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OPENING LEARNING ZONE Learnirw mKEYWORDS Arterial blood gases / Acid-base balance / Acidosis / Alkalosis / Compensation Zone

:

Provenance and Peer review: Unsolicited contribution; Peer reviewed; Accepted for publication September 2014.

Four steps to interpreting arterial blood gasesby KMA Rogers and K McCutcheonCorrespondence address: Karen McCutcheon, Lecturer, Queen's University Belfast, School of Nursing and Midwifery, Medical Biology Centre, 97 Lisburn Road, Belfast, BT9 7BL.

Email: [email protected]

This article examines acid-base balance and the interpretation of arterial blood gases (ABG). The article begins with a brief revision of related physiology, followed by a description of the primary disorders associated with acid-base imbalance. The normal ranges and the significance of abnormal ABG results are explored. The article concludes by providing an easy to follow four-step guide to ABG interpretation with practice examples presented in the CPD task section.

IntroductionMonitoring and detecting changes in acutely ill patients is one of the main responsibilities for perioperative practitioners. A recent review of NICE guidance on the acutely ill patient in hospital (CG50) stated that:‘there appears to have been a variation in uptake of the recommendations in various settings, with some recommendations being implemented more thoroughly than others' (NICE 2010). It is essential that all acute areas audit their practice in respect of this guidance and address any shortfalls. Failure to meet these recommendations can result in mismanagement of patient care and ultimately patient death.

One aspect of the NICE guidance includes the interpretation of ABGs. Although most perioperative practitioners are familiar with ABG results they are not necessarily competent in interpreting them. Coogan (2008) suggests that, following the implementation of NICE guidance (NICE 2007), it is vital that practitioners working in acute areas can interpret ABGs to ensure that patients are treated effectively and efficiently. This article provides practitioners with information on the physiological aspects of acid-base balance followed by a simplified four step approach to interpreting ABGs. This knowledge makes it possible for practitioners to assess their patients’ condition and provide appropriate care.

Increasingly acidic Increasingly alkalineA w-

0 1 2 3 4 5 6 7-0 8 9 10 11 12 13 14

Acidic Neutral AlkalinFigure 1 The pH scale

What is pH?pH is a measure of how acidic or alkaline (basic) a solution is. The pH scale ranges from 1 to 14. Pure water has a pH of 7.0, this is called neutral pH. Values less than7.0 are acidic; the lower the number the more acidic the solution. Values greater than7.0 are alkaline; the higher the number the more alkaline it is (Figure 1) (Rogers & Scott 2011a).

Blood pH is often measured in clinical practice, as it is difficult to measure intracellular pH. The pH of the blood should be maintained within the range of 7.35-7.45. Below this range the patient is described as being acidotic whilst above this range the patient’s condition is described as alkalotic. Maintaining the homeostasis of blood pH primarily requires coordination between the respiratory and renal systems. However, the body also operates a number of buffer systems, which act by resisting changes to blood pH. A pH of less than 6.8 or greater than 7.8 is incompatible with life.

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- ii* I.-A__ J.. 1| pfcl ■■ ■ H jReview NICE guidance Acutely ill patients in hospital: Recognition of and response to acute illness in adults in hospital (NICE 2007) www.nice.org.uk/ nicemedia/pdf/CG50FullGuidance.pdf.

Reflect on your personal skills and abilities in response to this document and consider any further training support you may reguire. Bring any areas for development to the attention of your line manager.

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46 March 2015 / Volume 25 / Issue 3 / ISBN 1750-4589

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The normal structure and function o f the human body requires a relatively stable environment in terms o f temperature, electrolyte concentration and pH

Cells are continuously metabolising and these metabolic processes require certain proteins called enzymes. Enzymes work best within a specific pH range, so for the enzymes to work effectively in metabolism the body must maintain the pH within the normal homeostatic range.

Intracellular and extracellular fluids must maintain a normal pH between 7.35 and 7.45; any deviation beyond this can disrupt metabolism. However, metabolism itself produces many acidic by-products which can alter the pH; therefore it is essential that there are mechanisms in place to resist changes in body fluid pH. The more hydrogen ions (H+) there are in a solution the more acidic it is and the less there are the more alkaline it is.

How does the blood become acidic?Acids and alkalis (or bases) are by-products of the body's natural metabolism and the regulation in the body is controlled through a number of mechanisms:

• Acids are ingested as part of the diet

• Digestion of dietary fats in the gastrointestinal system produces fatty acids

• Muscle activity produces lactic acid as a waste product

• The kidneys respond to changes in blood pH by regulating the excretion of acid or alkali ions into the urine

• Respiring cells produce carbon dioxide as waste which is removed from cells by capillary exchange of tissue fluid and blood plasma. When carbon dioxide combines with water it produces a weak acid called carbonic acid (H2C03); this acid has an important impact on blood pH levels. The circulating red blood cells contain an enzyme called carbonic anhydrase which quickly converts the carbonic acid into hydrogen ions (H+) and bicarbonate ions (HC03). Over 70% of blood carbon dioxide travels in the plasma as bicarbonate ions.

The normal structure and function of the human body requires a relatively stable environment in terms of temperature, electrolyte concentration and pH. When these factors are maintained at their optimum levels the body is said to be maintaining

Review anatomy and physiology of the renal and respiratory system.

Reflect on how this revision will improve your practice and care.


homeostasis. This is essential for the normal functioning of the body’s cells (Rogers & Scott 2011a).

The respiratory and renal systems, together with buffer mechanisms, play a vital role in maintaining homeostasis of the body’s acid- base balance through negative feedback mechanisms. The treatment of acid-base imbalances involves the stabilisation of hydrogen ion levels in the blood. Ultimately, these mechanisms do this by removing hydrogen ions from the body through exhalation or excretion in the urine.

The role of the respiratory system in maintaining pH homeostasisChemoreceptors are chemical-sensing cells which regulate the cardiovascular and respiratory systems by monitoring three important chemical characteristics of the blood: carbon dioxide (C02), oxygen (02) levels and acid concentration. When chemoreceptors detect a high concentration of carbon dioxide, they signal to the control centre in the brain to increase the heart rate and breathing rate to excrete the excess carbon dioxide from the blood. Conversely, when chemoreceptors detect low levels of oxygen in the blood they trigger an increase in respiration rate to boost blood oxygen concentration. If chemoreceptors detect a low blood pH the rate of respiration will increase. This simultaneously lowers blood carbon dioxide concentration, through the increased rate of exhalation, and increases blood oxygen levels due to increased inhalation. Consequently the blood pH will increase towards the normal homeostatic range which helps to regulate the body’s acid- base balance.

The pH of the blood (and cerebrospinal fluid (CSF)) are constantly being monitored by a homeostatic control mechanism (see Figure 2) to detect if the pH deviates beyond the normal range.

chemoreceptors Respiratory Lungs breathedetect blood pH centre increases quicker and

<7.35 rate and depth deeper, expellingof respiration excess C02

Recentors = Control centre = Effector = Homeostasischemoreceptors

in cartoid and aortic bodies

respiratory centre located in the

medulla oblongata of the brain stem

► Lungs --------► restored = blood pH returns to normal range

7.35 - 7.45

\ chemoreceptors detect blood pH

>7.35(perhaps due to

hyperventilation or artificial ventilation

Respiratory centre decreases

rate and depth of respiration

Breathing is relaxed, lungs

breathe normally, retaining CO2

(or rebreathing exhaled C02)

Figure 2 Respiratory regulation of blood pH - the homeostatic feedback mechanism

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Four steps to interpreting arterial blood gasesContinued

Alkalosis Acidosis

Figure 3 Acid-base imbalance due to respiratory acidosis

Alkalosis Acidosis

Figure 4 Renal compensation to correct the acid-base imbalance due to respiratory acidosis

The role of the kidneys in maintaining pH homeostasisThe kidneys can increase the pH of body fluids by excreting more acidic urine and can also decrease the pH of body fluid by excreting additional, alkaline urine.They do this by regulating the amount of two specific ions in the blood plasma: the alkaline bicarbonate ion and the acidic hydrogen ion. Bicarbonate ions are continuously filtered into the renal tubules from the blood plasma and will be reabsorbed back into the blood only as required.

Hydrogen ions are produced within the tubular cells from the combination of water and carbon dioxide and are secreted into the tubular filtrate for excretion in the urine or reabsorption into the blood. Under certain conditions, the body’s pH may become more acidic than normal. To correct this, the kidneys will secrete more hydrogen ions into the tubular filtrate and more bicarbonate ions will be synthesised from the filtrate. If the body fluids become too alkaline, more bicarbonate ions will be secreted into the renal filtrate and the amount reabsorbed back into the blood plasma will be reduced.

Buffer systemsA buffer system can be activated within seconds and is therefore considered the first line of defence in counteracting fluctuations in blood pH levels. One of the major buffer systems is the sodium bicarbonate-carbonic acid mechanism. When a buffering reaction occurs, the concentration of one member of the buffer pair, either the sodium bicarbonate or the carbonic acid, increases while the other decreases.

Acid-base imbalancesAcid-base imbalances (disorders) are characterised by changes in:

• partial pressure of carbon dioxide in the blood (the normal range for PaC02 is 4.7- 6.0 kPa)

• serum bicarbonate (HC03) levels (the acceptable range is 22-26 mmol/L)

• serum pH (normal blood pH is between 7.35-7.45).

The actual detected changes in the pH of the blood will depend on the degree of compensation present and whether multiple processes are involved.

Acidosis refers to a process of physiology that causes an abnormal accumulation of acid or a loss of alkali from the blood.

Alkalosis refers to a process of physiology that causes an abnormal alkali accumulation or acid loss.

In either situation the serum pH may or may not be abnormal due to the influences of physiological compensation mechanisms that may be involved in trying to counteract the abnormal change.

Acidemia describes a serum pH that is abnormally acidic (i.e. when the pH is less than 7.35).

Alkalemia is defined as a serum pH greater than 7.45.

The serum pH measurement identifies the primary process as either acidosis or alkalosis but, when considered on its own, an abnormal blood pH value does not identify whether the problem is respiratory or metabolic. The acid- base imbalance may then be classified as:

• Respiratory - when the pH change is principally due to a change in PaC02 (increase or decrease in ventilation); or


An increased level o f carbon dioxide in the blood is toxic to the body

• Metabolic - when the pH change is mainly attributed to an alteration in serum bicarbonate levels.

Signs and symptoms of acid-base imbalances

More than one primary acid-base imbalance may be present simultaneously. It is important to identify and treat each primary acid-base imbalance. Compensated or mild acid-base imbalances cause few signs or symptoms. Severe, uncompensated disorders have multiple cardiovascular, respiratory, neurologic, and metabolic consequences (See Figure 5 for possible causes of acid-base imbalances).

Respiratory acidosisRespiratory acidosis (RA) is an acid imbalance caused by breathing-related problems. When the breathing process in the lungs is impaired, the lungs are unable to excrete carbon dioxide effectively and the excess carbon dioxide forms an acid in the blood (Figure 3).

An increased level of carbon dioxide in the blood is toxic to the body. If carbon dioxide accumulates it will react with water in the body to produce carbonic acid which ionises to hydrogen ions and bicarbonate ions. Accumulation of hydrogen ions causes a low blood pH, this can result in respiratory acidosis (Rogers & Scott 2011b).

This situation can be compensated by the kidneys increasing production of bicarbonate ions which are alkaline and therefore act as a buffer to neutralise the acidic effects of the excess carbon dioxide (Figure 4).

Causes of respiratory acidosis

Respiratory acidosis can develop acutely (acute RA) or its onset may be gradual as lung function deteriorates (chronic RA). Therefore, a number of acute and chronic underlying respiratory conditions can cause RA (see Figure 5). These include:

• Chronic bronchitis (chronic RA)

• Asthma (chronic RA)

• Severe pneumonia (acute RA)

• Airway obstruction (acute RA)

• Airway swelling

• Foreign body airway obstruction (food, vomit).

Alternatively, RA may be caused by other non- respiratory conditions. These can include:

• Head injuries/brain tumours which may interfere with nerve signals sent from the respiratory centre in the brain to the lungs

• Neuromuscular diseases such as myasthenia gravis, muscular dystrophy or Guillain-Barre syndrome

• Conditions causing chronic metabolic alkalosis

• Drugs (acute RA)

• Anaesthetics, sedatives and narcotics

• Cardiac/respiratory arrest.

Respiratory acidosis displays a range of symptoms:

• Slow or difficult breathing

• Headache

• Drowsiness

• Restlessness

• Tremor

• Confusion

• Tachycardia

• Changes in blood pressure

• Swelling of the blood vessels in the eyes

• Cyanosis

If the underlying condition causing the respiratory acidosis is identified, treated and corrected, there may be no longterm effects. However if the underlying condition is due to lung disease or respiratoryfailure, the patient may require the assistance of a respirator or ventilator. In extreme cases, or if the condition is left untreated, the patient could experience coma or even death. Patients who are at risk of developing respiratory acidosis, such as those with chronic lung disease and those who receive sedatives and narcotics, should be closely monitored for the onset of acidosis.

Respiratory alkalosisA decrease in blood carbon dioxide concentration occurs due to excessive exhalation of carbon dioxide gas. This causes a decrease in hydrogen ions and leads to a rise in the pH of blood, resulting in respiratory alkalosis. Treatment aims to elevate C02 concentration (making blood pH more acidic). Although the incidence of respiratory alkalosis

is rare, a number of conditions can trigger a decrease in blood C02 levels (see Figure 5). These include:

• Pulmonary disease

• Stroke

• Severe anxiety (leading to hyperventilation)

• High altitudes.

Symptoms of respiratory alkalosis include:

• Dizziness

• Light-headedness

• Numbness of the hands and feet.

Clinical point - to distinguish between respiratory acidosis or alkalosis

Check the partial pressure of carbon dioxide (PaC02) in the blood.

Note: normal range for PaC02 is = 4.7- 6.0 kPa

> A value above this suggests the patient is suffering from respiratory acidosis

> A value below suggests the patient is suffering from respiratory alkalosis

Metabolic acidosisMetabolic acidosis is defined by an arterial blood pH of less than 7.35 with a plasma (serum) bicarbonate ion concentration of less than 22 mmol/L (the acceptable range is 22-26 mmol/L). Metabolic acidosis may occur due to (see Figure 5):

• Severe diarrhoea

• Excessive production of acids, as in diabetic ketoacidosis

• Failure of the kidneys to excrete hydrogen ions

• Poor perfusion, induced by shock.

Depending on the severity of the episode, hyperventilation may help to elevate the blood pH and hence make the blood less acidic by exhaling excess carbon dioxide.

Most of the symptoms of metabolic acidosis are attributable to the underlying condition causing the metabolic acidosis. However metabolic acidosis itself can quickly trigger a number of symptoms such as:

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Condition Cause pH HCOs- PaC02

Respiratory acidosis Hypoventilation ▼ Normal A

Respiratory alkalosis Hyperventilation ▲ Normal ▼

Metabolic acidosis • Ketoacidosis• Diarrhoea

• Renal insufficiency

▼ ▼ Normal

Metabolic alkalosis • Vomiting

• HC03~ retention• K+ depletion• Over transfusion• Ng suctioning

▲ A Normal

Figure 5 Acid-base imbalance summary

• Irritability

• Tremor/twitching

• Confusion

• Tachycardia

• Cardiac arrhythmias

• Hypotension

• Severe cases can lead to convulsions and coma.

Depending on how severe the condition is, hypoventilation can help to lower the blood pH by retaining carbon dioxide in the body, making the blood more acidic.

• Tachypnoea

• Confusion

• Lethargy

• Depression of consciousness (which can lead to coma)

• Cardiac arrthymias

• Severe metabolic acidosis can lead to shock or death.

In a situation of acute metabolic acidosis, the main focus is the correction of the underlying cause, therefore it is best to avoid giving bicarbonate, even if the pH is below 7.1, as this can lead to metabolic alkalosis. When the underlying illness is treated the body will correct the acid-base disorder and restore blood pH homeostasis through its own corrective mechanisms.

Metabolic alkalosisWhen the bicarbonate ion level in the systemic arterial blood is above 26 mmols/L, the condition is called metabolic alkalosis.This condition may occur due to (see Figure 5):

• Severe loss of gastric acid due to severe/ persistent vomiting

• Excessive intake of alkaline medications (such as antacid medication)

• Use of steroids or diuretic drugs.

Some symptoms of metabolic alkalosis are similar to metabolic acidosis and include:

• Slowed breathing initially

• Episodes of apnoea lasting 15 seconds or longer

• Cyanosis

• Nausea, vomiting, and diarrhoea

Respiratory Metabolic

Acidosis

Indicated by:

Compensated by:

> increase in PaC02

> increasing HC03'

> decrease in HC03

> hyperventilation (decreasing PaC02)

Alkalosis

Indicated by:

Compensated by:

> decrease in PaC02

> decreasing HC03

> increase in HC03

> hypoventilation (increasing PaC02)

Figure 6 Compensation mechanisms for acid-base imbalances

Step 1 Consider the normal parameters of an ABG report

Step 2 Perform an analysis of an ABG report describing your interpretation of each step and final overall interpretation.

The following are given as examples:

Example 1 Pa02 7.5kpa Sa02 88% pH 7.32 PaC02 6.7kPa HCO; 24mmol/L

Example 2 Pa02 12kpa Sa02 95% pH 7.49 PaC02 6kPa HCO/ 30mmol/L

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The kidneys help to maintain normal blood pH homeostasis by excreting hydrogen ions in urine

ABG component Normal value Acceptable range

pH 7.4 7.35 - 7.45

Pa02 12 kPa 11.5 - 13.5 kPa

Sa02 97% 93 -100%

PaC02 5.3 kPa 4.7 - 6.0 kPa

HC03- 22 mmol/L 22 - 26 mmol/L

Figure 7 Normal values and acceptable range for analysing ABG components.

CompensationAcid-base imbalances may trigger compensation mechanisms within the body that aim to normalise the blood pH and restore normal physiological homeostasis. Respiratory acid-base imbalances can result in metabolic compensation (triggering a change in HC03'), while metabolic acid- base imbalances may lead to respiratory compensation (causing a change in PaC02) (Figure 6). For example, metabolic acidosis may stimulate an opposing abnormality such

Step 1

Determ ine the patier pH value to identify t acidosis or alkalosis

i t ’s arterial blood he presence of

Step 2 \

Examine the PaC02 determine oxygen stt

7

ind Sa02 values to3tUS

Step 3 \

Study the PaC02 and to determ ine respirat aetiology

7

HC03‘ values ory or metabolic

Step 4 \

If either the C02 or H changes in the oppo the pH change, then compensating for the

7

CO3- value site direction to tha t system is pH change

Figure 8 Four step interpretation guide of arterial blood gas values

as respiratory alkalosis to maintain pH; this is compensation.

Disorders that develop slowly, such as chronic renal or respiratory failure, allow time for maximum compensation to occur and thus are accompanied by minimal changes in pH. Acute illness, such as a cardiac arrest, allow little or no time for compensation to occur, resulting in profound deviation in blood pH levels that may be fatal if immediate treatment is not sought. Compensatory mechanisms aim to counteract respiratory and metabolic imbalances; the method of compensation is influenced by the nature of the acid-base imbalance (Figure 6).

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watch the video: Arterial puncture for blood qas analysis, found at the University of Cambridge website www.sms.cam.ac.uk/media/1070935. Reflect on the role of the perioperative practitioner in this procedure considering the important aspects for the health and safety of the patient and practitioner.

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Treatment of acid-base imbalancesThere are three major mechanisms that help to rectify acid-base imbalances, mainly though the disposal of hydrogen ions. These are (1) buffer systems, (2) renal regulation and (3) respiratory regulation.

Buffer systems

Some substances in the body (such as proteins, bicarbonate ions) can react with hydrogen ions and neutralise the acidic effect o f excess hydrogen ions. These substances are called buffers and act quickly but temporarily. Buffer systems can therefore help in restoring the normal range of the pH of blood, but they do not eliminate hydrogen ions from the body. Subsequent metabolic responses can take hours or even days to normalise blood pH.

Renal regulation

The kidneys help to maintain normal blood pH homeostasis by excreting hydrogen ions in urine. Although this mechanism is slower, excretion in urine is the only way to eliminate acids from the body.

Respiratory regulation

Arterial C02 levels indicate the degree of pulmonary ventilation. To correct hydrogen ion imbalances a number of respiratory interventions may be appropriate: hyperventilation may be adequate for less severe cases of respiratory acidosis and may also be appropriate as a first aid response since altering respiratory rate and volume can normalise blood pH in minutes. Alternatively, in severe situations often when respiratory suppression is drug-induced, the respiratory stimulant doxapram may be administered (Greenstein & Gould 2009). However there are a number of side effects associated with this treatment such as hypertension and tachycardia, and it is contraindicated in patients with coronary heart disease, epilepsy or hypertension (JFC 2014). For respiratory alkalosis, hypoventilation may necessitate ventilatory support.

When a patient presents with a suspected respiratory or metabolic acid-base imbalance, the healthcare professional must first consult the patient's ABG values (see Figure 7).

The following four-step guide has been developed to systematically analyse the patient's ABGs against normal values for quick and accurate diagnosis and treatment (Figure 8).

March 2015 / Volume 25 / Issue 3 / ISBN 1750-4589 51

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ConclusionEfficient interpretation of arterial blood gases is the crucial element in the diagnosis and treatment of critically ill patients. Healthcare professionals have a responsibility to continually update their knowledge and understanding, to ensure that their patients receive optimum safe and effective care (NMC 2008, HCPC 2012). As with any skill, practise and competence are essential. This article provides the background theory to assist healthcare practitioners in understanding the principles of ABG interpretation. In practice, the application of the four step interpretation guide can assist in providing a speedy and accurate diagnosis of the patient’s condition.

ReferencesCoogan J 2008 Arterial blood gas analysis 1: Understanding ABG reports Nursing Times 104 (18) 28-29

Greenstein B, Gould D 2009 Trounce’s Clinical pharmacology for nurses Edinburgh, Churchill Livingstone Elsevier

Health and Care Professions Council 2012 Standards of conduct, performance and ethics London, HCPC

Joint Formulating Committee 2014 British National Formulary (BNF) 68 London, Pharmaceutical Press. Also available online (subscription required) from: http://www.medicinescomplete.com/ [Accessed January 2015)

National Institute for Health and Clinical Excellence 2007 Quick reference guide: Acutely ill patients in hospital Available from http://pathways.nice. org.uk/pathways/acutely-ill-patients-in-hospital [Accessed November 2014]

National Institute for Health and Clinical Excellence 2010 Review of Clinical Guideline (CG50) - Acutely i l l patients in hospital Available from: www.nice. org.uk/guidance/CG50 [Accessed November 2014]

Nursing and Midwifery Council 2008 The Code: Standards of conduct, performance and ethics for nurses and midwives NMC, London

Rogers K, Scott W 2011a Nursesl Test yourself in anatomy and physiology Maidenhead, McGraw-Hill Open University Press

Rogers K, Scott W 2011 b Nurses! Test yourself in pathophysiology Maidenhead, McGraw-Hill Open University Press

About the authorsKatherine M A Rogers PhD, PGCHET, BSc (Hons)

Lecturer, Queen's University Belfast, School of Nursing and Midwifery, Belfast

Karen McCutcheonRGN, MSc, PGCHET, ENB176

Lecturer, Queen’s University Belfast, School of Nursing and Midwifery, Belfast

No competing interests declared

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