Observation and monitoring of the patient requiring respiratory care
One should always be sure when assessing the patient’s respiratory function that first and foremost their airway is clear. Never forget the ABCDE approach to patient assessment. If their airway is not clear then this should be your priority before you go on to assess their respiratory function.
Common causes of airway obstruction would include food, vomit, inflammation, and in the patient with neurological impairment the tongue.
In order to be able to intervene appropriately it is important that the practitioner is able to do a proper and thorough assessment of the patient’s respiratory status at regular intervals.
There are a number of features which can be observed that may give the practitioner clues as to the patient’s respiratory well-being.
Normal respiratory observations
Firstly one needs to know the normal parameters for the respiratory observations. One should always bear in mind during such observation that the patient is an individual, and consequently may have a past medical history which will require the practitioner to adjust what they consider to be normal.
The normal respiratory rate vary slightly with age and gender. The range one would expect to see would be between 12 to 16 breaths per minute. The patient may be tachypnoeic which means that they are breathing faster than this. They may be having difficulty in breathing which is known as dyspnoea or there may be absence of breathing known as apnoea.
As well as the rate one also needs to assess the depth and pattern of their breathing. One should observe the chest wall to see that it is rising and falling equally and that the expansion looks good. A patient may be taking shallow breaths for a number of reasons, including pain, neurological illness, and possible splinting from the abdomen. Shallow breathing is often an ineffective way of breathing as the patient may not be exhaling their carbon dioxide effectively.
The practitioner should also look for the pattern of breathing will stop in normal breathing the diaphragm will only move a small amount, and the accessory muscles in the neck will not be used at all. However if the patient is having difficulty in breathing they will utilise their diaphragm which causes the abdomen to move outwards, and they will also utilise their access three muscle is helping to lift the thoracic cavity upwards. The patient breathing in this way is struggling for breath and probably need some assistance.
Skin and Colour
The practitioner should also assess the patient’s skin colour examining them for any signs of peripheral or central cyanosis. Peripheral cyanosis will manifest itself in the fingertips which can appear slightly blue. Central cyanosis, which is a much more serious condition as this means their vital organs may also be being deprived of oxygen, will show itself as blue lips and when you look under the tongue this will also appear blue.
The patient may also seem clammy and pale, which might be an indication of the degree of sepsis, a serious condition which need attending to immediately.
An often used indicator of how breathless the patient is is to ascertain how well they can speak. Can they speak in full sentences? Can they only complete short sentences, or are they only able to say one or two words at a time due to their breathlessness?
Finally one also needs to listen to some of the added sounds the patient may be making whilst breathing. Normal breathing is silent. The patient who is unwell may be making one of several sounds whilst trying to breathe.
A stridor may occur when there is inflammation or obstruction above the larynx. This can result in an inspiratory sound which is rather like the patient is being strangled. If you hear this you need to get help immediately.
A wheeze however can indicate inflammation or obstruction below the larynx. It is a common symptom for the asthmatic patient, or those with an acute bronchospasm. This is most often a musical like sound heard during exploration.
If the patient has a lot of secretions in their chest, possibly because of a chest infection, you may expect to hear these secretions as they breathe in and out. These often have a crackling like quality to them, and the patient may well be bringing up a lot of sputum.
An easy part of your assessment would be to utilise pulse oximetry.
Pulse oximetry is one of the most widely used monitoring techniques today providing a means on non-invasive, continuous monitoring of oxygen saturation. Its ease of operator use and interpretation ensure that it is commonly used in a variety of settings and, as the human eye seems to be poor at detecting cyanosis (Tooley 2005), provides back up to the practitioners experience. With an awareness of its limitations it can be a powerful tool in patient assessment.
The use of pulse oximetry has been discussed as early as the 1950’s (Stephen RC 1951) and started to be used regularly in clinical practice in the 1980’s (Tremper KK 1989). Today it is commonly used as either a finger or ear probe
Pulse oximetry utilises spectroscopy when assessing the saturation of the blood. As blood changes from oxygenated to deoxygenated it also changes colour and therefore absorbs light of different wavelengths.
The Beer-Lambert law states that there is a linear relationship between absorbance of light and concentration of the solution.
When monochromatic light (light of a specific wavelength) passes through a solution there is a relationship between the concentration of the solution and the intensity of the transmitted light. So, as the concentration of a solution increases, less light is transmitted through it and therefore more light is absorbed. Consequently there is a linear relationship between absorption and concentration whose other constant factors are path length and molar absorptivity (which will depend on the characteristics of the solution in question).
A = Absorbance
ἑ = molar absorbtivity
l = path length
When related to pulse oximetry this means that if one knows the absorbance of the wavelength then the concentration of the oxyheamoglobin can be calculated.
The figure opposite shows the absorption spectra, in the red to infrared range, of oxyheamoglobin and deoxyheamoglobin. The pulse oximeter has two light emitting diodes which transmit alternately at two different wavelengths, 660 nm (red) and 940 nm (infrared). At 660 nm the absorbance of oxyheamoglobin is less that that of deoxyheamoglobin and vice versa at 940 nm. The absorbencies can then be compared at these wavelengths for the saturations to be calculated.
The LED’s are switched alternately at 400MHz with off periods for both. This off period allows the photodetector to account for ambient lighting conditions, which may vary.
It is the pulsatile signal from the arterial blood supply which will supply the information. This is a pulsatile signal and can therefore be thought of as AC signal as compared to the DC signal of the light being transmitted from the surrounding tissues which, due to its non pulsatile contour, is more constant. The signal is processed so that the amplitudes of the two different signals are equalised, then the microprocessor calculates the ratio of absorption of the AC component at 660 nm with the ratio of absorption of the AC component at 940 nm.
Due to the potential different amplitudes of the AC and DC signals at the different wavelengths the processor within the pulse oximeter scales the DC signal to take account of this so that the AC signals can be compared. This normalising of the DC component allows the production of a ratio, L, which forms the X axis of the calibration curve shown below.
This curve has been created using arterial blood samples form healthy subjects, but as it would be unethical to obtain samples form subjects with saturations lower than 80% the curve is extrapolated to the lower values. The ratio is then used with the calibration curve to calculate the saturation.
Within the range of 70% to 100% saturations the reliability of pulse oximetry is believed to be good in the presence of oxyheamoglobin and deoxyheamoglobin. However if carboxyhemoglobin or methamoglobin are present in appreciable amounts then the accuracy may be suspect.
You can see from the graph above that metheamoglobin resembles deoxyheamoglobin at 660nm. A high concentration of MetHb in a hypoxic patient can artificially raise the saturations to 85% and similarly in the patient with 100% saturations the MetHb can cause readings of around 85%. MetHb can become raised with some local anaesthetics and nitrates.
Smokers or victims of carbon monoxide poisoning can have significant levels of carboxyheamoglobin in their blood and this resembles oxyheamoglobin in the red range. As a consequence there can be an overestimation of the saturations in this type of subject.
Because the calibration curve has to be extrapolated saturation values below about 80% are less accurate. However, as the patient is already considered hypoxic long before this saturation level is reached this inaccuracy is considered tolerable.
If vascular tone is changed there may be some effect on the accuracy of the oximetry as the plethysmographic signal may be altered or undetectable. This wave form is an important part of the calculation in order to generate the signal necessary. Hypertension, vasoconstriction, hypovoleamia or sepsis can all reduce the accuracy of the oximeter.
Skin pigmentation does not normally affect the readings whilst nail varnish may. The use of dyes such as methylene blue and indocyanine green can alter the absorption spectrum of haemoglobin.
Pulse oximeters are also prone to error from movement and vibration.