The Pitfalls of Diagnostic Labels: Life-saving Oxygen Therapy, COPD and Critical Illness

BACKGROUND

 

I feel there is a widespread misconception amongst medical students and medical practitioners resulting in patients with COPD occasionally being deprived of life-saving oxygen therapy during times of critical non-COPD illness.

 

Quite rightly we are taught at medical school to be aware of the risk of precipitating type II respiratory failure by giving uncontrolled oxygen therapy in patients with COPD. This is owing to a reliance on ‘hypoxic drive’ to maintain respiratory rate and therefore ventilation. So, if we give too much oxygen we obliterate the hypoxia and the patient’s central nervous system no longer feels the need to drive the respiratory musculature.

 

However, there is a crucial caveat that must be emphasised.  This theoretical risk of ‘obliterating hypoxic drive’ is not relevant to all patients with COPD. It may not be relevant to most patients with COPD. According to BTS guidelines those patients most dependent on hypoxic drive are those with resting hypoxia (i.e. on home oxygen), severe COPD (i.e. a high degree of airflow limitation on spirometry, poor exercise tolerance, and clinically severe disease suggested by frequent exacerbations), or a history of NIV use.

 

Therefore it is only these patients with severe disease that we should be wary of giving uncontrolled oxygen therapy to.  I don’t think it is good practice to deny all COPD patients of high-flow oxygen therapy as a rule.  We should also be very wary of labelling patients as having ‘COPD’ because this is often interpreted as ‘do not give high-flow oxygen’ by many practitioners.

 

Here’s some cases to illustrate this.

 

FLASH PULMONARY OEDEMA

 

I was asked by a ward SHO to review a breathless patient with oxygen saturations of 80% on air.  Via telephone I recommended that the SHO apply high-flow oxygen and I made my way over.  The SHO had applied high flow oxygen, but I learned as I arrived this came with disapproval from the nursing staff.

 

I arrived to find a 60 year-old gentleman severely tachypnoeic with a respiratory rate of 42 breaths per minute, saturations of 92% on high-flow oxygen and widespread crepitations on his chest. There was reasonably good chest wall excursion on inspection (i.e. he was shifting air pretty well) and no wheeze heard (i.e. no significant bronchospasm or airflow limitation). He was haemodynamically stable.

 

Treating as acute pulmonary oedema I prescribed IV furosemide and GTN.  Serial arterial blood gases (on high-flow oxygen, remember) revealed a pO2 of 9 kPa and a pCO2 of 6.6 kPa with a pH of 7.2. There was also an underlying metabolic acidosis – bicarbonate was 18 and base excess was minus 5.

 

The arterial gas remained the same on two gases performed 40 minutes apart with the patient on high-flow oxygen in the intervening period. Furthermore, this patient had an oxygen saturation of 95% on air prior to this episode of pulmonary oedema.

 

In other words, this gentleman was very unwell with profound hypoxaemia and whilst you would classify his ABG as showing type II respiratory failure, the pCO2 was only slightly raised (not high enough to account for a pH of 7.2) and it was not climbing. To all intents and purposes, both clinically and biochemically, this patient was suffering from a pulmonary diffusion problem (due to his lungs being water-logged) not an airflow obstruction problem.  He was given appropriate oxygen therapy but the important point to emphasise is he was monitored throughout this.

 

Unfortunately, because ‘probable COPD’ had been written in the notes all of the medical staff were reluctant to use high-flow oxygen, and I attracted looks of disbelief when I prescribed high-flow O2 and said he doesn’t need nebulisers (consider that he doesn’t have a wheeze, and also that nebulisers cannot be delivered with high-flow oxygen – so we would be giving him an unnecessary treatment and depriving him of life-saving oxygen).

 

Whilst he may have underlying COPD (life-long smoker) that is not his main problem now.  This patient had not been bothered with a cough, wheeze or phlegm production in the preceding 6 months. Considering that COPD is characterized by chronic cough, wheeze and phlegm there probably isn’t strong evidence for diagnosing severe/uncontrolled COPD clinically here.  But even if he does have COPD I still feel high-flow O2 is appropriate.  Of course it is not appropriate is to prescribe high-flow O2 and walk away, or to deny high-flow O2.

 

With the patient not improving I asked for help from our ITU colleague. He kindly attended and reviewed the patient.  I carefully documented that this gentleman had a stable pCO2 on high-flow oxygen, remained significantly tachypnoeic (i.e. not developing respiratory depression), and that his clinical picture was one of a pulmonary diffusion problem, not a ventilatory problem.

 

The ITU registrar reduced the patient’s inspired oxygen concentration to an FiO2 of 0.28 (or 28%, as opposed to the 85% which is approximately what a non-rebreathe mask delivers).  Unfortunately the patient’s O2 saturations dropped to around 80% at this point.  The oxygen was then increased back up to high-flow and the patient improved.  He was later taken for pressure support on ITU (respiratory exhaustion is another potential cause of worsening type II failure that I think was at play here, and is often not thought about due to our fear of patients ‘losing hypoxic drive’) and thankfully survived to hospital discharge some 5 days later.

 

RESPIRATORY ARREST SECONDARY TO NEUROMUSCULAR WEAKNESS

 

Consider another recent case.  A respiratory arrest call – a patient with muscular dystrophy and type II respiratory failure with a severe respiratory acidosis and hypoxia. The attending ITU registrar tried to reduce this patient’s inspired oxygen concentration simply because they had type II respiratory failure.  This critically ill gentleman should however be administered the high-flow oxygen and ventilatory support that he needs.  He had type II respiratory failure because his respiratory musculature was weakened by muscular dystrophy – this led to a severe hypoxia and acidosis.

 

This patient needed oxygen and ventilatory support, and so long as the arrest team is present there is no risk of him suffering from a loss of his hypoxic drive – a functioning arrest team would recognise this and manage it minute-to-minute.

 

UNILATERAL LUNG ATELECTASIS AND PNEUMONIA

 

In August I was called to a peri-arrest on the UK medical wards wherein a lady with a ‘history of COPD’ had been left with saturations of 80% on 5L of oxygen via nasal speculum.  I arrived to find that she had been left with this poor oxygen saturation for more than an hour and the nursing staff again had been very reluctant to increase her oxygen therapy due to her COPD label.  History and examination revealed a patient with a smoking history but no history of chronic cough or phlegm and an unlimited exercise tolerance (i.e. again poor evidence for COPD). On examination there were clinical features of collapse and consolidation of her whole right lung with excellent compensatory hyperventilation of her contralateral lung such that she had a respiratory alkalosis (i.e. her functioning left lung was so unaffected by any significant airflow limitation that it was managing to compensate for the total loss of her right lung).

 

With an urgent ITU consultation this lady received pressure support under their care and was later discharged home.

 

CONCLUSION

As a general rule all critically unwell patients should have their hypoxia addressed with adequate oxygen therapy – to maintain saturations of equal to or above 95%, regardless of a COPD history.  Of course this rule is subject to consultant review but we should not assume by default that patients with COPD should be left with low saturations.  We should assess the patient clinically and if they are at risk of losing hypoxic drive we should establish a plan for ongoing monitoring (serial ABGs and clinical assessment) and appropriate titration of oxygen therapy.  The alternative of leaving them with saturations of 88% during an acute cardiorespiratory crisis I do not feel is fair.

 

Essentially the bottom line is that you shouldn’t apply a high-flow oxygen mask and then walk away. Sadly too many of us are writing ‘COPD – Sats 88-92%’ in the notes without a more nuanced plan, which may be more dangerous than giving high-flow o2.

 

We should not withhold oxygen from any critically unwell patient without a good reason.  Of course if the patient is clinically well and stable it is probably acceptable to maintain oxygen saturations of 88-92%.

 

Remember that patients are not labels or diseases. Patients can be complex; they have past medical histories and current medical problems. Sometimes these are related, sometimes not. Do not take disease labels at face value, instead make an individual assessment.

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