PRACTICAL PROCEDURES Radial arterial lines Introduction arterial line. This is used to determine Intra-arterial cannulae in the radial artery collateral perfusion between the ulnar and are used for invasive arterial blood pressure radial arteries to the hand: poor collateral (IABP) measurement and for collection perfusion is said to be present in 12% of of blood for analysis. The radial artery is people. If ulnar perfusion is poor and a the preferred site for insertion because of cannula occludes the radial artery, blood low complication rates. Arterial lines are flow to the hand may be reduced. The the gold standard for accurate blood pres- test is performed by asking the patient to sure measurement. They may be used in clench their hand. The ulnar and radial intensive care and high dependency units arteries are occluded with digital pressure. and in anaesthetized patients undergoing The hand is unclenched and pressure over surgical procedures. An understanding of the ulnar artery is released. If there is good Figure 1. Two arterial cannulae. basic principles enables arterial lines to be collateral perfusion, the palm should flush used safely in these settings. in less than 6 seconds. In practice the use- the signal. The cannula is connected to fulness of this test is questionable. an arterial giving set. Indications n Arterial giving set. Specialized plas- The indications for a radial arterial line Equipment tic tubing, short and stiff to reduce are: n Arterial cannulae. Made from poly- resonance (see below), connected to a 1. Continuous, beat-to-beat blood pres- tetrafluoroethylene (‘Teflon’) to 500 ml bag of saline. sure measurement. Examples include minimize the risk of clot formation n 500 ml bag of saline. This is pressurized patients on the intensive care unit (Figure 1) they are short, with parallel to 300 mmHg using a pressure bag, i.e. (ICU) requiring inotropic support, or sides to minimize the effect on blood a pressure higher than arterial systolic patients with severe cardiovascular dis- flow distally. A 20G (pink) cannula is pressure to prevent backflow from the ease undergoing surgery. used in adult patients, a 22G (blue) cannula into the giving set. The arte- 2. Frequent arterial blood gas analysis for paediatrics, and a 24G (yellow) for rial giving set and pressurized saline in patients with respiratory failure, or neonates and small babies. Larger gauge incorporate a continuous slow flushing severe acid/base disturbance. cannulae increase the risk of thrombo- system of 3–4 ml per hour to keep the sis, smaller cannulae cause damping of line free from clots. The arterial giving Choice of arterial site The radial artery has low complication Figure 2. Radial arterial line. (Please state where Figure 2 should be cited in the text) rates compared with other sites. It is a superficial artery which aids insertion, and also makes it compressible for haemostasis (AQ This sentaence has been rephrased, is it okay?). The ulnar, brachial, axillary, dorsalis pedis, posterial tibial, femoral arteries are alternatives. Preparation Allen’s test is recommended by many textbooks before the insertion of a radial Figure 3. Direct cannulation of the radial artery. Dr Rachel Hignett is Specialist Registrar in Anaesthetics, Nuffield Department of Anaesthetics, The John Radcliffe Hospital, Oxford and Dr Robert Stephens is Academy of Medical Sciences/the Healthcare Foundation Clinical Research Training Fellow, Institute of Child Health, UCL, London WC1N 1EH Correspondence to: Dr R Stephens British Journal of Hospital Medicine, May 2006, Vol 67, No 5 M3 PRACTICAL PROCEDURES PRACTICAL PROCEDURES set and arterial line should be free from sure alarm limits should be set. Damping and resonance may distort the Davis PD, Parbrook GD, Kenny GNC (1995) Arterial pressure waveform Figure 5 Blood Pressure Measurement in Basic Physics air bubbles. The line is attached to a Thrombosis as a result of radial artery arterial pressure waveform ( ), and and Measurement in Anaesthesia. Butterworth- transducer. occlusion is rarely problematic. Natural Once inserted, an arterial waveform trace lead to inaccurate recording of systolic and Heinemann: 221–32 (AQ Please provide the n Transducer, amplifier and electrical history is recanalization of the arterial should be displayed at all times. This diastolic pressures. Mean arterial pressure town of publication) Moyle JTB, Davey A (1998) Physiological recording equipment. The transducer is lumen: confirms that the invasive arterial BP (MAP) is usually still recorded accurately. Monitoring: Advanced Monitoring Systems. In zeroed and placed level with the heart. n Partial occlusion occurs in 1.5–35%. (AQ Please define in full) monitoring is Any restriction in transmission of the Ward’s Anaesthetic Equipment, Saunders: 293–7 Risk factors for partial occlusion include set up correctly, and minimizes problems arterial pressure to the diaphragm of the (AQ Please provide the town of publication) Technique of radial line Scheer BV, Perel A, Pfeiffer UJ (2002) Clinical large cannula width, multiple attempts as a result of, for example damping (see transducer results in a damped arterial review: complications and risk factors insertion at insertion and long duration of use below). waveform (Figure 5). The waveform is of peripheral arterial catheters used for After cleaning the skin, lidocaine 1–2% n permanent total occlusion 0.09%; loss smoothed out without sharp changes dis- haemodynamic monitoring in anaesthesia and is infiltrated over the radial artery. There of digits is very rare. Risk factors for Peripheral and central arterial played. Damping is caused by the dissipa- intensive care. Medicine Critical Care 6: 198–204 are three common insertion techniques. digital loss include pre-existing arterial waveforms tion of stored energy and is the progressive Whichever technique is used, the cannula disease, prolonged hypotension and use Waveforms from a peripheral artery such diminution of oscillations in a resonant should never be forced along the artery as of vasopressors as the radial artery differ from those of an system. This may be the result of blood local damage may occur. Infection. Sepsis or bacteraemia secondary aortic trace (Figure 4). A peripheral trace clots in the system, kinking, compression 1. Direct cannulation (Figure 3). Insertion to infected radial arterial lines is very rare has a higher peak systolic pressure, a wider of air bubbles and viscous drag of saline technique similar to that of a venous (0.13%); local infection is more common. pulse pressure and a more prominent in the line. cannula. Extension of the wrist If the area looks inflamed the line site dicrotic notch, i.e. the systolic pressure in Resonance is where an oscillating system brings the artery closer to the surface. should be changed. Risk factors include the dorsalis pedis artery is higher than in (such as the transducer, diaphragm and Stabilizing the wrist in this position co-existing bacteraemia and long duration the radial artery, which is higher than in saline column in the arterial measurement either with tape or with the aid of an of use the aorta. This is because peripheral arter- system) oscillates at maximum amplitude assistant makes insertion easier. The Emboli. Air or thromboemboli may ies are smaller and less compliant than to an alternating external driving force radial artery is palpated. The cannula is occur. Care should be taken to aspirate air central arteries and therefore less disten- (such as the arterial pressure). Resonance inserted aiming to hit the middle of the bubbles. sible. is minimized by using a shorter, wider and artery at an angle of approximately 30 ° Accidental drug injection may cause stiffer connecting catheter. to the skin. When there is free flow of severe, irreversible damage to the hand. To Additional information from arterial blood back into the hub of the try to prevent this: arterial waveform Comparison with non-invasive cannula, the cannula sheath is advanced n No drugs should be injected via an arte- In addition to BP measurement, the shape BP over the needle into the artery. rial line of the waveform gives further useful infor- Arterial lines measure systolic BP approxi- 2. Transfixion. After obtaining a flashback, n The line should be labelled (in red) to mation: mately 5 mmHg higher and the diastolic the cannula is advanced through the reduce the likelihood of this occurring 1. Myocardial contractility. Indicated by BP approximately 8 mmHg lower com- posterior wall of the artery. The needle Inaccuracy as a result of damping effects the rate of change of pressure by unit pared to non-invasive BP (NIBP) measure- is removed and a syringe attached. The (see below) time (dP/dt) i.e. the slope of the arterial ment techniques. cannula is slowly withdrawn while aspi- Pseudoaneurysms. Rare. upstroke. rating. Once free aspiration is achieved, 2. Hypovolaemia. Suggested by a narrow Advantages of invasive blood the cannula is advanced proximally Mechanism of action waveform, a low dicrotic notch and a pressure measurement along the artery. This is a particularly The principles of invasive arterial blood peak pressure which varies with IPPV n Continuous blood pressure recording useful technique in paediatric patients. pressure measurement are the same in (AQ Please define in full) breaths n Accurate blood pressure recording even 3. Guidewire (Seldinger) technique. A the radial artery as in other arteries. The if the patient is ventilated, or with when patients are profoundly hypoten- guidewire may be used if advance- column of saline in the arterial giving set deep inspirations in the spontaneously sive ment of the cannula sheath over the transmits the pressure changes to the dia- breathing patient (also called an ‘arte- n Other information from arterial trace needle proves difficult, for example in phragm in the transducer. This produces rial swing’). such as ‘arterial swing’, indication of atheromatous disease. The guidewire an electrical signal which is displayed as an myocardial contractility.