HCO3

UNDERSTANDING AND INTERPRETING ARTERIAL BLOOD GASES

A LEARNING RESOURCE FOR INTENSIVE CARE NURSING STAFF

+ ← → ← → + Mairi Mascarenhas Clinical Educator ICU Raigmore Hospital Reviewed: February 2018

CO2 H2O H2CO3 H+

UNDERSTANDING ARTERIAL BLOOD GASES

An arterial blood gas is taken to measure the pH of arterial blood. The pH is a measurement of the acidity or alkalinity of the blood. It is inversely proportional to the number of hydrogen ions (H+) in the blood. The more H+ present, the lower the pH will be. Likewise, the fewer H+ present, the higher the pH will be.

The normal pH range is 7.35 – 7.45. In order for normal metabolism to take place, the body must maintain this narrow range at all times.

A pH < 7.35 is acidic. Changes in body system functions that occur in an acidic state include a decrease in the force of cardiac contractions, a decrease in the vascular response to cathecholamines, and a diminished response to the effects and actions of certain medications.

A pH > 7.45 is alkaline. And alkalotic state interferes with tissue oxygenation and normal neurological and muscular functioning. Significant changes in the blood pH above 7.8 or below 6.8 will interfere with cell functioning, and if uncorrected, will lead to death.

For cells to function normally, a control of the balance between acids and bases should normally exist. This is referred to as homeostasis.

An acid is a substance capable of providing hydrogen (H+). A base (alkali) is capable of accepting H+.

There needs to be a balance between the intake, production and removal of H+.

This balance is achieved by the kidneys and the lungs.

1.

The only 2 ways an acidotic state can exist is from either too much PaCO2 or too little HCO3.

The only 2 ways an alkalotic state can exist is from either too little PaCO2 or too much HCO3.

KEY CONCEPTS

How the pH is regulated:

1. The Respiratory (Lungs) Buffer Response:

A normal by-product of cellular metabolism is carbon dioxide (CO2).

CO2 is carried in the blood to the lungs, where excess CO2 combines with water (H2O) to form carbonic acid (H2CO3).

The blood pH will change according to the level of carbonic acid present.

This triggers the lungs to either increase or decrease the rate and depth of ventilation until the appropriate amount of CO2 has been re-established.

Activation of the lungs to compensate for an imbalance starts to occur within 1 to 3 minutes.

2. The Renal (Metabolic) Buffer Response:

In an effort to maintain the pH of the blood within its normal range, the kidneys excrete or retain bicarbonate (HCO3).

As the blood pH decreases, the kidneys will compensate by retaining HCO3 and as the pH rises, the kidneys excrete HCO3 through the urine.

Although the kidneys provide an excellent means of regulating acid-base balance, the system may take from hours to days to correct the imbalance.

When the respiratory and renal systems are working together, they are able to keep the blood pH balanced by maintaining 1 part acid to 20 parts base.

The balance is like a balance scale with carbon dioxide (CO2) on one side and bicarbonate (HCO3) on the other.

2.

CO2 HCO3

Respiratory acidosis:

Respiratory acidosis is defined as a pH less than 7.35 with a PaCO2 greater than 6kPa.

Acidosis is caused by an accumulation of CO2 which combines with water in the body to produce carbonic acid, thus, lowering the pH of the blood.

Any condition that results in hypoventilation can cause respiratory acidosis.

These conditions include:

Central nervous system depression related to head injury.

Central nervous system depression related to medications such as narcotics, sedatives or anaesthesia.

Impaired respiratory muscle function related to spinal cord injury, neuromuscular diseases, or neuromuscular blocking drugs.

Pulmonary disorders such as atelectasis, pneumonia, pneumothorax, pulmonary oedema, or bronchial obstruction.

Massive pulmonary embolus.

Hypoventilation due to pain, chest wall injury/deformity, or abdominal distension.

3.

Signs and Symptoms of Respiratory Acidosis

Pulmonary

Dyspnoea Respiratory distress Shallow respirations

Neurological

Headache Restlessness Confusion

Other

Dry mouth Tetanic spasms of the arms and legs.

Respiratory alkalosis:

Respiratory alkalosis is defined as a pH greater than 7.45 with a PaCO2 less than 4.6.

Any condition that causes hyperventilation can result in respiratory alkalosis.

These conditions include:

Psychological responses such as anxiety or fear.

Pain.

Increased metabolic demands such as fever, sepsis, pregnancy or thyrotoxicosis.

Central nervous system lesions.

Signs and Symptoms of Respiratory Alkalosis

Nervous

Light-headedness Numbness and tingling Confusion Inability to concentrate Blurred vision

Cardiovascular

Dysrhythmias Palpitations Diaphoresis

Other

Dry mouth Tetanic spasms of the arms and legs

4.

Metabolic acidosis:

Metabolic acidosis is defined as a bicarbonate level of less than 22mEq/L with a pH of less than 7.35.

Metabolic acidosis is caused either by a deficit of base in the bloodstream or an excess of acids, other than CO2.

Diarrhoea and intestinal fistulas may cause decreased levels of base. Causes of increased acids include:

Renal failure.

Diabetic ketoacidosis.

Anaerobic metabolism.

Starvation.

Salicylate intoxication.

Signs and symptoms of Metabolic Acidosis

Central Nervous System

Headache Confusion Restlessness Lethargy Stupor or coma

Cardiovascular

Dysrhythmias Warm/flushed skin

Pulmonary

Kussmaul respirations

Gastrointestinal

Nausea and vomiting

5.

Metabolic alkalosis:

Metabolic alkalosis is defined as a bicarbonate level greater than 26mEq/L with a pH greater than 7.45.

Either an excess of base or a loss of acid within the body can cause metabolic alkalosis.

Excess base occurs from ingestion of antacids, excess use of bicarbonate, or use of lactate in dialysis.

Loss of acids can occur secondary to protracted vomiting, gastric suction, hypochloraemia (excess of chlorides in the blood), excess administration of diuretics, or high levels of aldosterone.

Signs and Symptoms of Metabolic Alkalosis

Neurological

Dizziness Lethargy Disorientation Seizures Coma

Musculoskeletal

Weakness Muscle twitching Muscle cramps Tetany

Gastrointestinal

Nausea Vomiting

6.

Interpreting arterial blood gases: Arterial blood gases include measurements of hydrogen-ion concentration or pH, base excess, bicarbonate, partial pressure of carbon dioxide and partial pressure of oxygen. Before blood gases can be interpreted, it is necessary to familiarise with normal ABG values.

Normal blood gas range: pH range is 7.35 to 7.45 with the mean being 7.4:

A pH < 7.35 is acidic whereas a pH > 7.45 is alkalotic.

PaO2 range is 10.0 to 13.5kPa:

PaO2 measures only the partial pressure of oxygen in the plasma, but only about 3% of arterial oxygen is carried by the plasma, the majority (97%) is carried by haemoglobin.

Alveolar arterial gradients increase with age thus reducing the normal level of oxygen inolder people.

PaCO2 range is 4.5 to 6.0kPa:

The PaCO2 value indicates whether the patient can ventilate well enough to rid the body of the carbon dioxide produced as a result of metabolism.

A level > 6kPa suggests too little ventilation resulting in retention of carbon dioxide.

A level < 4.5kPa indicates too much ventilation resulting in carbon dioxide being ‘blown off’.

HCO3_ range is 22 to 26mEq/L:

The bicarbonate is reduced or increased in the plasma by renal mechanisms. A bicarbonate level < 22 defines metabolic acidosis which can result from ketoacidosis, lactic acidosis or diarrhoea. The cumulative effect is a gain of acids or a loss of base.

A bicarbonate level that is > 26 defines metabolic alkalosis which can result from fluid loss from the upper gastrointestinal tract (vomiting or nasogastric suction) diuretic therapy, severe hypokalaemia, alkali administration or steroid therapy.

Base excess and base deficit range is -2mEq/L to +2mEq/L:

A negative base level is reported as a base deficit whereas a positive base level is reported as a base excess.

7.

Steps to an Arterial Blood Gas Interpretation: The arterial blood gas is used to evaluate both acid-base balance and oxygenation, each representing separate conditions. Acid-base evaluation requires a focus on 3 of the reported components: pH, PaCO2 and HCO3. This process involves two basic steps: Step One Identify whether the pH, PaCO2 and HCO3 are abnormal. For each component, label it as ‘normal’, ‘acid’ or ‘alkaline’.

Step Two If the ABG results are abnormal, determine if the abnormality is due to the kidneys (metabolic) or the lungs (respiratory).

Now match the two abnormalities: Respiratory (lung problem) + Acidosis = Respiratory Acidosis.

8.

pH 7.30 (7.35 – 7.45) ACID

PCO2 7.3 (4.5 – 6.0) ACID

HCO3 26 (22 – 26) NORMAL

The two matching values determine what the problem is.

In this case, an ACIDOSIS

pH 7.30 (7.35 – 7.45) ACID

PCO2 7.3 (4.5 – 6.0) ACID = Lungs

HCO3 26 (22 – 26) NORMAL = Kidneys

Example one: John Doe is a 55 year old male admitted to ICU with recurring bowel obstruction. He has been experiencing intractable vomiting for the last several hours despite the use of antiemetics. His arterial blood gas is as follows: pH 7.50, PCO2 5.6, HCO3 33. Step one Identify whether the pH, PCO2 and HCO3 are abnormal. For each component label it as ‘normal’ ‘acid’ or ‘alkaline’. Step two If the ABG results are abnormal, determine if the abnormality is due to the kidneys (metabolic) or the lungs (respiratory). Now match the two abnormalities: Kidneys (metabolic) + Alkalosis = Metabolic Alkalosis

9.

pH 7.50 (7.35 – 7.45) ALKALINE

PCO2 5.6 (4.5 – 6.0) NORMAL

HCO3 33 (22 – 26) ALKALINE

The two matching values determine what the problem is.

In this case, an ALKALOSIS.

pH 7.50 (7.35 – 7.45) ALKALINE

PCO2 5.6 (4.5 – 6.0) NORMAL = Lungs

HCO3 33 (22 – 26) ALKALINE = Kidneys

Example 2: Jane Doe is a 55 year old female admitted to ICU with sepsis. Here is her arterial blood gas result: pH 7.31, PCO2 5.2, HCO3 17. Step One Identify whether the pH, PCO2 and HCO3 are abnormal. For each component label it as ‘normal’ ‘acid’ or ‘alkaline’. Step Two If the ABG results are abnormal, determine if the abnormality is due to the kidneys (metabolic) or the lungs (respiratory). Now match the two abnormalities: Kidneys (metabolic) + Acidosis = Metabolic Acidosis.

10.

pH 7.31 (7.35 – 7.45) ACIDOSIS

PCO2 5.6 (4.5 – 6.0) NORMAL

HCO3 17 (22 – 26) ACIDOSIS

The two matching values determine what the problem is.

In this case, an ACIDOSIS.

pH 7.31 (7.35 – 7.45) ACIDOSIS

PCO2 5.6 (4.5 – 6.0) NORMAL = Lungs

HCO3 17 (22 – 26) ACIDOSIS = Kidneys

Example 3: Jane Doe is a 34 year old female admitted to ICU with thyrotoxicosis. Her blood gas results are as follows: pH 7.50, PCO2 4.0, HCO3 24. Step One Identify whether the pH, PCO2 and HCO3 are abnormal. For each component label it as ‘normal’ ‘acid’ or ‘alkaline’. Step Two If the ABG results are abnormal, determine if the abnormality is due to the kidneys (metabolic) or the lungs (respiratory). Now match the two abnormalities: Respiratory (lung problem) + Alkalosis = Respiratory Alkalosis.

11.

pH 7.50 (7.35 – 7.45) ALKALOSIS

PCO2 4.0 (4.5 – 6.0) ALKALOSIS

HCO 24 (22 – 26) NORMAL

The two matching values determine what the problem is.

In this case, an ALKALOSIS

pH 7.50 (7.35 – 7.45) ALKALOSIS

PCO2 4.0 (4.5 – 6.0) ALKALOSIS = Lungs

HCO3 24 (22 – 26) Normal = Kidneys

Example 4: Jane Doe is a 19 year old admitted to ICU with a head injury. Her blood gas results are as follows: pH 7.38, PCO2 7.5, HCO3 35. Step One Identify whether the pH, PCO2 and HCO3 are abnormal. For each component label it as ‘normal’ ‘acid’ or ‘alkaline’.

Compensation:

So far the blood gases examined have not shown any evidence of compensation occurring. Let’s study what happens when an acid-base imbalance exists over a period of time.

When a patient develops an acid-base imbalance, the body attempts to compensate. Remember that the lungs and the kidneys are primary buffer response systems in the body. The body tries to overcome either a respiratory or metabolic dysfunction in an attempt to return the pH into the normal range.

Three types of compensation are possible: a patient can be uncompensated, partially compensated or fully compensated. How do we know when compensation is occurring?

Uncompensated states: in an uncompensated gas, the pH is abnormal and either the CO2 or HCO3 is abnormal. There is no indication that the opposite system has tried to correct for the other. Note: the pH remains outside the normal range.

Partially compensated states: in a partially compensated blood gas, the pH is abnormal and both the CO2 and HCO3 are also abnormal. One parameter will be deranged and following the same direction as the pH. This is the primary problem. The third parameter will be moving in the opposite direction in order to compensate for the primary disorder but it will not have changed enough to bring the pH back to the normal limits. Note: the pH remains outside the normal range.

Fully compensated states: in a fully compensated state the pH is normal but is usually ‘tending towards’ alkalosis or acidosis The two other parameters will be deranged and moving in opposite directions, one compensating for the other. The primary abnormality (metabolic or respiratory) is correlated with the abnormal pH (acidotic or alkalotic). Other clinical information will allow the clinician to interpret which parameter is compensating for which. Be aware that neither has the ability to overcompensate. Note: the pH remains within the normal range.

In the first three examples, the patients were uncompensated. In each case, the pH was outside of the normal range, the primary source of the acid-base imbalance was readily identified, but the third value (the compensatory buffering system) remained in the normal range.

Let’s return to the ABG results where there is evidence of compensation.

12.

pH 7.38 (7.35 – 7.45) NORMAL

PCO2 7.5 (4.5 – 6.0) ACIDOSIS

HCO3 35 (22 – 26) ALKALOSIS

Notice now, for the first time, that both the

PCO2 and HCO3 are abnormal.

This indicates that there is some degree of

compensation taking place. This will require a

slightly different approach to the blood gas

analysis.

Further review of ABG in example no 4: Jane Doe is a 19 year old admitted to ICU with a head injury. Her blood gas results are as follows: pH 7.38, PCO2 7.5, HCO3 35. Step One Identify whether the pH, PCO2 and HCO3 are abnormal. For each component label it as ‘normal’ ‘acid’ or ‘alkaline’.

As you may recall, in step one we determined that both the PCO2 and HCO3 were abnormal, indicating the presence of some degree of compensation.

Now we need to know 2 things. First are we dealing with an acidosis or an alkalosis? Secondly, how do you know which system (respiratory or metabolic) is the primary problem and which is compensating?

To determine this we need to go back and look as the pH in a slightly different way.

Step Two

If both the PCO2 and the HCO3 are abnormal, but the pH is in the normal range, look at the pH again.

Instead of using a ‘normal range’ of 7.35-7.45 as we have been doing, we are going to use the single value of 7.40 as our only ‘normal’.

Any pH < 7.40 is now going to be considered acidosis. Look at our pH in this example:

13.

pH 7.38 (7. 35 – 7.45) NORMAL

PCO2 7.5 (4.5 – 6.0) ACIDOSIS

HCO3 35 (22 – 26) ALKALOSIS

Notice now, for the first time, that both the

PCO2 and HCO3 are abnormal.

This indicates that there is some degree of

compensation taking place. This will require a

slightly different approach to the blood gas

analysis.

pH 7.38 (7.40) ACIDOSIS

PCO2 7.5 (4.5 – 6.0) ACIDOSIS

HCO3 35 (22 – 26) ALKALOSIS

The two matching values determine what the

problem is. In this case, an ACIDOSIS.

Step Three Now, for the two matching values, determine if the abnormality is due to the kidneys (metabolic) or the lungs (respiratory). Now match the two abnormalities: Respiratory (lung problem) + Acidosis = Respiratory Acidosis. Finally, we need to determine if the condition is partially or completely compensated.

In the above example, because the pH is 7.38 (within the range of 7.35 – 7.45) the condition is fully compensated.

14.

pH 7.38 (7.35 – 7.45) ACIDOSIS

PCO2 7.5 (4.5 – 6.0) ACIDOSIS = Lungs

HCO3 35 (22 – 26) ALKALOSIS = Kidneys

Sometimes, the system that is compensating (respiratory or metabolic) may either have not had

sufficient time to correct the situation, or is unable to completely compensate for the degree of

abnormality present.

If the pH is between 7.35 and 7.45, the condition is fully compensated.

If the pH is outside the range of 7.35 – 7.45, the condition is only partially compensated.

Remember, neither buffer has the ability to overcompensate!

KEY CONCEPTS

‘OTHER’ BLOOD GAS VALUES

Hct ( haematocrit) ~ this shows us how much red blood cells there are in a sample of blood – i.e. how watered down (or not) blood is. Normal range: 36 – 44%.

COHb (carboxyhaemoglobin) ~ affinity of Hb for carbon monoxide is 200 times greater than that of oxygen and impairs oxygen transport and release. This level can be high in heavy smokers. Carbon monoxide poisoning reduces the oxygen-carrying capacity of blood. Oxygen saturation is measured by using pulse oximetry or calculated by using the arterial oxygen partial pressure (PaO2) as measured with a standard arterial blood gas analyser. Oxygen saturation however does not take into account the presence of carboxyhaemoglobin. Gem4000 arterial range 0.5 -1.5 % O2Hb (oxyhaemoglobin fraction) ~ this measures just how much blood (Hb) is carrying oxygen compared to the total amount of Hb capable of carrying. Normal range: 94-98%

Lactate (lactic acid) ~ when cells no longer have enough O2 for ‘normal’ aerobic metabolism (cell hypoxia). Anaerobic metabolism takes over resulting in lactate production, leading to lactic acidosis. Gem400 arterial range: 0.0 – 1.3 mmol/L. MetHb ~ methaemoglobin is a compound formed from haemoglobin by oxidation of the iron atom from the ferrous to the ferric state. A small amount of methaemoglobin is present in the blood normally, but injury or toxic agents convert a larger proportion of haemoglobin into methaemoglobin, which does not function as an oxygen carrier. MetHb can arise as a result of hereditary RBC enzyme deficiency or exposure to certain drugs and chemicals can dangerously elevate levels. Gem4000 arterial range: 0 – 3%. sO2 (oxygen saturation) ~ measures the percent of hemoglobin which is fully combined with oxygen. HHb ~ deoxyhaemoglobin hemoglobin not combined with oxygen, formed when oxyhemoglobin releases its oxygen to the tissues. Gem4000 arterial range: 0.0 – 5.0% What other types of blood gas sampling are available? There are basically three main types of blood gas that may be taken:

1. Arterial.

2. Venous: a venous blood gas works in much the same way as an arterial gas – in that most of the values will be just as true in the veins as they are in the arteries. The pH and lactate should normally be the same. It is the gases ~ oxygen and carbon dioxide which will have swapped over since being in the arteries, so you need to remember that the oxygen will be at a lower value, and carbon dioxide will probably be higher than what is normal in arterial blood. Because bicarbonate is often calculated using the carbon dioxide concentration, this may also not be an accurate reflection of arterial blood.

3. Capillary: commonly used in children. A capillary tube is used to collect a capillary blood gas. The skin is

pricked and the blood is gathered one drop at a time.

15.

Lactate Normal value: 0.63 - 2.44mmol/l

Lactate acid is an intermediate product of carbohydrate metabolism and is derived from muscle cells and red blood cells.

During exercise, lactate levels may increase.

However, the liver can normally metabolise more lactate than is produced and can return lactate levels to normal within a few hours.

Elevated levels of lactate tend to lower pH with consequent disturbance of metabolism/protein structure, and beyond a tolerance level results in muscle fatigue or ‘cramp’.

Why should it be considered?

Hyperlactaemia is typically present in patients with severe sepsis or septic shock and may be secondary to anaerobic metabolism due to hypoperfusion.

The prognostic value of raised blood lactate levels has been well established in septic shock patients, particularly if the high levels persist.

Therefore obtaining serum lactate may be considered essential to identifying tissue hypo-perfusion in patients who are not yet hypotensive but who are at risk for septic shock.

It is important to recognise that severe oxygen deprivation of tissue results in a switch from aerobic to anaerobic metabolism.

Because lactate is the main product of anaerobic metabolism, it accumulates when there is oxygen deprivation.

Its concentration increases when its production by ischaemic tissue overwhelms its elimination by the liver and kidneys.

Hypoxia, seen in shock, congestive heart failure, (or any other condition that would cause problems in oxygen being picked up or transported in the blood), hepatic dysfunction, ischaemia and pulmonary insufficiency are all associated with increased serum lactate.

Resuscitation of hypo-perfused patients should be considered complete only when there is no evidence of ongoing anaerobic metabolism or tissue acidosis.

16.

Serum lactate is considered a sensitive indicator of occult shock and may be useful in patients with a significant mechanism of injury yet demonstrating vital signs within normal limits.