In the second part of our series about acute care medicine, Nicola Cooper explains the principles behind treatment with oxygen

Oxygen is one of the most important drugs you will ever use, but it is poorly prescribed by medical staff. In 2000, a colleague and I did two surveys of treatment with oxygen. The first looked at prescriptions of oxygen in postoperative patients in a large district hospital. We found that there were many ways used to prescribe oxygen and that the prescriptions were rarely followed. The second study surveyed 50 medical and nursing staff working in acute care. We asked them to name different oxygen masks and to say how much oxygen each delivered. We then asked them to decide which mask was most appropriate for a given clinical situation (see quiz).

When you should use oxygen

Patients need oxygen when they have hypoxaemia, acute hypotension, or respiratory distress; when they have trauma or other acute illness, carbon monoxide poisoning, or severe anaemia; during the perioperative period; and when they have taken drugs that suppress ventilation, like opioids.

Oxygen masks

Oxygen masks are divided into two groups.

Inspiratory flow rate

A person breathing normally inspires about 15 l/min of air--this is the inspiratory flow rate, which varies throughout respiration. Assuming a constant flow rate, if you give a person who is breathing normally a nasal cannula at 2 l/min, he or she would be breathing 2 l/min of 100% oxygen and 13 l/min of air (21% oxygen). The concentration of oxygen delivered to the patient is therefore (1 * 2 + 0.21 * 13)*100/15 = 31%.

In someone with respiratory distress, the inspiratory flow rate can reach 30 l/min. In this case he or she would be breathing 2 l/min of 100% oxygen plus 28 l/min of air (21% oxygen). You can see that the proportion of oxygen received is now much less. That is why nasal cannulas, simple face masks, and masks with a reservoir bag are called low flow devices (because the maximum flow is only 15 l/min) and deliver a variable concentration of oxygen.

High flow masks

High flow masks or Venturi masks (fig 1 and tables 1 and 2), on the other hand, are designed to overcome this problem. Oxygen is forced through a short constriction. This has the effect of increasing gas flow. The Bernoulli effect was described in 1778: fluid velocity increases at a constriction. Mathematically this can be written 1/2*v² + p = constant, where * is the density, v is the velocity, and p is the pressure. Total energy is conserved, so if the pressure of a gas falls and its potential energy falls then its kinetic energy increases and it gains velocity.

Fig 1 How a Venturi mask works

After gas is forced through the Venturi valve, the pressure drops suddenly due to the increase in area. The velocity or flow of the gas increases as a result, and air is entrained from either side of the valve. Venturi masks come with different sized colour coded valves. Each requires a certain gas flow to deliver the oxygen concentration stated. To change the oxygen concentration, you have to change the mask and flow. By delivering a constant mixture of oxygen and air at above the maximum inspiratory flow rate, changes in breathing do not affect the oxygen concentration delivered.

Why PaCO₂ rises

The causes of a high arterial carbon dioxide concentration (PaCO₂) are much misunderstood in relation to treatment with oxygen. Many doctors reduce the oxygen concentration in patients who have a high Paco₂ because they believe that treatment with oxygen may be causing the problem. In most cases, this is illogical and dangerous. "Carbon dioxide retention" is the term used for the phenomenon that occurs because of uncontrolled treatment with oxygen in patients with severe chronic obstructive pulmonary disease (and some other conditions such as kyphoscoliosis causing chronic respiratory failure). "Hypercapnic respiratory failure" is the term used to describe when PaCO₂ rises for other reasons.

The most common reason for a high PaCO₂ in any patient you will ever see, including those with chronic obstructive pulmonary disease (COPD), is getting tired, or ventilatory failure (fig 2). This is when the load on the respiratory muscles becomes too great for the strength in the respiratory muscles.

Fig 2 Ventilatory failure The most common reason for a high PaCO₂ in any patient you will ever see... is getting tired, or ventilatory failure

Most doctors would never reduce the oxygen concentration in a patient with severe asthma and a rising PaCO₂ because they recognise that ventilatory failure is the problem. But what about elderly patients with pneumonia, severe acute left ventricular failure, a compromised conscious level, or neuromuscular disease? To make matters more complicated, even patients with an exacerbation of COPD develop ventilatory failure, which is why we use non-invasive ventilation in such patients.¹

Carbon dioxide retention caused by treatment with oxygen is uncommon. The reasons are complex. It used to be thought that in patients with a chronically high PaCO₂, the chemoreceptors in the brain become blunted and the patient depends on hypoxaemia to stimulate ventilation. If they are given too much oxygen, their hypoxic drive is abolished, breathing will slow, and the PaCO₂ will rise as a result. However, some studies have since shown that minute volume and respiratory rate remain unchanged in such patients.² Likely explanations include:

• The release of hypoxic vasoconstriction (a normal lung response) with oxygen therapy causes a change in ventilation-perfusion in the lung. This results in increased dead space ventilation and PaCO₂ rises as a result
• Patients with COPD have a reduced ventilatory drive in response to hypoxaemia and hypercapnia. The reasons for this are unknown. Genetic factors play a part, as "normal" family members also demonstrate blunted responses. Acquired loss of ventilatory drive is also present in COPD. A different pattern of ventilation is observed in people who retain carbon dioxide, with lower tidal volumes but an increased respiratory rate. This is probably a compensatory mechanism designed to reduce the work of breathing
• The Haldane effect is sometimes mentioned. When haemoglobin is saturated with oxygen, the amount of carbon dioxide it can carry is reduced and the number of hydrogen ions in solution increases as a result.

We do know that patients with severe impairment of lung function are susceptible to carbon dioxide retention. It tends to occur when the forced expiratory volume in one second (FEV₁) is less than one litre. This is not so for most patients with COPD. Clues to the severity of someone's COPD include compensatory polycythaemia, functional history, drugs, and arterial blood gases.

Oxygen delivery

Remember that haemoglobin and cardiac output as well as the lungs mediate the delivery of oxygen to tissues. If you have a patient with pneumonia who is severely dehydrated, you can improve the delivery of oxygen by giving fluid more than you can by giving oxygen. This is because the resulting increased cardiac output has a sizeable effect. The equation for the delivery of oxygen (in millilitres of oxygen per minute) is haemoglobin concentration (g/dl) * oxygen saturation of haemoglobin * 1.3 * 10 * cardiac output. Each gram/dl of Hb carries 1.3 ml of oxygen when fully saturated. The factor of 10 converts decilitres to litres.

Examples of situations

You are called to see a 60 year old woman with COPD who has been admitted to the ward with pneumonia. She has deteriorated and her vital signs are as follows: drowsy, temperature 34^oC, blood pressure 80/60 mm Hg, pulse 110 beats/min, respiratory rate 30 breaths/min with feeble chest movements. The nursing staff have learned the A, B, C, D, E system of managing critically ill patients,³ and they have given 15 l/min oxygen via a reservoir bag mask before calling for help. A doctor changed her oxygen to a 28% Venturi mask because of the high PaCO₂ and history of COPD. Her oxygen saturations fall from 92% to 55%. Arterial blood gases show pH 7.2, PaCO₂ 9.0 kPa (69.2 mm Hg), PaO₂ 4.5 kPa (46 mm Hg), standard bicarbonate 26 mmol/l. What would your management be?

Answer

Although this patient has a diagnosis of COPD, this is a peri-arrest situation. The problem here is ventilatory failure, because you can see that the chest is hardly moving. Allowing the patient to be so hypoxaemic in this situation is illogical. You should also note that the standard bicarbonate is relatively normal in this woman, suggesting that the rise in PaCO₂ is an acute problem (if not, she has a metabolic acidosis as well--see the next article on blood gases). Remember the A, B, C, D, E system,³ give a high concentration of oxygen and call for help as soon as possible. Consider using a bag and mask. This patient requires ventilation.

Key points

Nicola Cooper specialist registrar in general internal medicine and care of the elderly, St James's University Hospital, Leeds
Email: nacooper@doctors.org.uk

1. Calverley PMA. Oxygen-induced hypercapnia revisited. Lancet 2000;356:1538-39.
2. Robinson TD, Freiberg DB, Regnis JA, Young IH. The role of hypoventilation and ventilation-perfusion redistribution in oxygen-induced hypercapnia during acute exacerbations of COPD. Am J Respir Crit Care Med 2000;161:1524-9.
3. Cooper N. Acute medicine: can you recognise a critically ill person? studentBMJ 2003;12:12-3. (January.)

See how well you do in this simple oxygen quiz

Questions

(1)Can you name each mask?

A 2 l/min


B 15 l/min


C 15 l/min


D 8 l/min

(2)What concentration of oxygen (in %) does each mask deliver, at the flow rate shown?

(3)Which mask is most appropriate for:
(a) Acute severe asthma
(b) A well postoperative patient
(c) Cardiogenic shock
(d) An acute exacerbation of COPD (chronic obstructive pulmonary disease)

(4)How much oxygen may be given in severe COPD?

Answers

A 2 l/minC 15 l/min
(1)A=nasal cannulas;
B=simple face mask (also called a Hudson or MC mask);
C=mask with a reservoir bag (or non-rebreathe bag);
D=Venturi mask

(2)A=variable (25%-40%);
B=variable (about 50%);
C=variable (60%-70%);
D=40%, as the label says

(3)(a)=C;
(b)=A;
(c)=B or C;
(d)=D

(4) It depends, there is no limit
B 15 l/minD 8 l/min

This series is based on the book Cooper N, Cramp P. Essential guide to acute care, London: BMJ Books, 2003 (ISBN 0 727 91 648 3.)'

The series so far

• Recognising critical illness. (Jan 2004) www.studentbmj.com/back_issues/ 0104/12.html

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