without any Hb.

The JW perhaps could counter by saying that if cardiac output was doubled than sufficient oxygen could be supplied at an oxygen pressure much less then the risky 3 atm AND also he could add that I have assumed a constant oxygen consumption (& attempted to maintain oxygen supply) rather than looking at also decreasing oxygen demand (eg sedation, intubation, paralysis & ventilation - but is this 'living'?). He would have a point BUT at these lower pressures than CNS toxicity may not be the problem but rather pulmonary oxygen toxicity -this would result in V/Q mismatch and some shunt. The shunt would impair the ability to oxygenate arterial blood despite the 100% O2.

So bottom line is Hb is absolutely necessary. The solubility of O2 is just too low and no satisfactory artificial O2 carriers have yet been developed.

The real point I think of what the JW fellow means is that there are ways to cope with a very low [Hb] and these involve a mixture of strategies to increase supply (both as dissolved & on Hb) AND to decrease demand. I have touched on this area in my Physiology text. The key points in maximising supply are stopping further bleeding, giving 100% O2 and maintaining a high cardiac output. Add in the ways of decreasing demand and it is quite feasible to maintain anaemic JWs at 3g% in ICU without much problem providing they have otherwise good cardiorespiratory function. The typical reaction though is to be too unduly pessimistic and not to consider the physiology involved. I recall that a case report written up of a JW with a Hb of about ?1g% (or was it 1.9g%) and who survived. However, from memory the physiological explanation was not detailed in the report.

Thanks again for your response. I had thought the term might be too obscure for anyone to know. 
Best wishes,
Kerry.

Dear Kerry,

Hufner's number is the volume of oxygen carried by one gram of Hb. It's value varies with the type of Hb being studied, and whether in vivo, or in vitro. The value of 1.34 mls oxygen per mmHg per dL of blood is most commonly used. It's physiological significance becomes obvious when one considers oxygen delivery to tissues.

Oxy delivery = Cardiac output * oxygen content     (where oxygen content = Volume on Hb + Volume dissolved)

Oxygen content = 1.34 * Hb conc * saturation + 0.003mlsOxygen per mmHg pO2

Combining these two equations :

Oxy Del = CO * [ (Hb*1.34*sat) + (0.003*P02) ]

The importance lies in the causes of hypoxia:

1. stagnant hypoxia : dec CO
2. anaemic hypoxia : dec Hb
3. Hypoxic hypoxia : dec Hb sat
4. Metabolic hypoxia : dec Hufners number

Hope this answers the question.

Darrel McNeil.
Box Hill Hospital
Victoria

Dear Kerry,

In 1894, at the University of Tübingen, Professor Dr. Gustav von Hüfner (1840-1908) found that 1g of haemoglobin could maximally bind 0.0598 mmol of oxygen gas. If one multipies this number by the molar volume of an ideal gas, 22.4 ml/mmol, one gets 1.34 ml/g.

It was only many years later that it was discovered, theoreticaly at least, that at full saturation, one molecule of haemoglobin tetramer (Molar Mass 64,500 g/mol) binds 4 molecules of oxygen.

Theoretically then 64,500 g Hb maximally binds 4,000 mmol oxygen or 1 g Hb binds 4000/64500 =0.062 mmol oxygen. Again multiplying by 22.4 ml/mmol gives 1.39 ml/g.

The International Union of Pure and Applied Chemistry (IUPAC) recommends a value of 1.36 ml/g in vitro at STP.

What about the 310K/273K gas volume correction factor for blood at 37 ºC?

Kind regards
Norman Barlev


Dear Norman,
Thanks for your interesting reply. I don't have the original papers as they were in German anyway but I once made some notes about this (which I am using as my source now) and these are different in a couple of minor points from what you mention:

For example:

[1] Hüfner¹ determined the oxygen capacity of Hb (1.336) but as I understand it he did not use oxygen to do this. He used carbon monoxide based on the idea that equimolar amounts of both bind to Hb so the oxygen capacity can be determined indirectly using CO. You mention Hufner using 'oxygen gas' rather than carbon monoxide. Is your reference clear on this?

[2] From my notes, the theoretical value of 1.39 derives from Braunitzer in his 1963 paper² and he used MW of Hb of 64,458.5 to determine this.

As regards the gas volume correction: I hadn't really thought about that as it does not apply to oxygen which is dissolved in a fluid or combined with Hb but only to oxygen in gas samples. However, when measuring the oxygen content of a blood sample, oxygen is released from Hb (by conversion to MetHb) and then the amount of oxygen is measured in the gas sample collected above the blood so the temperature and pressure of measurement are now important. 

Best wishes,
Kerry.

References:
1. Hufner G (1894) Neue Versuche zur Bestimmung der Sauerstoffcapacitat der Bluffarbstoffs. Arch Anat Physiol pp 130-176.
2. Braunitzer G (1963) Molekulare structure der hamoglobine. Nova Acta Acad Caesar Leop Card 26: 113-125.

11-Jun-2001

Thanks again to your team for a great Part I course. 

I have finally received a copy of the original 1894 article by Hüfner in German! It runs to over 40 pages! Your personal notes are entirely correct regarding his use of Carbon Monoxide as an index for oxygen binding capacity. After the exam is behind me I might tackle a translation of the article.

Kind regards
Norman Barlev

(First page of this article has now been posted on-line)

Dear Kerry,

i seem to recall that it is the volume of oxygen carried by 1 gram of haemoglobin. This is 1.39ml in vitro and 1.34 ml in vivo. With it we can estimate content!

Enjoyed the gas

Geoff Gordon 
(Townsville)