In the critical care transport setting, the importance of the arterial blood gas is frequently neglected. All too often I have watched as everyone from physicians to paramedics overlooked this important factor in trending patient care. We remember blood pressure and heart rate, but may neglect the importance of pH and other blood gas values. We learn about the importance of the arterial blood gas in school, but it may not come to mind when we’re actually called on to transport an intubated or critically ill patient. The oversight becomes even easier when sending providers are in a rush to move the patient and have neglected to draw or even order an ABG. Ideally, we would like to see serial ABG’s correlated with ventilator changes and pharmacological management prior to transport. Knowledge of this progression can provide vital insight affecting transport decisions and goal determination.
Paramedic education typically employs the traditional approach to acid-base eduction, which emphasizes the importance of hydrogen (H+) and bicarbonate (HCO3-). While the traditional approach is certainly adequate, there is another method to approach the acid-base question. This method was developed by Peter A. Stewart. Stewart’s system focuses on the manipulation of a set of “independent variables” to obtain desired changes in “dependent variables.” This is a brief introduction; Stewart discusses the method in detail in his book.
Recall the principle of homeostasis, which essentially states that the body will do whatever it can within reason to maintain a state of neutrality. It does so via the manipulation of independent variables in order to change dependent variables. Independent variables are those factors that we can manipulate directly, and include the partial pressure of carbon dioxide (PCO2), weak acids, and strong ion difference. Dependent variables are those factors that change based on the manipulation of the independent variables, and include hydrogen (H+) and bicarbonate (HCO3-). H+ and HCO3- are functionally limitless in the body, but it takes a change in one or more of the independent variables to come along and make them “kick in” and compensate.
One interesting and often overlooked independent variable is albumin, chief among the weak acids. If albumin levels are raised in the body the result is a lower pH, and if albumin is lower you will find a raised pH. The next thing to understand is the strong ion difference. It is here that we get the groundwork for our ion gap. The body’s electrical charge must remain neutral, which is to say the cations (positively charged ions) and anions (negatively charged ions) must remain in balance. We know that that pH is a measure of relative acidity, and is determined by hydrogen (H+) and balanced by bicarbonate (HCO3-). Although this is true, these two factors (H and HCO3-) actually account for only a small part of the pH balance. The big players in this game are sodium (Na+) and chloride (Cl-).
Na+ and Cl- are the primary cations and anions in the blood and as such they, rather than H+ and HCO3-, make the strongest contribution to pH. The image above demonstrates the relative distribution of cations and anions, and shows that there is a greater amount of Na+ than Cl-. The normal difference between Na+ and Cl- is 37. If the difference between them is less than this value, the result is acidosis. If greater, the result is alkalosis. When the body enters the shock state of anaerobic metabolism, it is these stronger ions that are affected by the lactate and ketoacids produced.
Finally, we will discuss the concept of base excess. The base excess is found by removing the respiratory component of the ABG, in order to remove any compensation that is taking place. The base excess or base deficit quantifies how much base or acid is required to get the pH back to neutral. So a base excess of 3 would require 3mm of base to even out. Conversely, an excess -3 (which is actually a deficit as it is a negative value) would require 3mm of acid to neutralize. A positive value base excess needs base, while a negative base excess needs acid.
This is an introduction to Stewart’s approach, and lays some theoretical groundwork. In part two, we will look at some practical application of this method.
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