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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