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Welcome! This series is an educational exercise to provide practice in interpreting blood gas results in a clinical context. Most of the case scenarios will have an emphasis on Anaesthetic or Intensive Care practice but interesting gas results on other patients that I come across may be included. Most of the results are ones you could typically come across in your daily practice so the emphasis is not on finding strange or extreme results though some unusual results may be included. Perhaps you have come across an 'interesting gas' - please feel free to send this along to me at kbrandis@bigpond.net.au. The style of analysis used here is that presented in my book "Clinical Acid-Base pHysiology".

Please consider the following clinical scenario and then consider the question & discussions that follow.

Also: Answer to the question in Gas of the Week No 4 is now posted

History
A 74 year old lady was taken to her community hospital by relatives because relatives said she was "pretty crook" (meaning respiratory distress worsening over the previous 3 days & associated with fever). She had a past history of severe chronic airway disease and continued to smoke heavily.

Examination
She was in significant respiratory distress with dyspnoea & peripheral cyanosis. She was drowsy & confused. Chest examination revealed poor air entry and widespread rhonchi. Resp rate 46/min, BP 120/80, temp 39.0°C. Urine testing was negative for ketones.

Management
She was commenced on a high concentration of oxygen via a venturi mask and her colour improved. She subsequently detoriorated so was intubated & ventilated and transferred to the Intensive Care Unit at our hospital.

Blood gases & electrolytes were collected at various times.

Blood gases & Electrolyte Results

Questions

[1] How would you analyse the initial set of results?
[2] Should blood gas results be 'temperature corrected'?
[3] Should we use pO₂ or should we use SO₂ as the best indicator of the adequacy of oxygenation in this patient? -Why?

Consultant (to Registrar)
How would you analyse the initial results? The are presented as two sets of values: one temperature corrected and one not. 

Registrar
My initial quick scan reveals acidaemia, severe hypoventilation (in view of hypercapnia) with adequate oxygenation.

Hypercapnia nearly always indicates hypoventilation. The only exceptions of note are:
* Malignant hyperthermia particularly when ventilation is controlled
* Significant amounts of carbon dioxide present in the inspired gas.

Either of these 2 situations could happen intraoperatively but this is not the situation here as the history excludes these unusual situations. 

The other key thing the deserves comment is that a pCO₂ of 90mmHg is life-threatening because of the obligatorily associated hypoxaemia if the patient is breathing room air. The pO₂ here is quite high; too high to be breathing room air. The patient has arrived at hospital in extremis and has been immediately commenced on oxygen (high F_IO₂) so the gases were not collected on room air. 

Consultant
If the patient had been breathing room air with this arterial pCO₂ level, what would the arterial pO₂ have been?

Registrar
The alveolar pO₂ can be calculated from the alveolar gas equation:

p_AO₂  = p_IO₂ - (p_aCO₂/R)

where R is the respiratory exchange ration (assume it is 0.8)
and p_IO₂ is inspired O₂ concentration which = (p_B - pH₂0) x F_IO₂

Now substituting known values of 760mmHg for barometric pressure (p_B) and assuming room air (F_IO₂) of 0.21:

p_AO₂ = [(760 - 47) x 0.21] - [87 / 0.8] = 41mmHg.

Now the arterial pO₂ will be lower than this even in health because of 'venous admixture'.

Consultant
Before you go on, could you tell me what you mean by 'venous admixture'?

Registrar
This term is used in slightly different ways by different people. I use the term to apply to the factors which cause the arterial pO₂ to be lower than than the alveolar pO₂.

The assumption I make is the arterial pO₂ is lower than alveolar pO₂  because of the addition of mixed venous blood to the pulmonary end-capillary blood. Pulmonary end-capillary blood has the same pO₂ as that in the alveoli it is in contact with because equilibrium is reached between the alveoli and pulmonary capillary blood. Typically this equilibrium is reached in about one-third of the time (0.25 sec) that the two are in contact (0.75 sec).

Now in reality of course the pO₂ of the blood causing this depression is rarely the the same as that in mixed venous blood, but we can calculate how much blood with the composition of mixed venous blood would have to be added to cause the observed depression in arterial pO2. This makes use of the shunt equation and the calculated value for this 'virtual shunt' is referred to as the 'venous admixture'. 

The two main contributors to venous admixture are:
* alveoli with low V/Q ratios
* shunting (eg bronchial venous blood, thebesian blood draining into the left heart).

 On room air, the typical (A-a)pO₂ difference (or 'A-a gradient') is 5 to 15 mmHg. So with this very high pCO₂ level, we would expect a pO₂ of about 30mmHg if the lungs were otherwise normal. This is why I said earlier that such a high pCO₂  is obligatorily associated with life-threatening hypoxaemia.

Of course, the lungs here are definitely not normal so if the pCO₂ was that high before arrival then the pO₂ would have been much lower - this suggests that the patient's ventilation has decreased since she was commenced on oxygen.

Consultant
Now you said that this patient is 'hypoventilating' and yet the history is that she was tachypnoeic (resp rate 44/min) and had significant respiratory effort. This doesn't sound like hypoventilation does it? 

Registrar
That's an interesting point that I don't think I had realised before. We use the terms hypo- & hyper-ventilation in 2 different ways:

[1] When clinically assessing a patient, 'hyperventilation' tends to refer to the amount of effort the patient is making or perhaps more accurately the amount of ventilation per minute; this is the minute volume of ventilation (MV). 

[2] When we use the term 'hyperventilation' in acid-base physiology, we are referring to the alveolar ventilation as this is what is important for pulmonary gas exchange. This is related to MV as follows:
MV = [resp rate x (V_D + V_A)]    where V_D is dead space volume & V_A is alveolar volume. 
If V_D is increased, then a high MV could be associated with a low alveolar ventilation.

In this particular case the clinical assessment is of much increased respiratory effort but clearly the alveolar ventilation is much decreased. Perhaps hyperventilating in the clinical sense of a high MV, but hypoventilating in the gas exchange sense of a low V_A - if this makes sense!
 
The equation that relates arterial pCO₂ to alveolar ventilation (and to the body's CO₂ production (Vco₂) ) is:

p_aCO₂  = k. (Vco₂ / V_A )   (where k is the proportionality constant)

Assuming Vco₂ is constant, then p_aCO₂ and alveolar ventilation are inversely related. If p_aCO₂ is doubled then alveolar ventilation must have halved. It is the change in alveolar ventilation that causes the change in arterial pCO₂.

The other point I thing you are wanting me to make is that the amount of respiratory effort by the patient as assessed clinically gives a clue as to the cause of the alveolar hypoventilation. 

If the cause is primarily pulmonary (as in this case) then the elevated arterial pCO₂ will stimulate the peripheral and central chemoreceptors resulting in stimulation of the respiratory centre. This results in respiratory muscle activity to attempt to increase alveolar ventilation and correct the hypercapnia. 

If the cause is primarily central depression (of the respiratory centre) such as occurs with an overdose of opiates or barbiturates than there will be little respiratory effort. This reduced respiratory effort is the cause of the hypercapnia. 

So the amount of respiratory effort gives a guide to the cause of the hypercapnia: central or pulmonary. This is useful to know in some situations but in mostly you should know what is likely from the history or even from just looking at the patient. Some cases are more complicated as a primarily respiratory cause may be complicated by central depression from hypoxaemia and hypercapnia. 

Consultant
You've made some good points. Now back to assessing the actual gas results. You have two sets of results from the single initial sample: which do you use? -and why?

Registrar
Well the patient's actual temperature is 38.8ºC. My initial thought is to use the gases which give the actual values for the actual temperature. The patient's temperature is entered into the blood gas machine before the analysis is done but I think the gases are actually measured at 37ºC in the machine. 

Consultant
Yes that's right. The blood gas machine is thermostated to 37ºC and all measurements are made at that temperature. The calomal reference electrode in the blood gas machine is very temperature sensitive. Results for other temperatures are calculated using a formula which is programmed into the blood gas machine. 

It has been found experimentally that the change in pH with temperature in a blood sample is almost linear. Anaerobic cooling of a blood sample causes the pH to rise, so the pH is higher at a lower temperature (& lower at a higher temperature). 

A common formula for temperature correcting blood gas results uses the Rosenthal correction factor. The Rosenthal factor is a change in pH of 0.015 pH units per degree centigrade change in temperature.

Registrar
The pH here is 7.215 at 37ºC. Using the Rosenthal correction factor, then it will be lower by (1.8 x 0.015) at 38.8ºC:

pH (at 38.8ºC) = 7.215 - [(38.8 -37.0) x 0.015] = 7.188

So the machine at the referring hospital is using the Rosenthal correction factor. 

Consultant
Similarly, there are formulas in the machine's programming to temperature correct the pO₂ and the pCO₂. The [HCO₃] is calculated from the pH and the pCO₂ using the Henderson-Hasselbach equation.

Registrar
The thing I'm a bit unsure about is how the pO₂ and the pCO₂ values can actually change with change in temperature. This is all happening in a closed system where there is no contact between the blood sample and the ambient air. So how can the gas values change? There is no entry of any O₂ or CO₂ from outside so the gas content of the sample cannot change. So how can the pO₂ and the pCO₂ in the sample change? Now I know the reported values change but is this a 'real' change? 

Consultant
Yes, I think that is not well understood by a lot of people. The way to find the answer though is to have a clear understanding of what the 'partial pressure' values mean. We are taught about using pO₂ and pCO₂ and become familiar with this but what does the partial pressure values really mean?

Registrar
Well, they tell you the 'tension' of the gas present in the solution and this tells you how much of the particular gas is present. 

Consultant
You say 'how much' so you are referring to content. Well, consider this: if the pO₂ of a blood sample was doubled from 100mmHg to 200mmHg, does the oxygen content also double?

Registrar
No, because the vast majority of the oxygen present is combined to haemoglobin (Hb) and at a pO₂ of 100mmHg the Hb is essentially fully saturated so doubling pO₂ from 100 to 200mmHg does not result in a doubling in total oxygen content. The saturated Hb cannot carry more.

I should clarify: what I meant was that the partial pressure of the gas in the solution tells you the amount dissolved in the solution. If the pO₂ increased from 100 to 200mmHg, then the amount (or content) present as dissolved oxygen will indeed be doubled (even though the total amount present would not be). The same is true for all gases: if the amount of gas dissolved in a solution doubles, then the partial pressure of that gas doubles.

Consultant
Good. What is the basis of our universal practice of referring to the amount of a gas dissolved in a solution in terms of 'partial pressure' (in mmHg or in kPa) rather then using the actual amount (in mls/dl) present. 

Registrar
This is just Henry's Law.

This says that "at equilibrium, the amount of gas dissolved in a liquid is proportional to the partial pressure of gas at the surface of the liquid" (One proviso is that this does not refer to gases which are infinitely miscible in the liquid.) The formula we derive from Henry's law is:

Amount of gas dissolved = k x partial pressure

(where k is the proportionality constant). 

Now the value of 'k' is very important as it is a measure of how soluble the gas is in the liquid. If k is high, the gas is more soluble. If k is low, then the gas is said to be less soluble. The value of k is different for different gases. This proportionality constant is important then for that reason and is referred to as the 'solubility coefficient' for the particular gas in the particular liquid. For example, the solubility coefficient for oxygen in water is 0.003 so:

Dissolved O₂  = 0.003 x pO₂ 

If pO₂  is in mmHg, then this will give you the amount of oxygen dissolved in mls O₂ per mmHg per 100mls blood.

Consultant
So how much oxygen is dissolved in arterial blood when the pO₂ is 100mmHg?

Registrar
Amount dissolved = 0.003 x 100 = 0.3 mls O₂ /dl of blood. 

Consultant
What is the physiological significance of this?

Registrar
Under normal conditions, the amount of oxygen carried as dissolved oxygen is very small (in comparison to the amount carried on Hb). This is a direct consequence of the very low solubility of oxygen in water. The way that this problem is overcome in the body is by using a specific oxygen carrying protein. Haemoglobin at a concentration of 15 g/dl can carry about 20mls of oxygen per dl of blood. This is over 60 times the amount of dissolved oxygen in the same volume of blood at a pO₂ of 100mmHg.

Under normal conditions, dissolved oxygen is not an important transport form for delivering oxygen to the tissues. Dissolved oxygen accounts for less than 2% of the normal 1,000 mls of oxygen delivered to the tissues per minute. 

Consultant
So dissolved oxygen is not important in the body then?

Registrar
Thats a trick question because the answer is this:

Dissolved oxygen is vitally important in the body. It is the form of oxygen that diffuses from the capillaries to the cells. It is the form of oxygen that reacts with cytochrome oxidase at the end of the electron transport chain in the mitochondria. This is what permits oxidative phosphorylation to continue. This reaction in mitochondria accounts for about 80% of the body's use of oxygen. (The many oxidases in the body account for much of the rest.)

Dissolved oxygen is not particularly important in the transport of oxygen in arterial blood. It is this one circumstance where its low solubility (& consequent small amount dissolved) is a physiological problem which is overcome by the presence of haemoglobin. Don't generalise to say that it is not important in the body though. It is only not very important in transport of the blood under normal conditions of [Hb] & cardiac output.

Consultant
 Now back to Henry's Law because we still haven't answered the question about how the partial pressure of a gas in solution can change even though the dissolved gas content does not. This is what you were trying to understand.

The key thing to remember is that because of the Henry's law proportionality, you can refer to gases using the gas phase concept of partial pressures as an indicator of the amount dissolved in the liquid even though this does not actually indicate the actual amount dissolved. The gas is present in a dissolved form; it is not there as a gas which can be measured by its partial pressure. The partial pressure refers to amount of the gas in the gas phase which would be in equilibrium with that amount of dissolved gas if they were in contact.

The amount of oxygen dissolved in water (or blood) at a partial pressure of say 50mmHg is different from the amount of CO₂ dissolved at the same partial pressure. The actual amount present is different because the two gases have different solubility coefficients. We can say that if the partial pressure of each were to double then the amount of each gas dissolved would be doubled but the dissolved amounts of each gas are different. (The additional confusing thing about these two gases is that there are other forms of carriage for each present as well. This affects the total amount present but we are only considering the dissolved content.)

The key thing that links the partial pressure (in the gas phase) to the amount of dissolved gas (in the liquid) is the proportionality constant 'k'. This is the 'solubility coefficient'. The final key fact to know is that this 'constant' is not constant!. It is temperature dependent and this provides the answer to your problem.

Registrar
So the partial pressure of the gas in the liquid can change even though the dissolved amount of gas does not. It is the value of the solubility coefficient that changes. In this patient then, the pCO₂ of 95.7mmHg at 38.8ºC represents the same amount of dissolved CO₂ as the pCO₂ of 86.9mmHg at 37ºC. The partial pressure changes but the amount present doesn't. This makes sense then as in a closed system, the total amount dissolved must be a constant but if the solubility coefficient changes value with temperature than the partial pressure must change (because it is equal to the product of 'k' & the amount dissolved). 

Consultant
The way we use Henry's law to indicate the amount of gas dissolved in a liquid is essentially an agreed 'convention'. We could have all agreed to only speak of the actual amount present but we didn't. This convention is undoubtedly useful.

The lack of understanding of what this is all about has lead to a couple of other absurd situations. For example, the somewhat common practice of referring to the pH₂O in a blood sample as being 47mmHg (at 37ºC) is quite annoying. Many texts do this. But how can water be dissolved in water??? Granted that this is absurd then it is quite silly to refer to a dissolved pH₂O. The use of pH₂O in this way is really to do with saturated vapour pressure and not Henry's law. It is not an indicator of the amount of H₂0 dissolved in the blood sample which is what this Henry's law convention is about.

Similarly there is no reason why the sum of all the partial pressures of dissolved gases must add up to 760mmHg. Clearly they never can if the pH₂O is not added in. Again this is a misuse of another gas law: Dalton's law of partial pressures. This law refers only to the gas phase. The misapplication of Dalton's law to liquids is what makes the use of a pH₂O value necessary. Perhaps then you could say it is a 'convention' also. Simpler to just say it is wrong.

Registrar
This then also applies to the use of the Henderson-Hasselbach equation. The change in the pCO₂ doesn't alter the calculated pH because the term in the denominator is the dissolved CO₂ (ie: k x pCO₂) and this doesn't change despite the change in pCO₂. 

I notice in the 2 results that the [HCO₃] is the same in both. But if the pH has changed, and the (k x pCO₂) term is constant, then if [HCO₃] is constant, the pKa must have changed. 

Consultant
Yes, that is so. Just to summarise the effects of temperature on a closed system:

pH increases with decrease in temperature (and the reverse with an increase in temperature). The amount is estimated using the Rosenthal correction factor.

pCO₂ decreases with a decrease in temperature. This is because gases are more soluble at a lower temperature due to a lower kinetic energy and thus less tendency to escape from solution. So if the total CO₂ content is constant as is true for this closed system, the pCO₂  decreases (and the solubility coefficient rises)

[HCO₃ ] is constant with changes in temperature.

pKa increases with a decrease in temperature

pO₂ decreases with a decrease in temperature for reasons as for pCO₂. There is an additional factor here as the haemoglobin oxygen dissociation curve shifts to the left with a decrease in temperature - this means Hb has a higher oxygen affinity at a lower temperature. These 2 factors (increased solubility & increased Hb affinity) make for a more complex situation then with CO₂. If pO₂ is high the Hb is already fully saturated and the change in affinity won't have much effect. At lower pO₂ levels, the increased affinity means some of the dissolved oxygen will bind to Hb causing a decrease in pO₂. This is additional to the decrease in pO₂ that occurs with the increased solubility of O₂ at the lower temperature. Total oxygen content does not change. 

These changes I'm talking about here are what's happening in the closed system of a blood-gas syringe and in the blood gas machine. In the patient at a temperature other than 37C, other effects are important. For example a decreased oxygen consumption and a decreased CO₂ production occur with hypothermia (in the absence of shivering).

Anyway, back to the patient. These are the changes occurring with the change in temperature but are they clinically important?

Registrar
Well I have to say no in this case. The pO₂  & the pCO₂  are different at the two temperatures but not enough to make any clinical difference. The patient is grossly hypercapnic but the obligatory hypoxaemia has been able to be overcome by administering high oxygen concentrations. The patient is in extremis though and deteriorated despite oxygen. She required intubation and ventilation. The deterioration was possibly due to central depression by the hypercapnia, the lessening of hypoxic drive following oxygen administration or simply exhaustion, or probably all of these factors.

The situation with temperature changes in blood gas values is more marked with quite hypothermic patients (eg if cooled to 20 to 28ºC while on cardiopulmonary bypass). Maybe the use of temperature corrected values would be more appropriate then. However, I understand that the current recommendation is to not temperature correct the values even at these temperatures. 

Consultant
Yes. This is the alpha-stat approach (which we don't have time to discuss here today). One problem with temperature correction of gas results is that we have no reference range for temperatures other than 37ºC. So how can we interpret what is normal or abnormal at a temperature different from 37ºC?

Temperature correction is necessary in some situations (eg to correctly determine the endtidal-arterial pCO₂ difference).

In this patient then, please analyse the initial non-temperature corrected results for me.

Registrar
Clearly a severe respiratory acidosis is present. The electrolyte results are not presented for this time. 

If this was an acute respiratory acidosis, the 1 for 10 rule would apply. This predicts a [HCO₃] of 28.5mmol/l. I think 90mmHg is about the limit for experimental confirmation of the expected amount that bicarbonate changes. By and large, [HCO₃] will always be below 30mmol/l if due to an acute respiratory acidosis alone.

I think this is clearly the wrong approach in this patient. This is an acute infective exacerbation of COAD and has been present for 3 days by the history. It would be useful to review any recent electrolyte results to see if 'total CO₂' was elevated as this would provide evidence of chronic CO₂ retention. Renal compensation is well under way I would say so the 'rule of thumb' for a chronic respiratory acidosis is the one to use. The 4 for 10 rule says the [HCO₃] increase by 4mmol/l for each 10mmHg chronic elevation in pCO2 and this takes say 3 or more days to reach its maximum. A pCO₂ predicts a [HCO₃]  of (24 + 18) = 42mmol/l.

The actual [HCO₃] of 33.8mmol/l is lower than this. The 2 possibilities to consider are:
* A coexistent lactic acidosis (or other metabolic acidosis) is present
* Insufficient time has elapsed for maximum compensation

Consultant
Now you need to apply the 'So what' test.

Registrar
OK then. The 'So what' test means to assess the clinical significance of something. In effect to say 'so what does this mean for the patient'. The test result is never an end in itself and should not be ignored.

So what does this mean? My clinical assessment suggests that this patient has had several days of illness but it is very unlikely she has survived for 3 days with a pCO₂ of 86mmHg at home because as discussed earlier this pCO2 means a severe hypoxaemia with pO2 of less then 30mmHg. More likely is that she has had slowly worsening gas exchange but mostly with a pCO2 lower than 86mmHg. A [HCO₃] of about 34mmHg would be full compensation for a pCO2 of about 65mmHg: this would allow a more tolerable pO2 level.

The lactate level was measured with the next set of gases 2.5 hours later after intubation. The value of 2.7mmol/l excludes a significant lactic acidosis and this is consistent with the above scenario. The anion gap is low and there is no clinical or biochemical evidence of another cause of metabolic acidosis (such as renal failure, ketoacidosis, diarrhoea)

Consultant
You mention the phrase 'full compensation' in your response. Is this what you mean?  

Registrar
Actually no, that was a mistake. The body's compensation for an acid-base disorder is virtually never 'full' in the sense of enough to return the pH to the normal range. So we should avoid the phrase 'full compensation'. What I meant was 'maximal compensation'. I understand this is a common error. 

Consultant
I think you explanation is very plausible. It would indeed be unlikely that a person ill with a pCO₂ of almost 90mmHg would be just sitting around at home for so long. The obligatorily associated severe hypoxaemia would be fatal. 

Extremely high pCO₂ values (say >100mmHg) occur only in patients receiving a high inspired oxygen concentration if only because they would undoubtedly be dead from hypoxia if breathing room air.

Another point to make is that a severe lactic acidosis is much more likely with poor perfusion than with hypoxaemia. The various autoregulatory changes allow the tissues to cope better with a low pO₂ than with a low perfusion.

Is it better to use S_aO₂ or p_aO₂ as an indicator of the adequacy of oxygenation in this patient?

Registrar
Well I tend to use p_aO₂ as the indicator of 'oxygenation'. This is a more sensitive indicator of the lung's ability to add oxygen to the blood when the inspired oxygen concentration is high. For example, if the patient is breathing 100% oxygen and her p_aO₂ is 100mmHg and saturation is 98% then there is a major defect in oxygenation but the saturation value is high and insensitive to this defect. If p_aO₂ is low, then S_aO₂ will also be low and either could be used.

On the other hand, looking at it from the tissues point of view, it is the 'oxygen flux' (tissue oxygen delivery per minute) that is most important. The key factors then are cardiac output, S_aO₂ and [Hb]. The proviso here is that there are not significant quantities of abnormal haemoglobins present (eg HbCO, metHb) as these do not carry oxygen. The ABGs at 0310hrs exclude this problem in this patient. 

Consultant
If there was a large amount of say HbCO present, then the S_aO₂ of the Hb would be normal but its oxygen carrying capacity would be severely reduced. In that case, S_aO₂ would be a poor indicator of the adequacy of tissue oxygen delivery. 

In general, p_aO₂ is a good indicator of oxygenation if considering pulmonary function, but S_aO₂ is generally a better indicator of tissue oxygen delivery (assuming of course that cardiac output & [Hb] are adequate).    

Registrar
But how can haemoglobin saturation be normal if a substantial percentage is in the form of carboxyhaemoglobin? 

The oxygen flux equation says:

Oxygen delivery/min = [ CO x [Hb] x  S_aO₂ x 1.34 ] + [ 0.003 x  p_aO₂]

If a lot of HbCO were present, oxygen flux must be decreased and the only way this can be accounted for in the equation is by a decrease in  S_aO₂. So how can the S_aO₂ be normal? 

Consultant
I see your point but nevertheless you are wrong. Have a think about it and we'll discuss it next week.

The above is a purely hypothetical dialogue which is presented for educational purposes.