The ins & outs of blood gas interpretation
Guest post by Dr Jono Holme, ACCS trainee
This is a tour through the real basics of blood gas analysis before quickly moving on to cover some more advanced concepts such as Anion Gap, A-a gradient and even Lactic acidosis.
ACID-BASE DISTURBANCES = H2O+CO2 ↔H2CO3 ↔HCO3–+H+
Hydrogen ions are the product of normal cellular metabolism. The body must continually dispose of acid to keep the arterial pH within a narrow range so that cellular metabolism, such as enzyme activity, is not disrupted. The pH is maintained in the body by several buffering systems, the main one being the carbonic acid-bicarbonate system above
Carbonic acid (H2CO3) links the respiratory and metabolic systems. The components can be varied independently of each other. So if the respiratory system falters, the metabolic system can attempt to compensate, and vice versa.
As Hydrogen ions are lost from the body, pH goes up. As they are maintained, pH goes down. This is the principle behind acid base balance
The respiratory system controls carbon dioxide. If breathing is rapid or deep, more carbon dioxide will be “blown off.” Carbon dioxide is acidic, so as it is removed the blood becomes less acidic and the pH increases.
Equally, if breathing is ineffective, slow, or shallow, carbon dioxide is retained and the blood becomes more acidic and pH decreases.
Respiratory changes have a rapid effect on the pH.
The kidneys are the metabolic system organ in the context of blood gas measurements, controlling the level of serum bicarbonate.
Bicarbonate is a base and an important buffer of hydrogen ions (H+) in the blood. The kidneys can regulate the amount of bicarbonate in the blood through increased production or reabsorption of these ions.
Metabolic changes are slower and may take several days to reach full effect.
Blood gas interpretation – key principles
Step 1: Is there an acid-base abnormality—that is, is the pH outside the normal range?
Step 2: Is the respiratory (PaCO2) or the metabolic (HCO3 -) component abnormal? Is it the primary cause of the acid-base abnormality?
Step 3: Is there compensation? Look at the other compensating system—for example, if it is a respiratory problem then the metabolic system should try to compensate and vice versa
Step 4: What does the PaO2 tell you? Remember that the PaO2 does not affect the acid-base balance, but it is important to review. Type I respiratory failure is when the patient has hypoxaemia in the absence of hypercapnia. Type II respiratory failure occurs when the patient has both hypoxaemia and hypercapnia, indicating hypoventilation.
A 60 year old man with a history of chronic obstructive pulmonary disease presents to the emergency department with increasing shortness of breath, pyrexia, and a cough productive of yellow-green sputum. He is unable to speak in full sentences. His wife says he has been unwell for two days.
On examination, a wheeze can be heard with crackles in the lower lobes; he has a tachycardia and a bounding pulse.
A six-year-old boy is taken to the emergency department with vomiting and a decreased level of consciousness. His breathing is rapid and deep (Kussmaul breathing), and he is lethargic and irritable in response to stimulation.
He appears to be dehydrated—his eyes are sunken and mucous membranes are dry—and he has a two-week history of polydipsia, polyuria, and weight loss.
A 75-year-old lady attends the ED 2 weeks after an NSTEMI. She is vomiting, diaphoretic and appears breathless.
She appears to be delirious, and you wonder if she may be septic given her tachypnoea and tachycardia, but she is afebrile with a normal WCC.
She can’t hear you properly because of some ringing in her ears which began recently….
A 28 year old girl attends the ED with 8 hours chest pain on a background of 48 hours dyspnoea. She is breathless on exertion only. She has no other past medical history apart from 2 prior miscarriages. She is on no other regular medications than the oral contraceptive pill. Her Oxygen saturation is 92% on air so she was started on Oxygen in triage. You do a quick, painless arterial gas (using warm local anaesthetic) to be sure . . .
A 78-year-old insulin dependent diabetic with poor glycaemic control presents to the ED with confusion in the context of a recently treated UTI. You remember the sepsis 6 pathway, and wonder if this raised lactate might be indicative of some of that sepsis stuff….
There is always more to learn about blood gases but it is important to get the basics right. Always remember to stop and look at the whole blood gas, that includes:
- sodium (yes it is an impressive lactate but the sodium is the reason why they are having seizures)
- potassium (your monitor is working, that is called a sine wave…)
- carbon monoxide (no wonder they aren’t feeling so good)
- met-haemoglobin (chocolate brown blood after a good dose of amyl nitrate?)
- haemoglobin (is that 15g/dL or g/L….)