In this video we look at a systematic approach to interpreting blood gases, particularly arterial blood gases, but we will also look at venous blood as well. Blood gases give us information on the gas exchange status, the acid base status of the patient, and can give a preliminary indication of other values as well, so they are a very useful test. in the acute setting. As with most investigations, the first step is to confirm that your ABG is that of the correct patient. This includes the date and the time and any previous blood gases to compare with.
You should also consider the ABG in the context of the clinical state of the patient. Linked to this, the gas should also indicate how much oxygen the patient was using when the sample was taken. which varies between different flow rates and different oxygen delivery devices. I then look at the partial pressures of oxygen and carbon dioxide.
This is effectively a measure of the amount of oxygen and carbon dioxide dissolved in the blood. Normal PO2 is above 10 kilopascals on room air, and if on oxygen should be 10 less than the fraction of inspired oxygen. For example, 2 litres of oxygen through a nasal cannula delivers around 28% oxygen, so the PO2 should be above 18 kilopascals. This is why the flow and oxygen delivery device needs to be recorded.
If the PO2 is below 8 kilopascals, this is considered respiratory failure, of which there are two main types. Type 1, where oxygen levels are low, but carbon dioxide levels are not elevated, or type 2 respiratory failure, where carbon dioxide levels are high, called hypercapnia. The normal range is roughly 4.5 to 6 kilopascals.
An easy way to remember this is type 2 has two gases affected. Type 1 is mostly caused by poor oxygenation of the blood, most commonly due to ventilation perfusion mismatch. for example pulmonary edema where ventilation is reduced and perfusion is normal or pulmonary embolism where ventilation is normal but perfusion is reduced. Other causes include high altitude, shunting and diffusion problems where oxygen cannot diffuse into the blood properly. Type 2 respiratory failure is present when the pCO2 is above 6 kilopascals.
which occurs as a result of reduced ventilation and therefore less exhalation of carbon dioxide from the lungs. We'll come back to this as it is closely linked to the acid base portion. I include hemoglobin and other variants of it such as carboxyhemoglobin and methoxyhemoglobin here as well because these are easy to overlook on an ABG when they actually give some information on the oxygen carrying capacity of the blood. The haemoglobin level may not be particularly accurate, but it can help give a rough idea of the presence or absence of a severe anemia.
Carboxyhaemoglobin is when carbon monoxide binds to haemoglobin instead of oxygen, and it does this 200 times more strongly than oxygen does, meaning less space is available for haemoglobin to transport oxygen. Normal values are typically less than 3% in non-smokers, and can be as high as 15% in smokers, although symptoms such as headache typically appear at around 10%. Next is the pH, which should normally be between 7.35 and 7.45. This is based on the concentration of hydrogen ions in a solution.
With higher concentrations of hydrogen ions, giving a lower pH indicating a more acidic environment and vice versa. pH values below 7.35 indicate acidemia while values above 7.45 indicate alkalemia. In general, abnormalities in the pH are divided into acidosis or alkalosis and respiratory or metabolic. On the ABG, There will be the partial pressure of CO2, which reflects the respiratory component, and the concentration of bicarbonate, which reflects the metabolic component.
with normal values approximately being 22 to 28 millimoles per litre. An important point to remember is that carbon dioxide forms carbonic acid in the blood, and so more carbon dioxide means more acid. Bicarbonate is an ion in the blood that mops up hydrogen ions, so more bicarbonate means less hydrogen ions and so less acidity.
Based on the pH, the partial pressure of CO2, and the bicarbonate level, we can determine whether a primary respiratory or metabolic cause is suspected. Respiratory acidosis is caused by reduction in ventilation of the alveoli, leading to retention of carbon dioxide. Metabolic acidosis is caused by either acid ingestion, increased acid production, or a loss of bicarbonate. or reduced excretion of acids.
An important factor in metabolic acidosis is the anion gap, which is the difference between the amount of positive and negative ions in the blood, given by sodium, minus chloride and bicarbonate. The normal anion gap is approximately 4 to 12 mmol per litre. This is mostly caused by albumin, which is not measured on the ABG. If the value is higher than expected, it suggests that another unmeasured substance is causing the acidosis. For example, in diabetic ketoacidosis, the unmeasured substance not on the ABG are the ketoacids.
Generally, the causes can be divided into lactate, toxins, ketones or renal, and the mnemonic mud piles can be used to remember some of the more specific causes. Causes of normal anion gap metabolic acidosis are generally gastrointestinal or renal losses of bicarbonate and renal tubular changes. Specific causes can be remembered with the mnemonic hard up. Respiratory alkalosis is caused by excess carbon dioxide being exhaled due to hyperventilation while metabolic alkalosis is caused by an overabundance of bicarbonate.
either through excess loss of acid or excess presence of bicarbonate. Remember that there are mechanisms in the body that work to keep the pH within the normal range. This is known as compensation. For example, if carbon dioxide is causing a change in the pH, the kidneys can adjust the amount of bicarbonate in the blood to counteract this, and it also works vice versa. If changes in bicarbonate are causing the change in the pH, the body can adjust ventilation to increase or decrease the carbon dioxide to compensate.
Generally, compensation through ventilation will happen faster than changes in bicarbonate that may take several days. We touched on lactate within acidosis, but lactate itself is an extremely useful marker as it is a by-product of anaerobic metabolism. and some is produced normally in the body and excreted mostly through the liver and kidney.
However, lactate can act as a marker for underlying disease, typically as a result of inadequate oxygen delivery to tissues, meaning they need to use more anaerobic metabolism than normal. Examples include muscular activity, for example in seizures where there is anaerobic muscular activity, hypoperfusion of tissues, especially in sepsis, or not enough oxygen in the blood that is being delivered to the tissues, for example hypoxemia or anemia. Some other causes for increased lactate that do not involve tissue delivery of oxygen include underlying diseases like hepatic or renal failure, diabetes, lymphoma or leukemia, or even drugs and toxins, for example salbutamol.
Often overlooked on ABGs are electrolytes. Typically, levels of sodium and potassium are provided, and so these can be an indication of if there are significant electrolyte abnormalities quickly. Some machines also give other values, such as creatinine or calcium. Blood glucose is also on the ABG.
That can again help to give more of an indication of an underlying problem. for example in patients with low conscious levels or who present with seizures. Here are some additional tips when it comes to blood gases. Venous blood gases are typically significantly easier to obtain. as they can be done during regular blood taking and are generally less painful.
So it's useful to know that the pH, lactate and bicarbonate on venous blood gas are accurate and very close to the arterial blood gas values. The partial pressure of oxygen will not correlate between the two, but partial pressures of carbon dioxide levels do to some degree. Evidence suggests that in cases of COPD, on non-invasive ventilation, the difference may not be clinically significant, which may save the need of repeated ABGs.
It's important that when taking a blood gas that all of the air is removed from the syringe when the blood is drawn, as if air is left in the syringe with blood, the gases will diffuse between each and invalidate the results. A normal partial pressure of carbon dioxide in a patient suffering an asthma attack. is a worrying sign as you would expect hyperventilation and therefore reduced carbon dioxide levels. A normal or high partial pressure of CO2 can indicate that they are tiring. Ventilation is decreasing and they are likely heading towards respiratory arrest.