Chris CameronClinical pharmacologist & General

PhysicianCCDHB

Oxygen- A prescribing Blindspot?

Ms J, 70yo

• Lives with partner, who has a recent diagnosis of breast cancer

• Works 3 days a week

• Weight 46kg

• Smoker (80 pack year history)

• Ex tol about 500m on flat

• Recent admission (May 17) for IECOPD

PMHx

• Severe COPD

– ICU admission July 2012.

– Spirometry Sept 2014: FEV1 0.66 litres (31%), FVC 61%, FEV1/FVC 42%

– Last seen by Resp Sept 2014

– CT chest (2014) Moderate centrilobularemphysematous change with hyperinflation of the lungs

RxHx

Bezafibrate 400mg PO nocte

Omeprazole 20mg PO mane

Salbutamol 100mcg 2 puffs inh BD and PRN q4h

Seretide 125/25 2 puffs inh bd

Spiriva 18mcg 1 puff inh od

Aspirin E.C. 100mg PO od

Cholecalciferol 1.25mg PO monthly

Dermol ointment, apply to psoriasis occasionally

Ensure liquid 1.5kcal/mL - chocolate, 1 BD

ICU admission 2012• Not known to be a CO2 retainer

• Slow respiratory wean - intubated for 12 days

• “Trial of BiPAP resulted in increased agitation and intolerance and so was stopped. Hypercapnic on further ABG's.”

• Documented ICU note that Ms J has significant respiratory disease (COPD with FEV1 0.77).

• “While she was successfully, though slowly, weaned from mechanical ventilation, her underlying lung pathologies are likely to worsen, especially if she continues to smoke. As such, she would be a poor candidate for ICU therapy if she presented to hospital in a number of months time; although they would be happy to discuss this further if the situation does arise”.

Blood gases in recent admissions

Date & time O2 given pCO2 PO2 pH

23 June 20120722

4L 44 (A) 74 7.34

1245 6L 67 (A) 108 7.18

Trial of BiPAP unsuccessful

Transferred to ICU, intubated and ventilated

1430 2L 54 (A) 56 7.27

5 July 2012 Extubated 5 July 2012

Date & time O2 given pCO2 pO2 pH

13 May 2017 unknown 47 (?V) 55 7.4

15 May 2017 unknown 55 (?V) 40 7.36

Final admission 14-17 July

• PC: Cough, fever, SOB → IECOPD, new pAF

• VBG: pH 7.382, pCO2 49.9, HCO3 29.6Bloods: CRP 9, WCC 8.1, Neuts 6.5ECG: Sinus tachycardia (132)with frequent PACs/PAF. No acute ischaemic changes.CXR: Heart not enlarged. Significant hyperinflation of the chest but no focal consolidation/evidence of failure.

Plan:1) Admit Gen Med2) Q2H obs - EWS adjusted3) Further IVF4) Continue IV Cefuroxime given Penicillin allergy5) Prophylactic Clexane6) Monitor heart rate but if does not settle with ABs and further fluid may need rate control -oral short acting metorpolol tartrate.7) Ca/PO4 and Mg added to bloods. (Unable to add TFTs - repeat bloods mane with TFTs).8) NRF form signed in discussion with patient.

Progress 14-16 July

• Ms J was steadily improving

• HR settling, still some pAF

• SW discussions re care for partner

Date & time O2 given pCO2 pO2 pH

14 July 201718.51 (ED)

45% 50 (V) 21 7.38(HCO3 29.6)

Then suddenly..

What happened?

• Ms J was started on 2L/min O2 in the early hours of 16 July when her O2 sats were 92% RA

• After about 4 hours she became restless and → ↑ HR (AF)

• She then had a seizure

• Transferred to HDB

Date & time O2 given pCO2 pO2 pH

16 July 2017 2L/min 120 (A) 70 6.99

Time O2 given pCO2 pO2 pH

0709 ? 114 (A) 65 7.08

0816 4L 105 (A) 71 7.08

Lessons to be learned

• No-one appreciated that Ms J was a CO2 retainer

• This was not on her problem list

• No O2 therapy was prescribed for her

• VBGs were used to guide therapy

• No O2 therapy documented on VBG

• No Venturi mask was used

• Signs of CO2 retention not appreciated

ABG vs. VBG in COPD

• Values from VBG:

• pH ABG=VBG

• pO2 ABG bears no relation to VBG

• pCO2 If VBG pCO2 <46, then ABG <46 usually

If VBG pCO2 >46, the ABG pCO2 is high, but ?how high

In COPD patients and others at risk of T2RF, ABGs need to be used to guide O2 therapy. Get some practice. VBGs are not useful in this setting.

Low pO2 can predict

CO2 retention

PML Guidance

14 July 2017 Venturi Mask

x

2L-4L/min

x

Do not administer oxygen unless discussed with registrar/SMO

Supplemental O2 is an FiO2 > 21% and is a drug

(remember RA=21% O2)Type of device Litres O2/minutes FiO2 inhaled When to use

Nasal prongs 1L/min 2L/min3L/min4L/min 6L/min

24% (0.24)28%33%41%45%

When low flow O2 neededin a patient without CO2 retention. If >4L/min humidification is recommended

Hudson mask 4L/min6L/min8L/min

24-28%31%35-40%

In hypoxic patients without CO2 retention

Venturi mask 2-4L/min2-4L/min4-6L/min6-8L/min9-10L/min10-12L/min12-15L/min

24%26%28%30%35%40%50%

In hypoxic patients with known CO2 retention, or at risk of CO2 retention

Non-rebreathermask

Upto 15L/min 60-90% In hypoxic patients without CO2 retention

Signs of hypercapnia

• Sedation, comatose

• Altered mental status, confusion, paranoia, seizures

• Muscle twitches

• Vasodilatation of the skin – flushed face, strong, bounding pulse

• Papilloedema

• Asterixis – an easy sign to elicit

What did we learn?

• Supplementary O2 is a drug

• Supplementary O2 must be prescribed

• All COPD patients are potential CO2 retainers

• Education of nursing and junior medical staff is needed