Oxygen therapy and Arterial Blood Gases

1. Oxygen therapy and Arterial Blood Gases Everyone loves their

2. Introduction As I was writing this new module on oxygen therapy and arterial blood gases an intensivist wrote to us at the medical school decrying the students’ poor understanding of the treatment of respiratory failure. He was concerned that they were still being taught to reduce oxygen to low (%) in patients with type II respiratory failure, despite the growing evidence that this is precisely what should not happen. Indeed, some students were actually being told to stop the oxygen when patients became more narcotised! The Intensivist’s concerns reflects the world of medicine which you are about to enter. As our medical understanding and therapeutic options increase seemingly exponentially it is our duty as teachers to keep up to date, pass on the latest thinking and not to perpetuate misunderstandings or out of date information. This is not to ignore that there is NO substitute for experience and a sharp mind! . It is your obligation as students and newly qualified doctors (hopefully) to push forward this knowledge, to ensure the next generation of students are given the care and teaching that you are now receiving and of course to ensure that you are practising the best medicine you can! Keeping up to date is really difficult but very necessary. By working your way through this module you should begin to acquire the knowledge and skills necessary to treat patients with oxygen.

3. Aims and Objectives This module is aimed at all students who are trying to improve their knowledge and skills around oxygen therapy and arterial blood gases. By the end of this module you should be • able to correctly prescribe appropriate oxygen therapy for patients on a drug chart. • able to understand the differences between the various oxygen delivery devices. • able to understand the different types of non-invasive ventilation systems (CPAP and BiPAP), their indications and contra-indications. • ready to attempt to take arterial blood gases (ABGs). • able to interpret a set of ABGs and institute appropriate therapy.

4. Challenging Practice Before you launch into this module • Write down the management of a 68yo man with an acute exacerbation of his COPD • What do his ABGs show? pH 7.11,PaCO2 12.9KPa, Pa02 6.6KPa, Sats 68%,HCO3- 14mmol/l,Base excess -8.3 • How does this change your management? • List the essential steps in performing a set of ABGs.

5. It is true … HYPERCAPNIA : MAKES YOU DROWSY - IT’S A SIGN OF VENTILATORY FAILURE AND EXHAUSTION,IT LEADS TO NARCOSIS … HYPOXIA KILLS YOU!

6. RULE 1: GIVE THEM OXYGEN – GIVE THEM LOTS OF IT – THEN CALL FOR HELP! • Hypoxia kills! Therefore give enough oxygen to correct the patient’s hypoxia. [At least to PaO2 > 8KPa or sats >91%] • If there is no improvement in their hypoxaemia, and/ or further worsening of their hypercapnia, the acidosis or their clinical condition this is an indication for NIV or indeed Intubation (if appropriate). • DO NOT STOP THEIR OXYGENdue to hypercapnia. • Treat underlying causes appropriately. • Remove exacerbating factors e.g. sedatives, opiates.

7. Prescribing Oxygen therapy • Like all therapeutic interventions MUST be prescribed • Usually specific chart or part of the drug chart • Very poorly done by PRHOs (and others!) • Should be based on what your therapy is trying to achieve, the ABGs and / or the oxygen saturation.

8. Indications for Oxygen • Principally HYPOXIA! • Poor ventilatory effort – post operative, sedation, confusion • Poor oxygen delivery – hypotension and shock, anaemia, heart failure • Acute illness and acidosis • In fact anyone who you are concerned is unwell!

9. Which System to use? (a) Low Flow oxygen devises - nasal cannula, simple face mask, ‘reservoir bags’ (A Tarantino classic) (b) High Flow oxygen devices - Venturi masks (c) Non-Invasive Ventilation (NIV) - CPAP / BiPAP

10. Low Flow Oxygen Devices • So called because they deliver Oxygen at less than Peak Inspiratory Flow rate (PIFR) • Not because you use a low flow oxygen rate! • PIFR Normal = 15 litres /minute • In a ‘Sick patient’ it may be up to 30 litres / minute

11. How much Oxygen do low flow devices deliver? • All deliver VARIABLE O2 dependent on the patient’s PIFR • They should NOT be used when you need to know the precise amount of O2 being delivered i.e. Patients with Type II respiratory failure • Nasal cannulae – O2 should be set between 1 – 4l/min; delivers between 25% and 35% O2 [when O2 set at 2l/min] • Simple Face mask - O2 can be set up to 15l/min; delivers up to 50% O2 • Reservoir Mask – O2 can be set up to 15l/min; work on the principle that O2 fills the reservoir bag in expiratory phase so allows extra O2 to be delivered in Inspiration. Delivers up to 60% O2

12. Consider two patients on nasal cannulae with 100% oxygen@ 2 litres / minutes Normal patient – PIFR = 15 l /min Inspired Oxygen = 2l/min (100%) + 13 l/min (21% - air) Sick Patient – PIFR = 30 l /min Inspired Oxygen = 2 l/min (100%) + 28 l/min (21% - air) Thus the percentage of 100% oxygen received by the sick patient 2/28 (7.1%) is less than half of that being delivered to the well patient 2/13 (15.4%) using the same delivery device.

13. So when should I use low flow devices? • Low flow devices may be used when it is NOT essential to know the precise amount of oxygen being delivered E.g. • Often used as the first delivery system in ‘resus’ or HDU patients. • Post-operative patients • Cardiac patients – chest pain • Mild – Moderate Asthma • Domestic / Long term oxygen therapy • ‘Unwell’ patients – septic, hypotensive, anaemic (although correcting hypotension and anaemia will also have major effects on Oxygen delivery)

14. High Flow - Venturi System • ‘Noisier’ than low flow devices • Deliver oxygen concentration higher than PIFR • Work on the Bernoulli Principle (1778) i.e. Draw air into the valve system because of the reduced pressure as the oxygen passes across the nozzle into the chamber of the mask. By varying the nozzle’s aperture, different mixes of oxygen:air are produced. • Deliver a predictable concentration of oxygen 24% - 60% • The masks are colour coded (The Rate and (%) are on the side of the valve which connects to the mask! Look for it and you won’t have to guess!)

15. Colour Coded Venturi Masks

16. When should I use a Venturi mask? • When knowing the precise amount of oxygen that the patient is receiving IS important • E.g. All patients with type II respiratory failure (RF) and patients who are ‘hovering’ on the border of type II respiratory failure e.g. the sick patient with type I respiratory failure. • Try to correct any underlying or exacerbating conditions. • Make sure that the oxygen and other therapies you are giving are being continually re-assessed. • If the patient’s hypoxia or hypercapnia are unchanged or worsen, or there is worsening acidosis despite oxygen therapy then assisted ventilation should be considered. This may be Non invasive or invasive depending on the clinical circumstances. • Worsening gases are an indication to rapidly seek senior help; think ‘my friends the anaesthetist and the registrar’!

17. Non-Invasive Ventilation

18. Can you believe it a Danish speaking tiger!

19. Why Non-invasive ventilation (NIV)? • Decreases the work of breathing by reducing the effort of the respiratory muscles and diaphragm • Augments alveolar ventilation and can improve alveolar gaseous exchange • Can reset the responses of the CNS respiratory centres to changes in PaCO2 • Increases PaO2 without increasing PaCO2

20. NIV - Indications • Acute Type I and II Respiratory failure • Chronic Type II RF / Sleep apnoea • Patients who are deemed not for intubation • Acidosis (pH 7.10 - 7.35 – although the lower limit is not strictly adhered to in practice) • Hypercapnia where PaCO2≤12KPa • Tachypnoea [RR>30] The patient should be • Co-operative (i.e. not too confused) • Able to maintain own airway with a good cough reflex • Haemodynamically stable They should show clinical +/or ABG improvement within 2 hours

21. NIV - contraindications • Competent patient declines or refuses • Previously documented wish not for further NIV (End of life decision in terminal disease) • Unco-operative or very confused patient • Haemodynamic instability • Respiratory arrest • Patients at high risk of aspiration • Facial trauma or surgery

23. Non-Invasive Positive Pressure Ventilation CPAP – Continuous Positive Airway pressure ventilation device BiPAP – Bilevel Positive Airway Pressure ventilation device Both can deliver oxygen at 4 – 25cm H2O above estimated Peak End Expiratory Pressure (PEEP)

24. (Little Bo) PEEP • PEEP = Positive End Expiratory Pressure • COPD patients have ‘air trapping’ in their airways and therefore do NOT completely empty them at the end of expiration. • This leads to ‘Intrinsic PEEP’ of approximately 5cm H2O • Thus you need to set the pressures of machine (NIV) above ‘intrinsic PEEP’ • Therefore in most COPD patients you normally start with the pressures set at 10cm H2O

26. CPAP or BiPAP? Consider the following scenario ..You are a first day house officer. The sister on the ward tells you that a COPD patient is becoming increasingly unwell and dyspnoeic at rest. What would you like to do next? Is it time for CPAP or BiPAP Doctor? The decision is yours ……what will you do next?

27. Do basic things well … then panic! • Do not ignore your basic mantra – History, Examination, Investigations, Management! • Put the patient on ‘lots of oxygen’ – rule (1) (e.g. 60%) – then THINK!; make a rapid assessment of the patient. • Find the notes – see what has been going on and if there are any clues from the team looking after the patient as to what should (and should not) be done. • Find out from the patient what is going on (it may be they are too breathless to speak – a bad sign!) • If they are very sick – get someone to get the crash trolley, put out a cardiac arrest call, Triple bleep the anaesthetist on-call. (NEVER leave a sick patient alone) – but make sure that this is appropriate (check the notes) • Examine the patient – look for/exclude causative conditions – pneumonia, LVF, pleural effusions, pneumothorax, new drugs on the chart (e.g. betablockers) • Basic investigations – CXR, ABGs, ECG, Bloods (as appropriate) • Institute initial management – then get help (if you haven’t already)

28. CPAP or BiPAP? • This sounds easy in practice but will you do any of these? Or will you be more like me on my first day and just look blankly and then smile sweetly at the sister and ask her what she would do. • NOTE - Insight into the fact you know nothing is one very important step above knowing nothing or being too arrogant to admit you know nothing! • Of course these are all many steps below knowing things and being able to do things!

29. As for CPAP or BiPAP … BiPAP indications • Primarily WORSENING TYPE II RESPIRATORY FAILURE e.g. • Acute exacerbation of COPD associated with ACIDOSIS • Worsening Type II respiratory failure in patients with chronic chest wall disease – e.g. neuromuscular or skeletal deformity • Decompensated sleep apnoea CPAP indications • Primarily type I respiratory failure due to e.g. • LVF • Chest wall trauma • You need an HDU type environment and specialist, trained nurses to place patients on either CPAP or BiPAP; This is rarely applicable to patients on general wards – move them! • Neither CPAP nor BiPAP should be chosen over intubation and ventilation where appropriate e.g. Type II respiratory failure in an asthmatic • These and similar complex decisions should NEVER be made by house officers in isolation!

30. On a drug chart please prescribe the oxygen therapy for the patients below; What delivery system will you choose? What other important management steps will you take? • A previously fit and well 61yo man who is 8 hours post hemi-colectomy. • A 23yo Asthmatic with an acute asthma attack who has O2 sats of 93% on room air. • A 67yo COPD patient with type II respiratory failure. • A 91yo man with severe gram negative sepsis and dehydration; O2 sats are 92% on air. • 17yo Asthmatic with type II respiratory failure.

32. For more detail visit … • http://www.brit-thoracic.org.uk/c2/uploads/NIV.pdf Recommendations on NIV • http://www.studentbmj.com/issues/01/04/education/94.php see also the references included for further explanations on oxygen therapy • http://www.studentbmj.com/issues/04/02/education/56.php This is part of an excellent series written by Dr Nicola Cooper on intensive care and includes Recognising the sick patient, Oxygen therapy, Fluid replacement, Inotrope therapy, ABGs (see next section) • http://www.emedicine.com/MED/topic2011.htm Covers Type II Respiratory failure • http://bmj.bmjjournals.com/cgi/content/full/317/7161/798?ck=nck The ABC guide to oxygen therapy; Still an excellent overview after nearly 10years!