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Maintaining the pH of blood is essential for normal bodily function. However, numerous clinical scenarios can result in disruption of the body’s acid-base balance. Monitoring of acid-base balance is done by testing patients’ arterial blood gases [ABGs]. The results of ABG testing will often influence the treatment that patients receive. Therefore, a basic understanding of how to interpret ABG results can be useful for pharmacists to help them clarify the clinical picture.
The basics of acid-base balance
The optimal physiological pH of extracellular fluid is 7.35–7.45. A pH outside this range can cause protein denaturation and enzyme inactivation [1]. Because pH is a logarithmic scale, a small change in pH reflects a large change in hydrogen ion [H+] concentration [1]. The following equilibrium equation is crucial to understanding acid-base balance:
H2O + CO2↔H2CO3↔HCO3–+H+
This equation shows that carbon dioxide [CO2] in blood dissolves to form carbonic acid [H2CO3], which dissociates to form acidic H+ [which can then combine with physiological bicarbonate to push the equation back to the left]. Blood pH depends on the balance of CO2 and HCO3 — a change in the amount of CO2 will not lead to a change in pH if it is accompanied by a change in the amount of HCO3 – that preserves the balance [and vice versa] [2]. It is the renal and respiratory systems that are responsible for maintaining the pH of the blood.
Respiratory mechanisms
One way that the body controls the pH of extracellular fluid is by increasing or decreasing the rate and depth of respiration and thereby the amount of CO2 expelled [ie, slow, shallow breathing retains more CO2 than fast, deep breathing].
Renal [metabolic] mechanisms
Another way that the body can control pH is via the kidneys, which occurs by either:
- Excretion of H+
- Renal tubular reabsorption of HCO3–
The kidneys can adjust the amount of H+
and HCO3– that is excreted in the
urine in response to metabolic acid production.
Compensation
When acidosis or alkalosis occurs [either through respiratory or renal mechanisms], the opposite system will attempt to rectify this imbalance; this is termed “compensation”. For example, if the kidneys fail to excrete metabolic acids, ventilation is adjusted in order to eliminate more CO2[2].
It is important to note that compensatory changes in respiration can
occur over minutes to hours, whereas metabolic responses take hours or days to develop[3].
Buffers
The body has three main buffers that
minimise any changes in pH that occur when acids or bases are added, namely haemoglobin, HCO3– and proteins. Haemoglobin is six times more powerful as a buffer than proteins[1]. However, HCO3– is the most important buffer in the blood and is the dominant buffer in the interstitial fluid. The intracellular fluid uses proteins and phosphate to buffer pH[3]. At an intracellular
level buffering occurs instantly, but the effect is small.
Arterial blood gas sampling
Monitoring ABGs can be useful to:
- Assess the effectiveness of pulmonary gas exchange;
- Identify the presence of metabolic acidosis and alkalosis;
- Identify critically unwell patients requiring urgent intervention;
- Guide treatment and monitor response.
Some causes of acid-base disturbances can be found in Box 1.
Box 1: Some causes of acid-base disturbance[3,4]
The following are the commonly reported parameters of ABG results [see
Box 2 for the normal reference ranges]:
- pH — to determine whether a patient’s blood pH is within physiological range;
- PaCO2 and PaO2 — the partial pressures of CO2 and oxygen in arterial blood, respectively HCO3– — indicates how much HCO3– is in the blood [and is therefore available as a buffer];
- Base excess [or deficit] — a measure of the excess or deficiency of base in the blood; by definition, it is the amount of base [in mmol] that would correct one litre of blood to a normal pH [if an excess, this is the amount of base needed to be removed for a normal pH, or if a deficit, the amount required to be added];
- Lactate — the end product of anaerobic glycosis [a rise indicates poor oxygenation and perfusion of tissues].
Other parameters commonly found on ABG reports are: haemoglobin, glucose and electrolytes [sodium, potassium, chloride and ionised calcium].
Interpreting the results
ABGs can be interpreted using a stepped approach:
Step 1 — check the pH
The pH should be assessed first. A pH of less than 7.35 indicates acidosis and a pH greater than 7.45 indicates alkalosis.
Step 2 — check the HCO3– and PaCO2
Having determined if the patient is acidotic or alkalotic, check the HCO3– and the PaCO2 to classify the results as follows:
- Metabolic acidosis: patients who are acidotic and have a HCO3– 28 [base excess >+2];
- Respiratory alkalosis: patients who are alkalotic with a PaCO2