1 Ankle Brachial Pressure Index

Transcription

1 1 Ankle Brachial Pressure Index Stuart Wind Video Time 3 mins 42 s Overview The Ankle-Brachial Pressure Index (ABPI) is a very useful non-invasive test which can quickly identify the presence and severity of peripheral arterial disease. As a result it s widely used by clinicians, and whilst different hospitals have different teams responsible for doing the test itself, such as vascular scientists or vascular nurses, at some point the task of performing an ABPI will fall to you. The other advantage with this test over some of the others mentioned in this book is that you can practise on a colleague. This is helpful as it frequently pops up in OSCEs. Performing it is easy; being able to look slick and perform the test accurately and reproducibly is another matter; this requires a little practice. Don t worry, the ABPI is a very simple test and can be fun to perform, since it often brings not only a sense of relief, but often some amusement for the patient who, upon hearing the amplified signal from the handheld Doppler, will realise that they are in fact alive with a strange sound in their leg. Nevertheless there are a number of pitfalls which you can fall into for instance with inverted champagne bottle shaped and oedematous painful legs, so please read on carefully. Procedure Make sure you have the correct patient, for the correct procedure. Check for any allergies (unlikely to be an issue in this procedure), and that you have confirmed the indication; excluded any contraindications; explained the procedure and taken consent. Now wash your hands! Assemble all of your equipment on a trolley away from the patient. You will need everything in the equipment textbox. Make sure that you have the appropriate COPYRIGHTED MATERIAL How to Perform Clinical Procedures, First Edition. Matthew Stephenson, Joshua Shur and John Black John Wiley & Sons, Ltd. Published 2013 by John Wiley & Sons, Ltd.

2 Indications Contraindications Complications Screening for or grading peripheral arterial disease Determining if the patient is suitable for compression bandaging (e.g. in chronic venous insufficiency) Ruling out arterial contribution to ulceration Assessing degree of success of revascularisation procedures Patients with excessive lower limb pain (e.g. reflex sympathetic dystrophy, severe circumferential ulceration, cellulitis) (relative) Recent (<6 months) DVT (relative) Nil significant kit for your patient, for example correct-sized blood pressure cuff. If the patient has any open wounds or ulcers you can cover these with dressings or sterile cling film as the test can be inaccurate if performed over bandaging. Emphasising the importance of lying flat should increase compliance in patients who have trouble being supine for long due to heart failure for instance. Being flat will avoid errors due to changes in blood pressure introduced by posture. There are few patients who can t lie flat for just a few moments but those who can t can have an extra pillow or the head end of the couch raised slightly. Furthermore, especially for the occasions when a patient has walked from afar to see you, make sure that they have rested for at least 10 minutes before commencing the test so that systolic blood pressure stabilises. Firstly you should take the brachial systolic pressure as this will give you Equipment you may need Handheld Doppler with a continuous wave transducer 8-10 MHz this is a standard Doppler probe found in most hospitals Coupling media, i.e. ultrasound gel Standard sphygmomanometer with a selection of different sized pressure cuffs Optional clingfilm/simple dressings (if you need to cover open wounds) an idea of what to expect at the ankle. The systolic pressure is the only important one, not the diastolic pressure. It s helpful to identify the brachial artery near the small groove at the medial edge of the distal biceps muscle; you should be able to palpate the artery here, and it s easier if the elbow is completely extended. Place the cuff high up on the upper limb aligning the air bladder (indicated on the cuff by an arrow) over the brachial artery. When you look at the cuff on the arm, approximately two- thirds of it should 2 How to Perform Clinical Procedures

3 This is the handheld Doppler machine with an arrow pointing to the volume control. His thumb is on the on/off button Poor Doppler angle (poor signal) Good Doppler angle (45 60 degrees to flow) Good signal Direction of flow The optimal angle for Doppler insonation be encased, if not then a larger cuff may be required. Now you re ready to insonate the artery with the handheld Doppler (insonate means to expose to ultrasound waves, that is, put the probe over the artery and turn it on). Now, about this machine it s incredibly easy to use, don t be scared of it. It only has one button (on/off switch) and one dial (volume control) that s it, you don t have to be a professor of vascular surgery to use one. Turn the machine on and cover the end of the transducer with ultrasound gel. Place the end over the artery facing into the anticipated direction of flow with a angle. You will know when you have found the artery because you will hear a distinctive pulsating sound, if you accidentally find a vein the sound could be described as similar to a windy storm. You may need to search a little to hear the sound clearly and in order to do so track the beam across the anticubital fossa medially or laterally (see the following Tip textbox for more on finding the arteries). Whilst you re principally concerned with the presence or absence of an arterial sound for the ABPI, it s also very helpful to understand the sound you re listening to, also called the waveform see later for more details. Please also note that the final ABPI result will be based on the assumption that the brachial systolic pressure is representative of the systemic pressure, therefore if the brachial artery waveform is not normal (for example if the patient has a subclavian artery stenosis) then it s a good idea to check the opposite brachial pressure and waveform and make a note of both readings, taking the highest as the most representative of the systemic pressure. Now you re ready to inflate the blood pressure cuff and occlude the artery. Whilst pumping try to keep the probe completely still to avoid losing the signal. Just before the artery occludes there is often a whoosh sound, which will give you a clue as to whether you ve slipped off the artery. It s easy to slip off the artery if you do don t worry just release some of Ankle Brachial Pressure Index 3

4 Tip One hand pumps up the sphygmomanometer, the other holds the Doppler probe in position. Looking slick at doing all of this takes more practice than you think the air from the cuff and readjust the probe until the signal returns, then adjust a little more so the signal is optimised. Once you can no longer hear a signal, the artery is occluded, inflate for a further mmhg to ensure it is completely occluded. Now the cuff should be deflated, but slowly, around 2 mmhg per heartbeat to ensure accuracy of the measurement. When the systolic blood pressure is reached the signal should return suddenly; at this point make a note of the pressure. The pressure usually returns a little lower than where it disappeared on inflation; it is this deflationary number you re interested in. Finally you should gradually release the cuff to restore blood flow to the arm. You re now ready to take the ankle pressure. The ankle pressure is taken from two of the three main arteries supplying blood to the foot: the dorsalis pedis artery (DPA) and the posterior tibial artery (PTA) (the third is the peroneal artery but you don t use this for the ABPI). The DPA can be located running Finding the arteries 1 Do not move the probe too quickly as you may skim over the artery in diastole and not hear the signal 2 Try to use your mind s eye to imagine the beam coming from the transducer. It will help you search for those elusive signals and help you to avoid making rapid sweeping movements by angling the probe too quickly 3 Making very fine adjustments in angle and probe location once a signal has been found will allow you to optimise the sound thus facilitate waveform identification and also help you to realise when you are slipping off the artery when inflating the cuff just laterally to the extensor hallucis longus tendon on the proximal half of the dorsal foot surface. The PTA is usually found just behind the medial malleolus. Now apply the cuff, it should be positioned so that the bottom of the cuff is just above malleolar level. Make sure that the edges of the cuff are parallel rather than skewed. What this means is that even if the leg is not cylindrical (which of course it rarely is), the cuff still should be, even if it means that it leaves a free space at the bottom of the cuff. Check the size is correct (i.e. two-thirds of the leg completely encased). When the cuff is applied properly take and record the pressures in the same way as for the brachial artery, remembering to use the same technique described above for finding the arteries. Now you have all the systolic blood pressure readings it s time to work out the 4 How to Perform Clinical Procedures

5 The dorsalis pedis artery The posterior tibial artery ABPI. The highest pressure of the two arteries in each foot is used in the calculation, you can discard the lower number: ABPI ( ) ( ) Anklepressure mm Hg = Brachialpressure mm Hg The following table gives an idea of the meaning of the ABPI clinically: Resting ABPI Disease severity 1.4 or greater Calcification likely Not suggestive of arterial disease Suggests minor arterial disease (likely causing claudication) Severe occlusive disease Less than 0.3 Critical ischaemia with likely rest pain and tissue loss Other notes Waveforms In some patients the lower limb arteries may become calcified due to disease, this is particularly common in patients with diabetes and chronic kidney failure. The calcification causes the arteries to stiffen and hence they will compress at a higher pressure than the true pressure leading to an artifactually raised ABPI. This is important to know as it means that in a diabetic patient with an ABPI of 0.8, it may in fact realistically be more like 0.4, the difference of which has clinical significance (see table). Luckily for us there is another way to assess whether there is healthy blood flow, by using the sound of the arterial pulse from the handheld Doppler otherwise known as the arterial waveforms. There are three main types of arterial waveform, represented as different audible sounds: Triphasic: Each pulse has three phases to it, a large higher pitch sound followed by a lower pitch sound then a second higher pitch sound which is often quieter and lower than the original. This is the sound of a normal healthy artery. Biphasic: Each pulse has two phases a higher pitch followed by a lower pitch. Biphasic flow is often found in patients with minor peripheral arterial disease, or it can be normal. Monophasic: Each pulse has one phase to it, this is seen in severe cases Ankle Brachial Pressure Index 5

6 Velocity m/s Triphasic Biphasic Monophasic Time Visual display of the waveforms of peripheral arterial disease. The sound may be described as either pulsatile or damped. Absent: Due to occlusion of the artery, or occasionally congenital absence (approx 2 3% of the population for the DPA for example). If a patient has a waveform which suggests peripheral arterial disease but a normal ABPI, the result should be considered suspiciously. However patients with an ABPI 1.4 are considered to have diffusely calcified arteries which may or may not be clinically symptomatic. OSCE checklist Assembles equipment correctly Positions patient correctly Exposes limbs Applies BP cuff to arm Applies ultrasound gel to probe or artery Insonates brachial artery Inflates the BP cuff until the signal disappears Inflates by a further mmhg Deflates gradually and notes systolic Applies BP cuff to ankle Applies gel over probe or artery Insonates dorsalis pedis Inflates BP cuff until signal disappears Inflates by a further mmhg Deflates gradually and notes systolic Repeats with posterior tibial artery Repeats with the other leg Wipes off gel and advises to re-dress Calculates ABPI Washes hands Thanks patient Documents procedure Provides postprocedure advice 6 How to Perform Clinical Procedures