Mistakes in arterial blood gas (ABG) interpretation are common in clinical practice. The following is a simplified explanation of ABGs, including a practical. The principles of oxygen transport and ventilation are core concepts for critical care nurses to understand in managing acutely and critically ill patients. Nurses. ARTERIAL BLOOD GASES MADE EASY i This page intentionally left blank Arterial Blood Gases Made Easy Second Edition Iain A M Hennessey MBChB ( Hons).
|Language:||English, Spanish, Japanese|
|Genre:||Science & Research|
|Distribution:||Free* [*Register to download]|
History. 2. What is the Oxygenation status. 3. What is the pH? Acidemia or Alkalemia? 4. What is the primary disorder present? 5. Is there appropriate. sO2. 70 - 75%. ABG EASY AS 1,2,3. NORMAL VALUES & DEFINITIONS 3 STEPS TO ABG INTERPRETATION made no attempt to help normalise the pH. Key points. Most doctors struggle with arterial blood gas (ABG) interpretation. ABG interpretation is easy. Break it down into steps. The first priority for the.
Ask the question: If, for example, the problem is an acidosis and the P a CO 2 is low, then clearly the respiratory system is attempting to compensate. Thus, one can conclude that the problem is metabolic similarly with other combinations.
Therefore, after looking at only two numbers pH and P a CO 2 , most of the interpretation is done. The other numbers actual bicarbonate [aHCO 3 ], base excess [BE], P a O 2 and so on might do nothing more than confirm this conclusion. However, they can sometimes add information about time course or provide information on additional derangements, but they will not contradict the conclusion that has already been reached.
What is perhaps surprising is that, after many years of looking at ABGs, those intelligent, enquiring minds have seemingly never once pondered that question. The problem with this measurement is that it is markedly affected by P a CO 2. It is this value that would provide a direct handle on what the metabolic system is doing. Base excess BE measures all bases, not just bicarbonate.
However, because bicarbonate is the greater part of the base buffer, for most practical interpretations, BE provides essentially the same information as bicarbonate.
The major advantage of BE is that its normal range is really easy to remember. One could probably have guessed that the expected value of BE was zero the clue is in the word: If one has established that problem is respiratory, then the BE can tell us something of the duration of the problem. If, for example, in a respiratory acidosis, the sHCO 3 has shown no sign of responding still within the normal range , the probable explanation is that there has not yet been time to respond ie the problem is an acute respiratory acidosis.
A respiratory acidosis with a low sHCO 3 would indicate a combined respiratory and metabolic -acidosis.
Remember that one cannot live for long with pH outside of the normal range. An abnormal pH means there has to be an acute component to the problem. It is sometimes thought that type 2 respiratory failure is simply a more severe version of type 1. However, this is not the case. Type 1 and type 2 respiratory failures are due to entirely different mechanisms. Type 2 respiratory failure is extremely an issue of ventilation, that is, the business of pumping air in and out of the lungs.
When underventilation occurs, for what ever reason eg muscular weakness or opiate overdose , the P a CO 2 will increase the definition of underventilation and P a O2 must decrease even if the lungs are perfectly healthy. Type 2 respiratory failure results from underventilation, which can occur even in the context of healthy lungs. In such circumstances, oxygen delivered to the lungs by ventilation is handled inefficiently and P a O 2 falls. However, provided that overall ventilation is normal, P a CO 2 is maintained.
When P a O 2 is low yet P a CO 2 normal, type 1 respiratory failure is present, and such a result implies lung or pulmonary -vascular disease. Type 1 and type 2 respiratory failure can occur simultaneously. Indeed, the combination is common in severe chronic obstructive pulmonary disease, for example. Given that the two conditions result from entirely different mechanisms, with implications for treatment, one should be able to distinguish between them.
When the only derangement is P a O 2 , clearly the failure is type 1. However, when the P a CO 2 is high, one has to work out whether the low P a O 2 can be accounted for by underventilation alone or whether there is an additional type 1 problem ie whether there is anything wrong with the lungs. To do this, one needs to measure the alveolar—arterial gradient, that is, the difference between the alveolar partial pressure of oxygen P A O 2 and the P a O 2.
In healthy young adults, the difference should be less than 2 kPa. If the patient is older, breathing higher concentrations of O 2 or over ventilating, then the gap can widen, although in healthy patients this would not usually be expected to be greater than 4 kPa.
If the alveolar—arterial gradient is higher than it should be, then a type 1 respiratory failure is present.
ABG interpretation is not difficult. Break down the task into steps and do them in order. For a more detailed review of arterial blood gas interpretation, see Ref 1.
Box 1 provides an example of a patient presenting with breathlessness, where ABGs form an important diagnostic test. National Center for Biotechnology Information , U.
Journal List Clin Med Lond v. Chronic: hyperventilation, latent tetany Common causes With raised anion gap: diabetic ketoacidosis, lactic acidosis, poisons e.
If the body has compensated for the disorder, the pH may be in the normal range. Acute changes in PaCO2 will alter the pH. There is a delayed response of PaCO2 to an acute change. For instance, during a breath-hold, the PaCO2 rises at a rate of only 2—3 mmHg per minute, hence patients with a very high PaCO2 usually have a long-standing disorder. Accordingly, even when treated the PaCO2 may take a long time to return to normal.
The state of arterial blood oxygenation is determined by the PaO2. This reflects gas exchange in the lungs and normally the PaO2 decreases with age. This is due to decreased elastic recoil in the lungs in the elderly, thereby yielding a greater ventilation-perfusion mismatch. Consequently, a PaO2 of 75 mmHg, which may be of concern in a young person, is usually unremarkable in an year-old.