VOL: 98, ISSUE: 40, PAGE NO: 53
Gary Porter-Jones, BSc, RGN, DipN, is respiratory specialist nurse, Ysbyty Gwynedd Hospital, Bangor, Wales
Regardless of the setting in which oxygen is delivered, it should be regarded as a drug. Its potency in treating hypoxaemia (a low concentration of oxygen in the blood) is often underestimated and, if given inappropriately, it can be lethal (Dodd et al, 2000). Patients must receive this therapy in an appropriate, safe and comfortable way. This depends on a sound understanding of why oxygen is being delivered, the methods of oxygen delivery and the nursing needs of the patient receiving it (Box 1).
Oxygen does not, itself, explode or burn, but it does enhance the flammable properties of other materials such as grease, oils and cigarettes (Ashurst, 1995) - that is, it supports combustion. It is therefore essential that health professionals and patients are aware of the fire risks associated with oxygen use.
Providing optimal oxygen therapy
Acutely breathless patients
It is crucial to provide optimal oxygen therapy to the acutely breathless patient, and for most patients the major risk is giving too little oxygen (Murphy et al, 2001). Insufficient oxygen therapy can lead to cardiac arrhythmias, tissue damage, renal damage and ultimately cerebral damage.
For example, most acutely breathless patients attended to by ambulance staff will have conditions such as asthma, heart failure, pneumonia, pleural effusions, pulmonary embolism or pneumothorax, and some may be victims of major trauma (Murphy et al, 2001). These patients will require a high concentration oxygen therapy (40%-60% in most cases, but some may require higher concentrations from a non-rebreathing mask), and this may need to be continued in hospital.
Some patients with COPD who experience an exacerbation of their condition are more at risk of death from hypoxia (a deficiency of oxygen in the tissues) than from hypercapnia (a high concentration of carbon dioxide in the blood) (Nerlich, 1997).
Patients with hypoxic drive
Some patients must not receive high concentrations of oxygen, as this can be lethal. Typically, these are patients with chronic obstructive pulmonary disease (COPD) who have a reduced sensitivity to the circulating blood CO2 level that is normally the main driver of respiration. In these patients it is the level of circulating oxygen (a hypoxic drive) rather than CO2 that stimulates their respiration. This is because their CO2 level has gradually been increasing during the course of this chronic disease.
Administering oxygen in too high a concentration to these patients will depress their respiratory drive because the need for oxygen is being met. This can lead to a further and increasingly dangerous rise in their circulating blood CO2, resulting in CO2 narcosis and then death.
Not all patients with COPD fall into this hypoxic drive category, and the only way to determine this is by sampling blood gases, either arterial blood gases (ABG), or through less painful methods such as capillary sampling - often taken from the ear lobe.
Capillary sampling is not used as often as it should be, but results correlate well with arterial sampling, and it is a more comfortable procedure for the patient (Pitkin et al, 1994; Dar et al, 1995).
Until the results of ABG or capillary sampling are established, patients known to have COPD and requiring oxygen therapy should be given oxygen at 24-28% initially, with blood gases determining any alteration to this concentration. The patient should be closely monitored.
Oxygen delivery devices
The patient’s condition and diagnosis should always dictate the delivery device used.
The terminology used to describe oxygen delivery systems is often confusing. They are essentially of two types - low-flow or high-flow devices. Low-flow devices provide variable or uncontrolled oxygen concentrations, while high-flow devices provide fixed or controlled oxygen concentrations.
Variables affecting the amount of oxygen the patient receives
What determines the amount of delivered oxygen which the patient actually takes into the lungs? There are a number of variables to consider:
- Room air contains 21% oxygen, so this is always the minimum that is available to the patient without supplementary oxygen;
- The system used to deliver the oxygen plays an important role;
- The patient’s breathing pattern: depth and rate (ventilatory minute volume - MV) which is the total volume of air breathed in and out in one minute;
- The ventilatory MV can alter from one breath to the next in the same patient;
- The flow rate set at the oxygen outlet port (providing 0-15 litres/minute of pure dry oxygen).
As there are a number of variables determining the amount of oxygen the patient actually receives, as many as possible need to be controlled to deliver a known and accurate concentration of oxygen in a controlled manner (such as in the hypercapnic COPD patient). In the other patients, in whom a strictly accurate FiO2 (fraction of oxygen in inspired gas) is not so important, a device that provides uncontrolled or variable oxygen can be used.
Simple masks - often referred to as medium concentration (MC) or variable performance masks
With this type of delivery device (Fig 1), the concentration of oxygen delivered depends on the patient’s breathing rate and depth, and each breath is diluted by air drawn in from the atmosphere in a manner dependent on the patient’s breathing pattern. This is because the average adult patient has a peak inspiratory flow rate (PIFR) that is greater than the range of settings on the flow meter at the oxygen outlet port (these usually only go up to 15 litres/minute).
Every breath in inhales more gas than is flowing from the oxygen flow meter, so the balance is sucked in from the atmosphere. Therefore, 100% oxygen from the outlet port is being diluted with 21% oxygen from the air sucked in through the holes in the mask and around the mask, as it is not an airtight fit. However, this happens in a variable way because the patient’s minute volume is variable. This makes the concentration of oxygen being inspired by the patient variable from one breath to the next.
For example, giving a patient oxygen at two litres per minute via a variable device provides anything between 24% and 35% oxygen concentration, depending on each individual inspiration (Bazuaye et al, 1992). Furthermore, when the flow of oxygen is set low at the outlet port (for example, below five litres per minute) there is insufficient flow to flush out from the mask all of the CO2 that the patient expires with each breath, so there is rebreathing of some of the CO2 that has accumulated in the mask. Turning the flow rate up in an attempt to flush out the CO2 would result in too high an FiO2 for hypercapnic COPD patients. This is a factor that makes these systems unsuitable for patients with type II respiratory failure (low blood oxygen concentration with a raised CO2) (Bateman and Leach, 1998).
These masks are suitable for patients when it is not important to know the precise oxygen concentration - for example, during postoperative recovery, patients with angina, cardiomyopathy, myocardial infarction and some patients with respiratory disease. However, some argue that they are of limited use (Foss, 1990).
The normal flow rate of oxygen is usually six to 10 litres per minute and provides a concentration of oxygen between 40-60%. This is why they are often referred to as MC (medium concentration) masks, as 40%-60% is considered to be a medium concentration of oxygen. It is unlikely that the FiO2 will increase if the flow rate is increased above 10 litres per minute, and a non-rebreathing mask should be considered if a higher FiO2 is desired (Nerlich 1997).
The manufacturers of these masks usually provide guidance on suggested flow rate settings and the resultant ‘approximate’ FiO2 on the packaging.
Not all patients can tolerate a mask or they may find it inconvenient, as it covers most of their face. In this situation, nasal prongs (also referred to as nasal cannulae or specula) are a useful alternative (Box 2).
Nasal prongs are convenient and simple to use and are generally considered by patients to be comfortable and less claustrophobic. They allow patients to talk and eat without interrupting their oxygen therapy. Some patients can also continue to receive oxygen in this way while they are receiving nebulised bronchodilators via an air compressor.
Nasal prongs are low-flow or variable devices, so the exact FiO2 is not known. Typically used at a flow rate of one to four litres per minute, they can deliver an oxygen concentration between 24-40%.
If the flow rate is increased to six litres per minute or more, discomfort from dried mucous membranes results, with little enhancement of FiO2. This is because at six litres per minute the anatomic reservoir (oropharynx and nasopharynx) is already full, so there is no appreciable increase in FiO2.
It is important that patients have patent nasal passages and that the prongs are correctly fitted, if they are to benefit from oxygen delivered by this method (Fig 2). Patients who are mouth breathers - and most adults are (Bolgiano et al, 1990) - can still benefit from nasal prongs. Airflow in the oropharynx will pull oxygen from the nasopharynx but the FiO2 may be lower than if they were nose-breathing. Either way, it is only possible to ‘estimate’ the FiO2, as this is a variable device.
Other low-flow masks
Other low-flow masks that deliver a variable concentration of oxygen include the non-rebreathing mask, which is often found in ambulances and A&E departments.
Fixed-performance masks (also called Venturi masks, high-airflow-with-oxygen enrichment masks, controlled oxygen masks or air-entrainment masks)
Some patients require low concentrations of oxygen and knowing the exact FiO2, and keeping this constant are important. Fixed-performance masks are the devices of choice in this situation.
The fixed-performance mask incorporates a Venturi device (Fig 3) that keeps the oxygen concentration constant regardless of the oxygen flow rate or the patient’s breathing pattern (minute volume). Venturi devices come as individual colour-coded barrels that are attached to a suitable mask (such as a Ventimask). The barrel used depends on the oxygen concentration required and ranges from 24-60%.
There are also adjustable Venturi devices with a dial that is turned to provide the desired FiO2 at the given flow rate.
Venturi devices maintain a constant and precise concentration because they have a plastic body with a small jet hole through their middle. The body of the Venturi also has holes through which air can pass. As the oxygen from the outlet port is driven through the small jet hole its velocity increases, the pressure around it drops and it entrains (draws in) room air through the holes in the body of the device (this is a basic law of physics known as Bernoulli’s principle).
This room air (containing 21% oxygen) mixes with the 100% oxygen being driven through the jet and dilutes it to the concentration written on the side of the colour-coded Venturi barrel. It keeps this concentration constant regardless of the flow rate because, if the flow rate at the outlet port is increased, so too is its velocity at the jet. As this happens, the pressure around the jet drops and it entrains more room air (Bernoulli’s principle), thus maintaining the desired dilution.
The entrainment of room air and its addition to the oxygen flow increases the overall flow to the patient (this is why they are called high-flow devices). The flow delivered is two to three times more than the patient requires for breathing each minute (this high flow also helps to flush out expired CO2 from the mask so that rebreathing does not occur).
The minimum flow rate required to deliver the given concentration of oxygen is also written on the Venturi barrel.
Some breathless patients with high respiratory rates may be more comfortable and better oxygenated if the flow rate is set above the minimum recommended flow rate on the Venturi. This will not harm the patient because the FiO2 remains the same but the flow rate can be increased in order to exceed the patient’s peak inspiratory flow rate (Murphy et al, 2001).
If the flow rate at the outlet port is set below the minimum recommended on the Venturi barrel the patient still receives the given concentration, but with a reduced flow. A hyperventilating patient with a high peak inspiratory flow rate may entrain room air (thus diluting the concentration), so nurses should always set the flow rate to at least the minimum recommended on the Venturi barrel.
A Ventimask is a large-capacity (280ml) mask that attaches to a Venturi barrel. There is evidence to suggest that a large-volume Ventimask is more reliable in ensuring a constant FiO2 than smaller capacity Venturi masks (Cox and Gillbe, 1981).
Other high-flow systems
Other high-flow systems include large volume air entraining nebulisers/humidifiers, which operate on the same principle.
Oxygen therapy can dry the mucous membrane of the upper respiratory tract (URT), causing soreness. It can also cause pulmonary secretions to become stickier, making them difficult to expectorate. The patient may also feel generally dehydrated. Nurses should always consider humidification for patients requiring prolonged oxygen therapy and for those requiring a high FiO2. At lower flow rates (for example, up to four litres per minute) the URT provides enough humidification and, unless contraindicated, the patient should also be encouraged to drink more fluids.
Nurses should be aware that humidification alters the oxygen concentration provided by a Venturi mask, as the water vapour may condense in the jet hole, thus altering the FiO2 (Bolgiano et al, 1990; Calianno et al, 1995). Sterile water should always be used and changed daily to reduce the risk of infection. Although cold water can be used, devices are available for producing warm humidification, which is more effective.
Assessing the effectiveness of oxygen therapy
As with any intervention, evaluating the effectiveness of oxygen therapy is essential. Arterial oxygen saturation (SpO2), measured by pulse oximetry, and the arterial partial pressure of oxygen (PaO2), measured by blood gas analysis, remain the principal clinical indicators for initiating, monitoring, and adjusting oxygen therapy (Bateman and Leach, 1998).
While measuring SpO2 is useful in monitoring the state of oxygenation (and the trend of readings is more valuable than one-off readings), only blood gas analysis provides accurate information on the pH, PaO2 and PaCO2. This is why it is considered the gold standard in evaluating effectiveness of oxygen therapy (Howell, 2001).