Anaesthesia explained

Hypoxia kills.

As the house officer, you must prevent your patients from turning blue. Having to manage the patient with acute hypoxia causes anxiety among many doctors. We hope that this article will dispel common misconceptions that you may have about hypoxia and its treatment and give you the confidence to carefully prescribe oxygen (yes it’s a drug) for those in need.

Oxygen—one of life’s little necessities

All metabolising cells in carbon based life forms need oxygen to metabolise energy rich substrates to maintain cellular integri- ty and function. Without it, the cell turns from aerobic to the vastly inefficient anaerobic metabolism. Only one eighth of the energy is produced at the expense of producing a lot of lactic acid. Cells do notfunction well in an acidic environment and tend to die.
Our dependence on oxygen has its own inherent problems. Firstly, most of our cells are far away from a reliable source (the air). It is also fairly insoluble in water.
To help overcome these problems, we have a transport network (the cardiovascular system) for getting the oxygen from the air to the cells via the lungs. Unfortunately, the liquid that circulates in this system is water based. However, the oxygen is carried by a specialised protein which it has a high affinity for haemoglobin (see "Anaesthesia explained: oxygen-haemoglobin dissociation curve" on www.studentbmj.com).

Carbon dioxide stimulates breathing

Under normal circumstances, hypoxia is not a stimulus to make you breathe. It is the partial pressure of carbon dioxide that stimulates the respiratory centre. Receptors sensitive to carbon dioxide are found in the medulla and are bathed in cerebrospinal fluid, which has fewer proteins than plasma and little buffering ability so any minute changes in the partial pressure of carbon dioxide are rapidly sensed at the receptors and prompt changes in the respiratory pattern.
A low partial pressure of oxygen in the blood (PaO 2 ) will stimulate breathing only via the peripheral chemoreceptors (carotid and aortic bodies) when the PaO 2 is less than 8kPa. The arterial oxygen levels are then well below the safety range for adequate tissue oxygenation (see “Anaesthesia explained: oxygen-haemoglobin dissociation curve” at www.studentbmj.com). This is a last ditch attempt used by the body to stimulate breathing

Understanding the oxygen cascade

Oxygen transport from the air to the mitochondria of individual cells occurs as a series of steps. The oxygen cascade refers to the progressive decrease in the partial pressure of oxygen (PO 2 ) that occurs from the air (21kPa) to the cells (0.5-3kPa) (see "Oxygen cascade" web page).

Respiratory failure

This is a failure of oxygenation. Its causes may be remembered by consideringthe different steps of the oxygen cascade (see table 1).

Identifying patients at risk of hypoxia

The aetiology classes for hypoxia given in table 2 is fine for written exam answers or to remind you of the many possible diagnoses when dealing with a patient with hypoxia. It is important, however, to identify and pay attention to those at risk so that you may prescribe oxygen (yes, it needs to be written up on the drug chart) and keep your patients pink. Do not forget to prescribe adequate analgesia as painful wounds cause patients to take inefficient breaths, particularly after abdominal or thoracic surgery. Others at risk include older people, smokers, and people with a chest infection.

Hypoxic respiratory drive

There is a group of patients, who represent a small subset of those with chronic lung disease. As a result of long standing lung damage, their alveolar ventilation is inadequate and they tolerate grossly abnormal arterial blood gases. The central chemoreceptors become tolerant of a high partial pressure of carbon dioxide and the kidneys compensate for the respiratory acidosis by retaining bicarbonate (HCO 3 ) so the arterial pH is about normal. These patients rely on hypoxia to make them breath (hypoxic respiratory drive). If such a patient is given too much oxygen respiratory drive will be lost. He or she will not breathe adequately, and the partial pressure of carbon dioxide in arterial blood may rise to dangerous levels caus-ing progressive loss of consciousness and eventually apnoea. Once the patient stops breathing, hypoxia returns but it is not adequate to overcome the depressive effects of the high CO 2 levels, and unless the patient is artificially ventilated he or she may die.
If you think the patient has hypoxic drive he or she may still benefit from oxygen treatment, but it must be given in a controlled manner. Venturi masks are the delivery method of choice (see below)

General management of hypoxia

Appropriate management of hypoxia depends on treating the underlying cause while providing supplemental oxygen as necessary.

Physiotherapy

Alveolar hypoventilation and collapse can be corrected by mobilisation and drainage of secretions. Posture is important—sitting the patient up enhances breathing by increasing diaphragmatic descent and therefore tidal volume.

Giving supplemental oxygen

There are two types of oxygen delivery system—variable performance and fixed performance.

Variable performance oxygen delivery systems

These are called variable because it is impossible to predict the true inspired oxygen concentration (FiO 2 ). Although the system delivers oxygen at a given rate, the concentration delivered is dependent on the patient’s pattern of breathing (respiratory rate and tidal volume). If the patient is taking shallow, rapid breaths then theconcentration of inspired oxygen will be similar to that of air; if he or she takes deep, slow breaths then it will be much higher.
This system should be used with extreme caution (if at all) in patients needing a precisely delivered inspired oxygen concentration, such as those who depend on hypoxia to drive their breathing.
Simple face mask—for example, Hudson â mask: Oxygen from the supply passes via a tube into the mask where it mixes withoxygen of the ambient air drawn in through holes in the side of the mask. This system cannot deliver very high concentrations of oxygen, and often gives the patient a sensation of being smothered when worn.
Non-rebreathing mask with reservoir bag: A reservoir bag improves the maximum FiO2 to up to 60%. The reservoir fills up with oxygen during expiration and is breathed in during inspiration.
Nasal cannulae: Oxygen passes through two prongs that are inserted just inside the anterior nares and are supported on a light frame. Oxygen can be comfortably supplied at a rate of 1-4 l/min resulting in a FiO 2 up to 30%. Cannulae are generally well tolerated: there is no discomfort of a mask and the patient can talk, eat, and have access to their face. This is important when oxygen needs to be given continuously for long periods, such as for patients with advanced pulmonary disease. However, they are a variable performance system and the dry gas may cause crusting of nasal mucosa secretions. Oxygen should be humidified if possible.

Fixed performance oxygen delivery systems

These systems deliver a precise concentration of inspired oxygen (FiO 2 ) which is unaffected by the patient’s breathing pattern.
High airflow enrichment masks—for example, Venturi masks: These masks use the Venturi principle: oxygen enters the mask as a jet and entrains a constant flow of airvia holes at the bottom of the mask. When used correctly they will deliver a known FiO 2 greater than the patient’s inspiratory flow rate (typically 35 l/min) so the patient does not inspire air from outside the mask. Precise FiO 2 of 24%, 28% and 35% can be achieved. These masks are used for treating patients requiring controlled oxygen therapy.

What is controlled oxygen treatment?

Controlled oxygen treatment is used in those who need supplemental oxygen but rely on their hypoxic drive to continue breathing. The concentration of oxygen is progressively increased stepwise—for example, by using Venturi masks—and arterial blood gases are measured after 10 minutes. If the patient’s PaO 2 goes up without an accompanying increase in PaCO 2 it is safe to proceed to the next concentration up. If you see an increase in PaCO 2 it is a sign that he or she is going into respiratory failure because you have removed their hypoxic drive to breathe. Go back to the previous inspired oxygen concentration and prescribe this concentration on the drug chart to be given continuously.

The rules

(1) Give oxygen to the patient with hypoxia. Give as much as you can at first, and then reduce it, guided by blood gas measurements.
(2) Oxygen treatment will work only if the patient has a patent airway and is breathing. If there are problems withairway and breathing which do not respond to basic life support measures, involve someone who can sort them out (usually the anaesthetist or the team from your intensive care unit).
(3) Definitive treatment of hypoxia depends on the underlying cause. Giving oxygen is a holding measure.

1 Treacher DF, Leach RM. Oxygen transport—basic principles. BMJ 1998;317:1302-6.
2 Leach RM, Treacher DF. Oxygen transport—tissue hypoxia. BMJ 1998;317:1370-3.
3 West JB. Respiratory physiology—the essentials. Baltimore:Williams and Wilkins, 1985.

Review questions

(1) What stimulates you to breathe?
(2) What are the indications for giving patient oxygen?
(3) What are the problems associated with using a simple facemask?
(4) What simple measures, apart from oxygen treatment, should you take when called to see a patient who has hypoxia after abdominal surgery?

Answers

(1) Partial pressure of oxygen in arterial blood (PaCO 2 ).
(2) Blue patient; hypoxaemia as measured by arterial blood gases; hypoxia as measured by pulse oximetry; chest infection; after major surgery—in the postoperative period; during a cardiac and respiratory arrest; hypotension (systolic BP <100 mmHg); myocardial infarction or ischaemia; high oxygen demand—sepsis, shivering.
(3a) A simple face mask is a variable performance system (it cannot deliver a precise FiO 2 ) so can be dangerous in people with chronic lung disease and a hypoxic drive to breathe.
(3b) Face masks are poorly tolerated: the patient may have the sensation of being smothered by the mask; it leaves a dry mouth and nose because of high dry gas flow; patients feel as if they are in a wind tunnel; patients are unable to eat, drink, or talk while the mask is in place
(4) Sit the patients up, relieve their pain, and check that tight dressings or abdominal distension are not impeding breathing.

Nina Ruth Lewis, Surgical house officer, King’s Mill Hospital, Mansfield

Jo Fitz-Henry, consultant anaesthetist, King’s Mill Hospital, Mansfield


studentBMJ 2001;09:85-128 April ISSN 0966-6494