Essential critical care skills 6: arterial blood gas analysis

Copyright EMAP Publishing 2022 This article is not for distribution except for journal club use

Clinical Practice

Review Critical care

Keywords Blood gas/Critical care/ Patient assessment

This article has been double-blind peer reviewed

In this article...

The key principles of blood gas analysis using a five-stage approach What the indications are for blood gas analysis and the role of compensation Which red flags to look out for and the interventions required

Essential critical care skills 6: arterial blood gas analysis

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Key points

Arterial blood gases give an indication about a patient's oxygenation, ventilation, acidbase balance and metabolic status

They are used to assess patients who are critically ill, especially when they are receiving respiratory support

Arterial blood gas analysis should follow a systematic approach

Arterial puncture can be a painful experience, and should only be undertaken by staff who are trained and competent to do so

Author Daniel Harris is senior practice development charge nurse, critical care, Cambridge University Hospitals NHS Foundation Trust.

Abstract Blood gas analysis allows the assessment of patients' oxygenation, ventilation, acid-base balance and metabolic status. This article, the sixth in a series on essential critical care skills, outlines the indications and key principles of arterial blood gas analysis using a systematic five-stage approach. It also reviews compensated and uncompensated blood gases, considers the patient experience and provides an introduction to those interventions that can be undertaken.

Citation Harris D (2022) Essential critical care skills 6: arterial blood gas analysis. Nursing Times [online]; 118: 4.

Blood gas analysis is used in the holistic care of patients who are critically ill or deteriorating and must be available in all critical care units (Faculty of Intensive Care Medicine and Intensive Care Society, 2019). Samples can be arterial, venous or mixed venous; arterial blood gases (ABGs) are taken from arteries and give a real-time indication of the patient's oxygenation, ventilation, acidbase balance and metabolic status. An ABG can be taken by sampling blood from an arterial line or performing an arterial puncture (if staff are trained and competent to do so). This article focuses on ABGs. A glossary of key terms is given in Box 1.

Blood gases are taken using point-ofcare devices when a result is needed quickly (namely, from sampling to analysis in minutes). Small changes in a patient's CO2 or HCO3 levels can alter their pH; the body will try to maintain homeostasis by altering these levels by breathing more or less, or altering the level of HCO3 produced from the kidneys. Blood gas analysis features throughout guidelines from the British Thoracic Society (BTS) on oxygen use in adults in healthcare and emergency settings, written by O'Driscoll et al (2017).

As with any numerical analysis, a blood gas test should be used together with a holistic assessment of the patient using the systematic airway, breathing, circulation, disability, exposure approach (Resuscitation Council UK (RCUK), 2021). A systematic approach to analysing blood gases should also be used. Basic analysis of ABGs is part of step one of the adult critical care competency framework by the Critical Care Networks ? National Nurse Leads (CC3N) (2015).

Indications ABGs are used to assess patients in critical care, where many have a degree of respiratory support ? for example, invasive ventilation, nasal high-flow oxygen, non-invasive ventilation or humidified oxygen (RCUK, 2021). Only staff trained and competent to perform arterial punctures or access arterial lines should undertake this skill; they should be trained, for example, by undertaking the CC3N's step one competencies or a local competency assessment. Nurses should, at all times, follow local policies and procedures, as well as the Nursing and Midwifery Council's (2018) Code.

ABGs can provide reassurance that a patient is ventilating their lungs adequately

Nursing Times [online] April 2022 / Vol 118 Issue 4

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Clinical Practice Review

Copyright EMAP Publishing 2022 This article is not for distribution except for journal club use

Published in association with British Association of Critical Care Nurses

Box 1. Glossary

FiO2 ? faction of inspired oxygen Hb ? haemoglobin HCO3 ? bicarbonate K+ ? potassium ion kPa ? kilopascal Na+ ? sodium ion PaCO2 ? partial pressure of carbon

dioxide PaO2 ? partial pressure of oxygen SaO2 ? oxygen saturation; percentage

of oxygen bound to haemoglobin

Box 2. A common ABG result

PaO2 ? 12.5kPa PaCO2 ? 4.8kPa pH ? 7.4 HCO3 ? 23mmol/L Base excess ? 1 SaO2 ? 97% Hb ? 9.4g/dL K+ - 4.0mmol/L Na+ ? 136mmol/L Lactate ? 1.2mmol/L

ABG = arterial blood gas.

or their respiratory function is adequate, in conjunction with pulse oximetry (O'Driscoll et al, 2017). ABGs also assess the metabolic needs of a patient who is critically ill, allowing for timely escalation if needed. A change or deterioration in a patient's condition should trigger a blood gas sample to be taken; this could be when titrating mechanical ventilation settings, such as tidal volumes, respiration rate or FiO2 (Lian, 2010).

Principles of blood?gas analysis What is measured ABGs have five key pieces of information: PaO2 in arterial blood; pH; PaCO2 in arterial blood; HCO3; Base excess;

A common blood gas result is shown in Box 2; reference values are given in Table 1.

Systematic assessment of blood gases A systematic, five-stage approach should be used when analysing blood gases (RCUK, 2021). It can be used in conjunction with a flowchart (Fig 1) to work through an ABG systematically. Answers to the following questions should be ascertained: How is the patient? Does senior help

need to be requested? Does the patient have hypoxaemia?

Table 1. ABG values and clincial significance in critical care

Measure Normal values Clinical significance

PaO2

10.7-13.3kPa 80-100mmHg

45mmHg = respiratory acidosis 5 out of 10. Local anaesthetic was rarely offered (n=2/66), but it can help to reduce pain. Matheson et al's (2014) study showed that infiltration of lidocaine reduced pain scores, but the study was small in scale (n=40) and only partially randomised.

Topical anaesthetics can be used, but require time to work and, therefore, can delay the sample being taken (Aaron et al, 2003). BTS guidance recommends the use of local anaesthesia for all ABG sampling (O'Driscoll et al, 2017).

Use of an arterial line can decrease repeated stabbing (see part three of this series for more information).

Conclusion ABG analysis can form part of a holistic review of an adult who is critically ill. Only trained, competent staff should take an ABG, and results should always be interpreted by a competent practitioner. NT

References Aaron SD et al (2003) Topical tetracaine prior to arterial puncture: a randomized, placebo-controlled clinical trial. Respiratory Medicine; 97: 11, 1195-1199. Crawford A (2004) An audit of the patient's experience of arterial blood gas testing. British Journal of Nursing; 13: 9, 529-532. Critical Care Networks ? National Nurse Leads (2015) National Competency Framework for Registered Nurses in Adult Critical Care: Step 1 Competencies. CC3N. Faculty of Intensive Care Medicine and Intensive Care Society (2019) Guidelines for the Provision of Intensive Care Services. FICM/ICS. Joynt GM, Choi GYS (2016) Blood gas analysis in the critically ill. In: Webb A et al (eds) Oxford Textbook of Critical Care. Oxford University Press. Khan F et al (2012) P236 the use of local anaesthesia for arterial blood gas sampling ? a multicentre survey. Thorax; 67: Suppl 2, A167-168. Lian JX (2010) Interpreting and using the arterial blood gas analysis. Nursing Critical Care; 5: 3, 26-36. Matheson L et al (2014) Reducing pain associated with arterial punctures for blood gas analysis. Pain Management Nursing; 15: 3, 619-624. Nursing and Midwifery Council (2018) The Code: Professional Standards of Practice and Behaviour for Nurses, Midwives and Nursing Associates. NMC. Nuttall G et al (2016) Surgical and patient risk factors for severe arterial line complications in adults. Anesthesiology; 124: 3, 590-597. O'Driscoll BR et al (2017) BTS guideline for oxygen use in adults in healthcare and emergency settings. Thorax; 72: Suppl 1, ii1-ii90. Resuscitation Council UK (2021) Blood Gas Analysis and Pulse Oximetry. RCUK. Wilkinson IB et al (2017) Oxford Handbook of Clinical Medicine. Oxford University Press.

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