pCO_{2} in: mmHg kPa
blood
plasma
standard values
calculated values
0
∞
pH:
pCO_{2}:
[HCO_{3}^{-}]:
β:
Base excess: mmol/L
Disruption type: Normal State
Bicarbonate Buffer:
Nonbicarbonate Buffer:
Compensation:
none
after a few minutes
after 4-6 hours
after 3-5 days
This chapter of our journey mentions the first two approaches to acid-base interpretation, the Boston and the Copenhagen approach. The Boston approach is based on pCO2, HCO3- and pH; it relies on empirical rules for the interpretation of acid-base disturbances. In the late 1950's, Siggaard-Andersen introduced base excess, a new parameter meant to quantify the metabolic part of acid-base disturbances. This became known as the Copenhagen approach. There is to this day (if we are correctly informed) fierce disagreement between proponents of these two approaches, and as humanity rarely misses an opportunity to bestow catchy names upon anything interesting, this feud became known as The Great Trans-Atlantic Acid-Base Debate.1,2
The little piece of artwork above is called a Davenport diagram, brought into the world by Horace W. Davenport. We get that it may seem somewhat complicated and difficult to grasp at first, but in the midst of all that baffling complexity lies a road to a better understanding of acid-base physiology, which is, as far as we know, the goal of your visit.
The x-axis shows the pH, which is nothing more than the negative base logarithm of the hydrogen proton concentration (in the nanomolar range) in the system, and the y-axis shows the bicarbonate concentration and simultaniously the change in the non-bicarbonate buffer concentration. On the diagram there are two functions: one is linear and the other exponential, referring to the strength of the non-bicarbonate and the bicarbonate buffer systems, respectively. The intersection of these two functions represents the pH and bicarbonate concentration in the system given the parameters you determine.
It is an incredibly useful tool and we suggest you take your time and take in everything it has to offer. If you are still nervous about starting to learn from it, you can try affectionally calling it »Dave« and pretending that it is a friend trying to explain some interesting concepts to you.
This specific Davenport diagram is special in the sense that it is interactive, meaning that the system will change depending on the changes you make to the input parameters. If you don't understand them fully, you can either click on them or scroll down to the Glossary section.
The input parameters are the ones left to the diagram. As soon as you change one of them, the functions of the diagram will also change. Below the diagram, the output parameters will display some interesting values which will come in handy when you try to understand what you have actually done.
There are also two lines under the graph that keep track of the buffering share for the bicarbonate and the nonbicarbonate buffer systems.
The type of acid-base disturbance, should your actions cause one, will also present itself. But don't you worry too much, you'll be given a chance to correct or, more accurately stated, »compensate« for the disturbance in the force.
Sorry, we meant system. The disturbance in the system.
The values under the graph will then represent the new, compensated values of the system.
Of course, all of this is, as is much of the modern world, powered by some pretty fancy math and we felt that we owed you at least the basic equations we used. They are all written out in the section Mathematical disclosure and you are more than welcome to check them out.
And that is more or less it. Now you can first read through all of the Glossary below or start exploring your options above and learn on the go. Your call.
pCO_{2} - the partial pressure of blood CO_{2}. By default, it is presented in mmHg, but you can change that to kPa. Changing this parameter means inferring a respiratory acid-base disturbance. Together with the pH and HCO_{3}^{-} it is used in the Boston approach to determine what kind of acid-base disorder is present. More
HCO_{3}^{-} - the bicarbonate concentration. It is one of the possible metabolic markers of acid-base disturbances and if you are a fan of the Boston approach, together with the pCO_{2} and pH it is all you really need. Changing it will infer a metabolic acid-base disturbance (lowering it will result in a lower pH; elevating it will result in a higher pH). More
ß - the buffer capacity, specifically of the non-bicarbonate buffers. This is, in its essence, the slope of the linear, non-bicarbonate function on the Davenport diagram. Altough the function is strictly mathematically speaking a negative one, ß is written out as a positive value, for didactic purposes. Here you have more options. It can either represent the ß for plasma (discarding the influence of hemoglobin) or the ß for whole blood. You can set the parameters governing it to standard values or be a little more creative and determine them yourself (within reasonable boundaries, of course). Also, there are two special scenarios prepared, one where ß is set to 0 (discarding the non-bicarbonate buffer system completely), and the other, where ß is infinite. More
pH - the main measure for the acid base status. Need we say More?
BE - the base excess. This parameters represents the excess quantity of base in the system. If it has a negative value, it is also called a base deficit and represents the the deficit quantity of base in a system. It is loosely defined as the quantity of acid or base required to return in vitro blood to normal pH (7,4) under standard conditions. It was introduced in 1958 and has since then been revised multiple times, generating standard base excess and corrected standard base excess, all in the pursuit to give more accurate information. In this section, we have decided to go old-school with just base excess, and we used two different equations for it (explained in the Mathematical disclosure). It is argued that it is a better parameter for evaluating the metabolic part of acid-base disturbances. More
Albumins - the blood albumin concentration. Albumins are one of the carriers of the non-bicarbonate buffer capacity, but that is not everything they do. More
C_{Hb} - the blood hemoglobin concentration. It is a major player, non-bicarbonate buffer capacity wise. More
Phosphates - the blood phosphate concentration, which also contributes to the non-bicarbonate buffer capacity.
and
$$\beta(B)= C_{Hb}(B) \cdot \beta_{Hb}+\beta(P)$$ determines the slope of the linear function representing the non-bicarbonate buffer capacity. B means whole blood, P means plasma. $$BE(P)=\Delta [HCO_3^-]+\beta(P) \cdot \Delta pH$$and
$$BE(B)=(1-\frac{C_{Hb}(B)}{C_{Hb} {}^\circ}) \cdot (\Delta [HCO_3^-] + \beta(B) \cdot \Delta pH)$$ Calculations for determining the base excess (BE) in the simulated system. B means whole blood, P means plasma. CHbo is a constant.Projekt sofinancirata Republika Slovenija in Evropska unija iz Evropskega socialnega sklada.