What are blood gases?
A blood gas (arterial, venous or capillary) is a powerful test that allow clinicians to obtain a lot of information in a short period of time. It requires very little blood and can be run on a point-of-care device. The results are available within a few minutes and may guide the initial management of critically ill infants and young people.
Blood gases can be obtained from a central or peripheral vein (venous blood gas or VBG) or from an artery (arterial blood gas or ABG). Obtaining ABGs tends to be painful but often provides a more accurate indication of oxygenation when compared to VBGs.
Interpretation can be complex and nuanced. There are many approaches, formulas, and equations that you can master but in this post we are providing a very basic approach to interpretation of VBGs with some clinical scenarios to get you started.
Keep in mind:
- Be cautious not to interpret any lab value in isolation.
- Always consider the clinical context when interpreting results.
Components of a Blood Gas
|pH||7.35-7.45||Determine acidosis vs alkalosis|
|pCO2 Partial pressure of dissolved carbon dioxide||35–45 mmHg||Respiratory component|
|pO2 Partial pressure of dissolved oxygen||varies||Not particularly useful in VBG.|
|HCO3 Bicarbonate||22-26 mEq/L||Metabolic component|
|Base Excess/Deficit||-2 to +2||Amount of acid/alkali needed to to get pH back to normal|
If negative=deficit If positive=excess
Some blood gases also include information such as sodium, potassium, calcium, haemoglobin, haematocrit, glucose, and lactate.
Ultimately, everything comes back to pH and the concept of acids and bases. Physiologically, the body wants to maintain optimum pH for cellular functions and has two main buffering systems to do so:
- the lungs
- the kidneys
Lungs help regulate CO2 and kidneys help regulate HCO3.
Step 1: Determine acidosis or alkalosis
This is most easily accomplished by looking at the pH.
- If pH is <7.35, the patient is acidotic.
- If pH is > 7.45, the patient is alkalotic.
*Caution*: it is possible to have a normal pH with two disturbances that balance each other out.
Step 2: Determine if the acidosis or alkalosis is respiratory, metabolic, or both.
Examine the pCO2. In a hyperventilating patient the pCO2 should be less than 35 mmHg (4.6 KPa) because they are blowing off carbon dioxide, leading to a respiratory alkalosis. Whereas in a patient who is not ventilating well the pCO2 may be greater than 45mmHg (6 KPa) because they are retaining carbon dioxide.
Examine the HCO3 or bicarbonate. If the patient has a normal PCO2 an elevated bicarbonate signals metabolic alkalosis while a lower bicarbonate signals metabolic acidosis.
If there is a metabolic acidosis, calculate the anion gap [Na+ – (Cl– + HCO3)]. Use your favourite mnemonic to remember the causes.
Step 3: Is there compensation?
When there is imbalance in acids and bases, the body attempts to correct the imbalance.
- If there is a metabolic acidosis (less HCO3), respiratory rate might increase to help decrease pCO2. Usually the quickest compensatory mechanism is respiratory.
- If there is a respiratory acidosis (increased pCO2), the kidneys may be triggered to hold onto more HCO3. This compensatory mechanism tends to be slower.
|Disorder||pH||Primary Problem||Compensatory Mechanism|
|Respiratory*||↓ pH||↑ pCO2||↑ HCO3|
|Metabolic*||↓ pH||↓ HCO3||↓ pCO2|
|Respiratory**||↑ pH||↓ pCO2||↓ HCO3|
|Metabolic**||↑ pH||↑ HCO3||↑ pCO2|
Let’s take a look at some simple clinical scenarios to practice blood gas interpretation in the importance of interpreting them in a clinical context. We have purposely simplified these to focus only on the pertinent components.
Joanie is a 6 year-old girl presenting with abdominal pain, vomiting, and lethargy. Her parents are concerned that although she has been eating and drinking a lot, she has lost weight in the past few weeks and seems to be urinating frequently. On the physical exam, she is tired-appearing. Her breathing is deep and rapid. She is also tachycardic. Her lungs are clear to auscultation bilaterally. You get a venous blood gas:
This is a metabolic acidosis with respiratory compensation. This patient is also hyperglycaemic.
Joanie has a new diagnosis of diabetic ketoacidosis. She is acidotic with a bicarbonate level <15 demonstrating a metabolic acidosis. Her body is trying to compensate for the acidosis by Kussmaul breathing and eliminating CO2 which is why her PCO2 is low.
Peter is a 5 year-old boy with asthma presenting with an acute asthma exacerbation in the setting of a respiratory virus for the past two days. On initial presentation, he is breathing rapidly and wheezing in all lung fields. You start treating him with a nebulized albuterol and IV corticosteroids. When you next check on him, he seems more sleepy but is breathing less rapidly and you can no longer hear any wheezing on his lung exam. A VBG is obtained:
Although these lab values roughly lie within the normal ranges, this blood gas should be worrisome in this clinical context!
In a patient with asthma and respiratory distress who was initially breathing rapidly, we would expect the pCO2 to be low because he was hyperventilating. A normalising pCO2 on a patient with a severe asthma exacerbation should make you concerned that he is tiring out and retaining CO2. A silent lung exam should also bring up red flags that he is no longer moving air in his lungs. You need air movement to hear wheezing!
Diego is a 2 year-old boy presenting with fever, cough, and lethargy. He has had rhinorrhea and cough for the past week. In the past 3 days, he has had very poor appetite and fevers with temperature of up to 39.8°C. He appears very lethargic and is tachycardic with poor perfusion. A VBG is obtained:
This is a metabolic acidosis with some degree of respiratory compensation. However, the lactate is high, suggesting a degree of poor perfusion.
Diego is likely in septic shock given his fever, tachycardia, poor perfusion, and altered mentation with the elevated lactate and acidosis. His body is likely trying to compensate by reducing the acidosis through some degree of hyperventilation.
Keep in mind that sepsis is a clinical diagnosis. The blood gas may help, but lab work should not be necessary for diagnosis.
Have a high suspicion for sepsis and treat it early with fluids and antibiotics!
Paige is a 16 year-old girl with muscular dystrophy. She uses a wheelchair to move and depends on bilevel airway pressure (BiPAP) at night when she is sleeping. In the past two days, she has developed a worsening cough and congestion. A chest radiograph is obtained that does not show any consolidations. On examination, she is not in any respiratory distress. A VBG is obtained:
There is a respiratory acidosis with metabolic alkalosis.
Patients with a history of neuromuscular weakness and inefficient ventilation may exhibit a degree of baseline respiratory acidosis from CO2 retention.
Given the chronic nature of Paige’s respiratory acidosis, her body has compensated by retaining more bicarbonate. These blood gas values are not alarming in this clinical context.
Oliver is a 2 year old boy presenting with a first-time seizure. He has been seizing for at least 15 minutes. After he received multiple doses of benzodiazepines and a loading dose of levetiracetam, his seizure activity stopped. Unfortunately, he also stopped breathing and was promptly intubated and placed on a ventilator. A VBG obtained shows:
There is a respiratory acidosis due to prolonged hypoventilation from seizure activity. The glucose is elevated but can be expected in the setting of a seizure.
Patients presenting after prolonged seizure activity will likely exhibit respiratory acidosis and elevated pCO2. There is no time for the kidneys to compensate in this acute setting so bicarbonate should be normal. Many patients may spontaneously recover after a post-ictal period.
Oliver was in status epilepticus and required intubation. We can help lower his pCO2 by increasing the respiratory rate or tidal volume on the ventilator.
Pay attention to the electrolytes on the blood gas in seizing patients as hypoglycaemia and hyponatraemia may be the cause of the seizure and should be corrected.