Updated: Aug 19, 2019
As technicians, we are typically the first medical professional to handle patients presenting for emergencies. Unstable patients require quick intervention and often times every minute counts. As the first line for these patients, it is very important for technicians to understand the indications for blood gas sampling and what the results indicate, as well as what and when to bring to our doctor’s attention, who may be busy looking at the big picture of our patients overall status.
A blood gas is a blood sample, either venous or preferably arterial, that shows us a snapshot of what is currently happening in our patients. While arterial is considered the gold standard, a venous sample is a good place to start. Arterial blood draws are painful and often difficult to obtain. Regardless of the method used, venous or arterial, it should be continued on the patient, to be able to monitor trends. A blood gas sample gives us information about oxygenation status, electrolyte balance and respiratory, as well as metabolic function. The primary values that should be reviewed each time a sample is pulled are pH, pCO2 ( carbon dioxide), HCO3 (bicarb), SO2 (oxygenation saturation), lactate, K, Na, and glucose. Base excess and anion gap are generally available as well, depending on your analyzer, however, is not 100% necessary to understand what is happening in your patient.
So when should we consider obtaining a blood gas sample? Any patient who’s oxygenation status, electrolyte balance, respiratory and/or metabolic functions are in question would be a good candidate for pulling a blood gas.
pH is the value that indicates the acid-base status of our patients. Normal values are between 7.35 to 7.45. Any number less than 7.35 is considered acidotic, while any number greater than 7.45 is considered alkalotic. If the patient’s pH is abnormal, they are considered either acidemic or alkalemic.
Acidosis refers to the pathophysiological processes that cause net accumulation of acid in the body. Metabolic acidosis has a decreased plasma HCO3 and a decreased pH. While acidemia refers to the actual pH of the extracellular fluid (ECF). Alkalosis is the opposite, referring to the net accumulation of alkali in the body. Specifically in a metabolic alkalosis there is a higher concentration of HCO3 and an increased pH.
For acid base status, the next value that should be considered is pCO2. pCO2 is the measurement of carbon dioxide in the bloodstream. Carbon dioxide acts as an acid because when it reacts with water, it produces carbonic acid. Cells produce CO2 as a waste product and it is carried to the lungs to be expired. As gas exchange occurs, O2 is passed into the bloodstream and CO2 is expired. A hypoventilating patient will have an increased CO2, however, that is not the only reason for an increased CO2. Primary lung diseases that block gas exchange may lead to an increased CO2. These include pneumonia, CHF, asthma and ventilation-perfusion (V-Q) mismatch due to shunting. Respiratory acidosis may also be a physiologic compensation to metabolic alkalosis. As HCO3 increases the body attempts to compensate by increasing the PCO2. Every increase of 1 mmol/L HCO3 should result in an increase of 0.7mmHg PCO2.
HCO3, commonly referred to as bi-carb, is calculated by the blood gas analyzer. Because it is a calculated value, it is the total of many metabolic processes that can alter hydrogen ions. HCO3 will change with changes in PCO2 and may not accurately reflect a metabolic component of respiratory imbalances. Whereas BE (base excess) , while still a calculated value, is an alternative measure that is independent of changes in PCO2.
Lactate is synthesized from pyruvate and can accept a hydrogen ion to form lactic acid. At a normal pH is is completely dissociated, however, lactic acid does have an acidifying effect on plasma. There are 2 main types of lactic acidosis, type A and type B. Type A is the hypoxic form of lactic acidosis that occurs when there is inadequate oxygen delivery to the tissues. This can occur for many reasons, including poor perfusion and hypoxemia, as well as from a dramatic increase in oxygen demand, such as from seizures, heat stroke or exercise. Type B lactic acidosis is non-hypoxic and occurs when there is abnormal mitochondrial oxidative function, such as can occur during liver or renal failure, sepsis, toxins or drugs, diabetes mellitus and hypoglycemia. A normal lactate level is less than 2 mmol/L. A persistently elevated lactate is a poor prognostic indicator, as levels return to normal quickly after aerobic metabolism is restored.
When reading a blood gas it is important to take a methodical approach to interpretation. The four primary steps are:
Is an acid base disturbance present?What is the primary disturbance?Is the secondary or adaptive response as expected? (simple or mixed)What underlying disease processes are responsible for the acid base disturbance?
To determine if an acid base disturbance is present we need to look at the pH. Is it within the normal range? If not, then there is certainly a disturbance. Depending on the value of the pH the disturbance is either acidosis or alkalosis. Again, acidosis is any pH below 7.35, while alkalosis is any value over 7.45.
What is the primary disturbance? To decide where the primary disturbance lies we need to evaluate the PCO2 and the Bi carb. Is the PCO2 elevated, while the Bi carb is normal? Then there is a respiratory disturbance. If the Bi carb is low and the PCO2 is normal, then there is a primary metabolic disturbance.
To determine if there is a secondary or adaptive response, we need to evaluate the primary values. For example, if the PCO2 is elevated, and the bi carb is also elevated, what is the pH? If the pH is normal then we know that there is a primary respiratory acidosis with metabolic compensation. Metabolic compensation is chronic and may take days to effect pH, so there is most likely a chronic disease process. However, if the PCO2 and bi carb are both below normal values and the pH is normal, we can determine that there is a primary metabolic acidosis with respiratory compensation. Due to how quickly CO2 moves in and out of the blood stream, a respiratory response can happen within minutes to hours.
There are also mixed acid base disturbances where the pH is not normal. Depending on whether your patient is acidotic or alkalotic will determine which is the primary and secondary disturbance. The pH will shift in the opposite direction of the primary disturbance.
So what underlying diseases do we need to consider? HCO3 is excreted through the kidneys, but may also be lost during excessive diarrhea, so renal disease, toxins that effect the kidneys and small bowel diarrhea are all on the list of considerations. Hypoxemia can be the result of low fraction of inspired O2 (which rarely occurs in veterinary medicine), but can be the result of a decrease in barometric pressure such as on a non pressurized flight; or more commonly an improper inhalant anesthetic technique. Hypoventilation can occur under anesthesia quickly due to a low respiratory drive or due to toxin or paralytic agent. Diffusion impairment, can occur when there is a barrier to diffusion, such as with pneumonia, primary lung disease or congestive heart failure. V-Q mismatch and right to left shunting can also cause hypoxemia by perfusing non oxygenated blood.
Overall, there are many diseases that can cause acid base disturbances and the overall status of the patient should be considered. A blood gas is another piece in the puzzle to determine what may be wrong with you patient and a valuable trending tool to measure therapeutic effectiveness.
Dibartola, Stephen. Fluid Electrolyte and Acid Base Disturbances in Small Animal Practice (Fluid Therapy in Small Animal Practice) 4th Edition. Saunders, 2011
Silverstein, Deborah and Hopper, Kate. Small Animal Critical Care Medicine, Second edition. Elseiver. 2015