(Red) 660nm (Infrared) 910nm 10 Perfusion In hypothermia, hypotension, hypoperfusion, peripheral vasoconstriction (both iat rogenic and primary) and venous congestion, the pulsatile component of the signal can be of insuf \201 cient amplitude and quality, such that the reading is inaccurate or absent. Hbo2 Hb Movement Movement of the probe or cable by the patient or attend- ing staff can cause artif acts, loss of signal and errors. The most recent pulse oximetry algorithms have markedly im proved performance in low perfusion states and movement. 0.1 Wavelength (nm) 600 700 900 1000 Fig. 6.3.1 Absorption spectra of light as used in the pulse oximeter. The transmitted light travels through the tissue to a photo- detector where the measurement s take place. In the shorter wavelength region (red), oxy-Hb absorbs less light than deoxy- Hb, with the reverse being true in the infrared region (longer wavelength). At the photodet ector, the transmitted light is electronically processed and the relative absorption of the blood at the two wavelengths compared. The isobestic point is the wavelength at which two substances absorb a particular wavelength of light to the same extent and was used in some oximeters to correct for Hb concentration. It can also be used as a reference point where l ight absorption is independent of saturation. The pulse oximeter is designed to measure sol ely arterial oxygen saturation, which requires that the signals from other tissues and capi llary and venous blood be \201 ltered out. This is done using the pulsatile nature of art erial blood. Arrhythmias Atrial \201 brillation can make the measurement of maximum and minimum absorp tion more difficult and prone to error. Electrical interference Surgical diathermy is a common cause of loss of saturation reading. Abnormal haemoglobins Carboxy-Hb has a similar absorption coef \201 cient to oxy- Hb, tren ding towards a falsely high reading at 96%. In the case of carbon monoxide (CO) poisoning, e.g. in smoke inhalation, it is essential to remember this point. Where there are suspecte d high levels of carboxy-Hb, an arterial blood co-oximeter measurement should be taken to c on- \201 rm the correct carboxy-Hb and O2 levels as the SpO2 reading may be misleading. M ethaemoglobin (MetHb) has a similar absorption at both emitted wavelengths, giving 95 CHAPTER 6.3 Pulse oximetry This delay is also seen when oxygen saturations improve some seconds after \200\230pinking up \200\231 of a patient following an interven- tion to treat cyanosis and low saturations . This time lag has been shown to be reduced when an ear probe is used instead of a \201 nger probe. Despite the expert opinion perception of the advantage of continuous monitoring , there is no published trial evidence of alteration of outcome with pulse oximetry. a saturation reading of 84%. These readings are the same regardless of the true oxy-Hb sat uration. Recently launched multi-wavelength pulse oximeters can now measure car- boxy-Hb an d MetHb, and consequently have improved accuracy in SpO2 measurement. Other intravascular s ubstances A number of substances can give falsely low saturations when present in tissues a nd circulating blood, e.g. methylene blue and indocyanin green. Clinical applications Critical care unit It is now normal practice to have all patients con tinuously monitored using a minimum of an ECG and a pulse oximeter. Bilirubin This has a similar absorption to deoxy-Hb, giving a falsely low reading. Ambient light Lights (e.g. in theater or an anesthetic room) can affect saturation readings especially if light is bright, direct or \202 ickering (appearing pulsatile). Anesthesia Pulse oximetry has become a mandated standard both during and after general and regional anesthesia Ward care, imaging units, endoscopy and patient transportation Continuous oxygen saturation monitoring is becoming more commonplace especially in higher risk patients, and during tra nsportation and procedures involving sedation. Nail polish and nicotine These can absorb infrared light and cause inaccuracies. Nail polis h should therefore routinely be removed pre- operatively. If this is not possible or if the patient has false nails, the pulse oximeter probe can be placed side to side across the \201 nger, avoiding the nail-bed. Resuscitation Pulse oximetery can be used during CPR to give a general assessment of adequa cy of perfusion. Physical risks Pressure damage There have been case reports of pressure necrosis in patient s being monitored for long periods, more commonly if poor tissue perfusion is a factor. Investigations Saturation monitoring can be used during sleep studies to investigate sleep apnoea as well as in cardiac investigations such as exercise tolerance tests. This simple t o use, portable, non-invasive device has become one of the most widely used monitors in hos pital medicine. Whilst it has its limitations, it has proved an invaluable addi- tion to cl inical observation and other forms of monitoring. Magnetic resonance imaging (MRI) Burns have been reported due to electrical induction when non-MRI-compatible probes have been used. Clinical problems Complacency SpO2 readings in the \200\230normal range \200\231 can be fa lsely reassur- ing. Because of the sigmoid shape of the Hb dissociation curve, the pulse ox imeter can be relatively poor at detecting a signi \201 cant fall in PaO2 particularly at the steeper portions of the curve (Fig. 6.3.1). Further reading Aitkenhead AR, Smith G, eds. Textbook of anesthesia, 3rd edn. Churchill Livinstone, London 1996. Yentis, SM, Hirsch, NP, Smith, GB. Anesthesia and intensive care A \200\223Z, 2nd edn. Elsevier, 2000. Jubran A. Pulse oximetry. Review article. Critical Care 1999; 3: R11 \200\2237. Time lag There can be a lag of up to 30s between clinical evidence of cyanosis and pulse ox imetry detection of a drop in saturation. This page intentionally left blank 97 Chapter 7 Cardiovascular monitoring Chapter contents ECG monitoring 98 Arterial pressure monitoring 102 Insertion of central venous catheters 10 4 Common problems with central venous access 106 Pulmonary artery catheter: indications and use 108 Pulmonary artery catheter: insertion 110 Echocardiography 112 Clinical application of echocardiography in the ICU 116 Doppler 118 Pulse pressure algorithms 120 Non-invasive methods 122 Measurement of preload status 124 Detection of \202 uid responsiveness 126 98 E ASHLEY ECG monitoring ECG monitoring is carried out in order to measure heart rate, rhythm and conduction disturb ances, and to monitor pacemaker function. It is essential for the detection of myocardial i schaemia and may also give an indication of electrolyte imbalance. aVF I aVL aVR aVL aVR II III \210\22290 \210\22260 \210\222120 aVL II 5 mm \210\22230 III \210\222150 aVR 0.2 sec 180 5 mm 0 0.5 mV R aVF I aVF I +150 +30 +60 +120 0.04 sec 1 mm 1 mm 0.1 mV +90 aVR III II aVL 25 mm/sec 10 mm/mV aVL III II aVR I aVF III II P-R seg- ment T aVF P S-T segment U Fig. 7.1.2 ECG leads. Q P-R interval Augmented unipolar leads The reference electrode is formed by combining leads I, II and III , i.e. the center of Einthoven \200\231s equilateral triangle, where the potential differen ce is zero. right arm aVR left arm aVL left leg aVF S-T interval S QRS interval Q-T interval Fig. 7.1.1 Normal ECG con \201 guration. Unipolar chest leads The reference electrode is formed by the combined aV leads. V1 4th int ercostals space, right sternal edge V2 4th intercostals space, left sternal edge V3 midway between V2 and V4 V4 5th intercostals space, left mid-clavicular line V5 5th intercostals s pace, left anterior axillary line V6 5th intercostals space, left mid-axillary line An ECG \200\230lead \200\231 refers to the potential difference recorded between two de \201 ned points. The standard calibration for an ECG is 25mm/s and 1mV/cm. If the electrica l current is moving towards the positive electrode, then the de \202 ection of the oscillo scope is upwards, if it is away, the de \202 ection is downwards. The frequency range (cyc les per second) of an ECG is 0.5 \200\2234Hz, i.e. a heart rate of 30 \200\223240bpm. The E CG can be broken down into sine waves or harmonics, by Fourier analysis. A minimum of 10 ha rmonics have to be displayed to produce an ECG waveform, requiring a band- width between 0. 05 and 40Hz. The bandwidth has to be widened to 0.05 \200\223150Hz to enable ST segment ana lysis. Low and high frequency \201 lters improve the waveform dis- play; the low frequenc y \201 lter diminishes baseline drift caused by movement and breathing, but exaggerates S T segment changes. The high frequency \201 lter reduces wall power-source noise, but prev ents visualization of pace- maker spikes and interferes with QRS and J point recognition. B iological ECG signals are small and require ampli \201 cation with a differential ampli \201 er to minimize noise. Placing electrodes on bony prominences minimizes interference fr om muscle activity and the EEG. The signal is then trans- ferred to an oscilloscope for con tinuous monitoring, or to a printer for interpretation. Adjusting the gain on an ECG monito r may exaggerate or minimize ischaemic changes. It is therefore important to interpret ST e levation or depres- sion when the standard gain of 10mm/mV is selected. Lead selection In the operating theater, a single lead ECG display is used, because theater monitoring employs a three-lead ECG con \201 guration. It is possible to switch between l ead I, II and III. During cardiac surgery or on ITU, a \201 ve-lead electrode cable is us ed. This allows the display of seven ECG leads at the same time, including I, II, III, aVR, aVL, aVF and a single unipolar lead, e.g. the lateral chest lead V5. Three-lead ECG Lead II or a modi \201 ed V5 lead should be displayed for detec- tion of ar rhythmias, because it monitors down the axis of left ventricular depolarization in patients with a normal cardiac axis. If a pre-operative ECG shows an axis devia- tion, it is best t o display the lead that monitors most closely to the axis of depolarization, i.e. lead I in left axis deviation and lead III in right axis deviation. Coronary ischaemia is best monit ored via a lateral chest lead. This is not possible with a three-electrode ECG, but a modi \201 ed V5 lead can be obtained by placing the right arm lead on the manubrium sterni and the left leg electrode in the standard V5 position. Lead II must be selected on the monitor . This is known as Standard leads I Between right arm and left arm II Between left leg and right arm III Betwe en left leg and left arm 99 CHAPTER 7.1 ECG monitoring Interpretation and detection of arrythmias Sinus arrythmia One P wave per QRS complex, cons tant PR interval, beat to beat change in RR interval, with respiration in young people. the CM 5 lead and is superior to an individual limb lead for the detection of coronary isch aemia. Five-lead ECG Leads II and V4 or V5 should be selected for continuous monitoring. This comb ination has 90% sensitivity for the detection of inferior or anterior coronary ischaemia Sinus bradycardia Heart rate <40bpm. Often poorly tolerated, resulting in hypotension. Epicardial ECG monitoring After cardiac surgery. an atrial ECG can be monitored using the t emporary epicardial atrial pacing wires that are placed on the heart. This is useful for th e detection of atrial arrhythmias. Similarly, temporary ventricular pacing wires can be use d to monitor the ventricular ECG. Epicardial ECG monitoring is also used during off-pump co ronary artery surgery, during which the axis and amplitude of a conventional ECG may change due with positioning of the heart. Oesophageal or endotracheal electrodes have also been u sed during surgery. Sinus tachycardia Heart rate >100bpm. Can precipitate myocardial ischae- mia or left ventri cular failure. First degree heart block The PR interval represents the time for depolarization from the si noatrial (SA) node to the ventricle. This is usu- ally 0.2s. A prolonged PR interval of >0. 2s signi \201 es \201 rst degree heart block. This may be a sign of coronary artery dise ase, rheumatic heart disease, digoxin toxicity or elec- trolyte imbalance. ST segment analysis and diagnosis of ischaemia Subendocardial ischaemia Subendocardial isch aemia is \200\230demand ischaemia \200\231. i.e. it pro- duces ST segment depression of >1 mm (0.1mV), horizon- tal or down-sloping. It is measured 60ms after the J point. It is non- localizing to coronary distribution, and leads V5 and V6 are the most sensitive for detecti on. Second degree heart block Mobitz type II PR interval of conducted beats is constant, but oc casionally a P wave is not followed by a QRS complex, because the excitation fails to pass through the AV node and the bundle of His Wenkebach phenomenon Progressive lengthening of the PR interval, and then failure of conduc tion of an atrial beat. The following beat is con- ducted with a normal PR interval. 2:1 or 3:1 block Two or three P waves per QRS complex. Normal and con- stant PR interval in the conducted beats. Wenkebach is usually a benign rhythm, but Mobitz type II, 2:1 or 3:1 block may herald complete heart block in the context of myocar- dial infarction (MI). R S-T Segment Complete heart block P waves march through the ECG at a constant rate, whilst the ventricul ar excitation is a slow ventricular escape rate. There is no relationship between the P wav es and the QRS com- plexes, which are wide due to their ventricular origin. Complete heart block occurs acutely in the context of an MI or may be chronic due to \201 brosis around the bundle of His. T P U Q S J Point Paroxysmal atrial tachycardia Originates above the ventricles from a site other than the SA node, but including the AV node. The rate is 150 \200\223 250bpm and is regular. There is one P wave per QRS com- plex, although the P wave may be hidden in the QRS complex or the T wave. It will compromise the patient if left untreated. PR INTERVAL QRS INTERVAL Atrial \201 brillation No P waves, irregularly irregular ventricular rate, with a normal QRS morphology. Is usually associated with signi \201 - cant cardiac disease and can compr omise cardiac output, due to inef \201 cient ventricular \201 lling. It is best diagnose d from the arterial waveform, which is irregular, with beat to beat variation in cardiac ou tput. Fig. 7.1.3 J point recognition. Transmural ischaemia This is supply ischaemia and produces ST segment elevation. Signi \201 cant ST elevation is considered to be elevation of 1mm (0.1mV) in two limb leads or 2m m (0.2mV) in two V leads. It is measured at the J point and localizes coronary artery distr ibution. It has been shown that ECG monitoring on the ICU has a low sensitivity for detecti ng myocardial ischaemia, and fre- quent 12-lead ECGs are superior. Atrial \202 utter The atrial rate is between 250 and 350bpm, with a \200\230saw- toothed \200\231 P wave appearance. The AV node cannot conduct at this rate so there is a variable block, i.e. 2:1, 3:1 or 4:1, resulting in a ventricular rate of 150, 100 or 75bpm depend- ing on the block. A narrow complex tachycardia with a rate 100 E ASHLEY of 150bpm on the ITU is likely to be \202 utter and is readily treated with a DC shock of 50J. Ventricular \201 brillation A rapid irregular rhythm resulting from the discharge of impu lses from one or more foci in the ventricles. There are no P waves, no QRS complexes, only bizarre erratic ven- tricular activity. There is no cardiac output, and immediate cardiover sion is required. Junctional tachycardia This arises from the area around the AV node. The P waves may not be seen, but the QRS complex is a normal shape, because the ventricles are activated normally down the bundle of His. Further reading Hampton J. The ECG in practice, 4th edn. Churchill Livingstone, 2003 Martin ez EA, Kim LJ, Faraday N, et al. Sensitivity of routine intensive care unit surveillance fo r detecting myocardial ischaemia. Critical Care Med 2003; 31: 2302 \200\2238. Davis PD, Par brook GD, Kenny G. Basic physics and measurement in anesthesia, 4th edn. Butterworth \200 \223Heinemann, 2002. Ventricular tachycardia Rapid irregular depolarization from one or more ventricu- lar foci. There are no P waves; the QRS complexes are wide, irregular and vary slightly in shape. It may or may not produce a cardiac output, is life-threatening and requires immediate treatm ent This page intentionally left blank 102 E ASHLEY Arterial pressure monitoring Arterial cannulation allows for direct monitoring of the arterial blood pressure and also a rterial blood gas sam- pling. It is essential in intensive care patients with cardio- vascu lar or respiratory failure. It is used during anesthesia for major surgical cases including cardiac and vascular sur- gery and situations where major blood loss, cardiovascular insta bility and pharmacological manipulation of the cardio- vascular system is anticipated. It i s used in situations where non-invasive blood pressure is unreliable, e.g. arrythmias. Arterial cannulation The radial artery is the most commonly used artery for arterial cannul ation, as the hand receives collateral blood \202 ow via the ulnar artery. Direct cut-dow n onto an artery may sometimes be necessary in severely hypovolaemic patients, but this may be more dif \201 cult than anticipated. Aortic pressure can be measured post-cardiopulmon ary by-pass with a needle placed directly into the aortic root. A non-invasive blood pressu re monitor should be available to corroborate the mean arterial pressure. Fig 7.2.1 Arterial cannulation. Alternative sites for arterial cannulation include: \200 Allen \200\231s test Allen \200\231s test was originally described for assessing arterial b lood \202 ow to the hand in thromboangiitis obliterans. The test is now used to assess ul nar arterial blood \202 ow prior to radial arterial cannulation, or harvesting of the rad ial artery for coronary artery surgery; the ulnar and radial arteries are compressed at the wrist and the patient asked to clench tightly and open the hand. The hand appears white an d blanched. Pressure over the ulnar artery is then released and the color should return to the palm within 5 \200\22310s. Delayed return of palmar blood \202 ow, >15s is abnor- mal and may predict a risk of ischaemic changes if the radial artery is cannulated. The same t est can be used prior to ulnar arterial cannulation, but instead collateral circulation via the radial artery is assessed by releasing the pressure over the radial artery. However, o verall consensus is that Allen \200\231s test is not discriminatory at a particular cut-off time. This does not imply that it never should be per- formed, but suggests that it should be replaced by more objective tests, such as Doppler ultrasound. Brachial artery This is an end artery supplying a large part of the forearm, therefore comp lications such as thrombosis will have severe consequences. Axillary artery This is used in neonatal practice. Ulnar artery The ulnar artery should not be used if the radial artery i s occluded or has been damaged by previous cannula- tion attempts. Femoral artery This is f requently used in babies and children. A longer catheter is required especially in obese pa tients. It can easily become occluded if the hip is \202 exed and is more susceptible to infection. Dorsalis pedis artery Blood pressure will be 10 \200\22320mm Hg higher than in t he central circulation. Complications of arterial cannulation Ischaemic complications are much more likely in the s hocked or hypotensive patient. \200 Cannulation \200 Awake or asleep: if the patient is awake, explain proce- dure and in \201 ltrate with loca l anesthetic. The radial artery can usually be found between the ten- don of the \202 exo rcarpi radialis and the head of the radius, but beware aberrant anatomy Position the patien t \200\231s wrist in dorsi \202 exion, but do not over \202 ex. An assistant can hold the wrist, or a roll of swabs and tape may be used Insert the cannula slightly distal to the r adial pulse at 45 to the skin. When a \202 ash-back is obtained, advance the needle and the cannula further to compensate for the length of the bevel of the needle. Then carefull y slide the cannula off the needle and advance it into the artery. Trans \201 xion can be used especially in babies or small children, and may have a higher success rate. However, t he disadvantages include arterial damage and hae- matoma formation. The radial artery can a lso be cannu- lated using a Seldinger technique. This technique involves passing a Seldinge r wire into the atery via a needle and then exchanging the needle for a plastic cannula pri or to withdrawing the wire. Distal Ischaemia caused by arterial spasm, thrombosis or embolus Tissue necrosis Bleeding I nfection False aneurysm \200 \200 \200 Arterial pressure monitoring In order to monitor arterial pressure, the pressure energy wit hin the arterial cannula has to be transduced to pro- duce an electrical waveform. This req uires a system that consists of the following. A \202 ush system: this consists of a bag of \202 uid pressurized at 300mm Hg, a drip set and a \202 ow constrictor. This \202 ushes the cannula with heparinized or normal saline at a rate of 3 \200\2234ml/h. Excessive \202 ushing of an arterial cannula should be avoided, especially in babies and children, to prevent air or debris entering into the arterial circulation. Flushing with a syringe m ay cause retrograde cerebral emboli. 103 CHAPTER 7.2 Arterial pressure monitoring of the system is outside the range of frequencies present in the blood pressure waveform, t he problem is avoided. The resonant frequency can be raised by using a shorter stiffer arte rial cannula. A hyper- resonant trace will produce ele- vated systolic pressure readings an d decreased diastolic pressure values, but the mean remains accurate. Addition of a rubber bung or an air bubble (caution with air) into the circuit can compensate for underdamping. A damped trace occurs if there is restriction of transmis- sion of the blood pressure from the artery to the trans- ducer diaphragm. This can be caused by clots, kinks, air bubbles a nd excessively long or compliant tubing. The systolic pressure will be decreased, whereas t he diastolic pressure will be elevated. Again the mean pressure remains relatively accurate . Arterial cannula: 20 or 22G parallel-sided stiff Te \202 on cannula. Connecting catheter: short and stiff saline- \201 lled to reduce resonance. The number of 3-way taps in the sys tem should be kept to a minimum. Transducer dome or diaphragm: this detects the small move- ment of saline to and fro along the catheter. The diaphragm receives this \202 uctuating pressure energy, and converts it into an electrical signal, the amplitude of which depends on the degree of deformation of the dome. The transducer is placed at the level of the hea rt and zeroed in this position. Zeroing is carried out by exposing the transducer to atmosp heric pressure through an open 3-way tap and pressing \200\230zero \200\231 on the monitor . Electrical monitor and connections: the input transducer leads to an ampli \201 er and r ecorder. The frequency range of the arterial pressure waveform is between 0 and 40Hz. The m onitor must be able to respond adequately to this range of frequencies. Difference between central and peripheral arterial pressure The stiffness of the arterial t ree increases with increasing distance from the aortic valve. The blood pressure wave becom es narrower and increases in amplitude in more peripheral arteries. Therefore, even in a su pine patient the systolic pressure in the dorsalis pedis artery is higher than in the radia l artery, which in turn is higher than in the aorta. The diastolic pressure similarly decre ases peripherally and the pulse pressure widens. The effect of temperature and inotropes on the systemic vascular resistance can there- fore in \202 uence the recording of arterial blood pressure. Non-invasive blood pressure measurements do not always correlate with the i nvasive blood pressure reading, but the mean pressures should be similar. The nurses will f avor the value which suits the target blood pressure! Normal arterial pressure waveform morphology Digital readouts of systolic and diastolic blood pressure are displayed as a running averag e which is updated at frequent intervals. The MAP is a calculated value (diastolic pressure + 1/3 pulse pressure). 150 ) g H m m ( 100 e r u s s e r P Using the arterial pressure trace to estimate preload Systolic pressure variation (SPV) occ urs in the mechani- cally ventilated patient, where changes in intrathoracic pressure and l ung volumes produce cyclical variations in blood pressure. An SPV of >10mm Hg is an indicat or of hypovolaemia. This \200\230swing \200\231 on the arterial pressure trace can be used to monitor the response to a \202 uid challenge. Similarly the area under the arterial p ressure trace is an indication of cardiac output (see Chapter 7.15). 50 0 .5 1.0 1.5 Time (secs) Fig. 7.2.2 Normal arterial pressure waveform. Resonance and damping An arterial pressure waveform consists of a range of sine waves with different frequencies which are superimposed, producing the arterial pressure trace. The pr ocess of analyzing complex wave patterns into a series of simpler sine waves is known as Fo urier analysis. The pressure measuring system possesses a resonant fre- quency at which osc illations can occur. If this is <40Hz, it falls within the range of the frequencies present in the blood pressure waveform and the sine wave is superim- posed on the blood pressure w aveform, producing a spiky, distorted, hyper-resonant trace. If the resonant frequency Further reading Davis PD, Parbrook GD, Kenny G. Basic physics and measurement in anesthesia , 4th edn. Butterworth \200\223Heinemann, 2002. Jarvis A, Jarvis CL, Jones PRM et al. Relia bility of Allen \200\231s test in selec- tion of patients for radial artery harvest Ann Tho rac Surg 2000; 70: 1363 \200\2235. Langton JA, Stoker M. Principles of pressuer transducers, reso- nance, damping and frequenc y response. Anaesth Intensive Care Med 2001; 2: 186 \200\22390. 104 S BODENHAM & A GRATRIX Insertion of central venous catheters Indications Central venous access is almost universal in critical care patients. Indication s include: \200 \200 \200 \200 \200 \200 \200 \200 The internal jugular vein runs from the jugular foramen at the base of the skull (immediate ly behind the ear) to its termination behind the posterior border of the sternocla- vicular joint where it combines with the subclavian vein to become the brachiocephalic vein. Throu ghout its length it lies lateral, \201 rst to the internal and then to the common carotid arteries, within the carotid sheath, behind the ster- nomastoid muscle. Many approaches to the internal jugular vein have been described. Ultrasound will demonstrate the close assoc ia- tion of the vein and carotid artery. Choose a site for punc- ture where the vein does n ot lie directly over the artery. A typical approach is from the apex of the triangle formed by the two heads of the sternomastoid: \200 \200 \200 monitoring of CVP drug administration total parenteral nutrition \202 uid resuscitation i nsertion of temporary pacing wires insertion of pulmonary artery catheters dialysis lack of peripheral venous access. Contraindications These are relative, but include inability to identify land- marks, limite d sites for access, previous dif \201 culties or com- plications, severe coagulopathy, thr ombocytopenia and local sepsis. In addition, if an awake patient is unable to lie \202 at , central venous cannulation may be impractical without assisted ventilation. Slightly extend the neck. Turn the head slightly to the opposite side. Palpate the carotid artery at the level of the cricoid car- tilage. Look for the internal jugular vein pulsatio n. If com- pressed, the internal jugular can usually be seen to empty and re \201 ll. To l ocate the vein, introduce the needle from the apex of the triangle at an angle of 30 and aim towards the ipsilateral nipple. The vein lies typically within 1.5 \200\2232cm of the s kin surface. Often when attempting to puncture the vein it collapses under the pressure of the needle and puncture is not recognized. The vessel may then be located by aspirating as the needle is slowly withdrawn. Blood will be aspi- rated as the needle tip passes back int o the vein, which re \201 lls once the pressure has been removed. Ultrasound guidance for vascular access It is now increasingly recommended that ultrasound should be used to guide all central venous access. Ultrasound allows: \200 direct visualization of the vessels (artery and vein) and their associated structures ident ification of thrombosis, valve or anatomical abnormalities identi \201 cation of best targ et vessel \201 rst-pass cannulation in the midline of a vessel directly avoiding other vi tal structures visualization of guide wire and cannulae entering vein reduction of puncture -related complications. \200 External jugular vein The external jugular vein lies super \201 cially in the neck, run- n ing down from the region of the angle of the jaw, across the sternomastoid before passing d eep to drain into the subclavian vein. It can be used to provide central venous access, par ticularly in emergency situations when a simple large-bore cannula can be used for the admi nistration of drugs and resuscitation \202 uids. Longer central venous cath- eters can be sited via the external jugular, but the angle of entry to the subclavian vein often leads to inability to pass guide wires centrally and results in a high failure rate. \200 It is likely that the risk of catheter-related sepsis and throm- bosis is reduced by limiti ng the number of needle passes with ultrasound. Arteries can be distinguished from veins by their round cross-section, their non-compressibility and their pulsatility. Veins, in cont rast, show respiratory \202 uctuation and are easily compressible. In order to maintain s terility during vessel puncture the ultrasound probe should be placed in a sterile plastic sheath. Sterile ultrasound gel is required both inside and outside the sheath. The use of u ltrasound requires practice. You should seek instruction before attempting to use it on a p atient. You should also be famil- iar with the landmark approaches to the central veins. Subclavian vein Subclavian vein cannulation is associated with a higher inci- dence of comp lications, particularly pneumothorax, and a higher incidence of incorrect line placement th an internal jugular cannulation. It is, however, more comfortable for the patient long-term and the site can more easily be kept clean. The subclavian vein is a continuation of the a xillary vein. It runs from the apex of the axilla behind the poste- rior border of the clav icle and across the \201 rst rib to join the internal jugular vein, forming the brachioce phalic vein behind the sternoclavicular joint. \200 Internal jugular vein Right internal jugular vein cannulation is associated with a lower in cidence of procedural complications and higher incidence of correct tip placement than othe r approaches. It is especially appropriate for patients with coagulopathy or those patients with lung disease in whom pneumotho- rax may be disastrous. It may be best avoided in thos e patients with carotid artery disease or those with raised ICP because of the risks of car otid puncture and of impaired cerebral venous drainage. Internal jugular cannu- lation is a ssociated with a higher incidence of catheter infection than subclavian cannulation but bot h have a much lower infection rate than the femoral approach. Position the patient supine (some people advocate placing a sandbag between the patient \200\231s shoulder blades, which allows the shoulders to drop back out of the way). Identif y the junction of the medial third and outer two- thirds of the clavicle. Introduce the nee dle just beneath the clavicle at this point, and aim towards the clavicle until contact wit h bone is made. 105 CHAPTER 7.3 Insertion of central venous catheters dle once it has passed beyond the end of the bevel: it may shear off. Use a scalpel blade t o make a small nick in the skin. Hold the blade up and cut away from the wire. If provided, pass the dilator over the wire into the vein. Then remove it, leaving the wire in situ. Pa ss the cannula over the wire into the vein. Make sure that before you push the cannula forw ard the wire is visible at the proximal end. Hold on to the wire at all times, to prevent i t being lost inside the patient For an average adult patient the central venous cannula via the right internal jugular vein does not need to be inserted more than 12 \200\22315cm. Ch eck markings on the cannula. Many are 20cm long and do not need to be inserted up to the hu b. Draw back blood, check the color, pulsatility and the pressure of the back \202 ow of blood, \202 ush all the lumens of the line with heparinized saline and lock off the 3-way taps. At this point the patient can be leveled. Suture the line into place using the ancho rage devices provided and cover with an adhesive sterile dressing. If you appear to have mi ssed the vein on the \201 rst pass, pull back slowly while maintaining suction on the syr inge. You often \201 nd you have gone through the vein and can \201 nd it on withdrawal . Attach a transducer and display the waveform on the monitor. Dispose of your sharps and c lear away your trolley. Obtain CXR to verify central position of the line and check for com plications, including pneumothorax and haemothorax. Document the procedure in the patient \200\231s notes. To locate the vein, redirect the needle closely behind the clavicle and towards the suprast ernal notch. Ultrasound can be used to guide puncture of the vein using a more lateral appr oach. The axillary vein can be identi \201 ed under the pectoral muscles at a depth of 3 \200\2234cm in the average patient. Longer catheters (20cm left and 25cm right) are require d by this approach. Supraclavicular approaches can also be used using both landmark- and ul trasound-guided techniques. Femoral vein The femoral vein lies medial to the femoral artery immedi- ately beneath the i nguinal ligament. It is particularly useful for obtaining central access in small children and in patients with severe coagulopathy. \200 \200 Palpate the femoral artery. To locate the vein, introduce the needle 1cm medial to the femo ral artery close to the inguinal ligament. It is a common mistake to go too low where the s uper \201 cial femoral artery overlies the vein. Ultrasound should be used to identify the vessels (long saphenous vein, deep and super \201 cial femoral arteries) and ensure that the vein is punctured near the inguinal ligament where the artery and vein lie side by side . Procedure Central venous catheterization is almost universally achieved using a catheter ov er a guide wire (Seldinger) technique. This is associated with a lower incidence of incorre ct line placement and complications than cannula over needle techniques. \200 \200 For internal jugular, external jugular and subclavian veins, position the patient supine wi th 10 \200\22320 head down tilt. This distends the vein to aid location and helps prevent air embolism. Monitor ECG in case of dysrhythmias. Universal precautions. Use aseptic tech nique, sterile gown and gloves. Prepare sterile \201 eld. Prepare all equipment. Check wi re passes through the needle freely. Attach 3-way taps to all open ports of the cannula. Fl ush the lumens with heparinized saline. Inject local anesthetic to the entry site. Do not f orget to anesthetize suture sites as well. Identify the target vessel by ultrasound and/or landmark technique. Using a 10ml syringe (partially \201 lled with saline) and nee- dle e nter the central vein by the chosen approach, main- taining suction on the syringe at all t imes. Pass the guide wire through the needle. This should pass freely and without any force into the vein. Watch for arrhythmias. Never pull the wire back through the nee- Position on chest X-ray The catheter should lie along the long axis of the vessel and the t ip should be in the superior vena cava (SVC) or at the junction of the SVC and right atrium , but ideally out- side the pericardial re \202 ection. The pericardium lies below the car ina, so ideally catheter tips should be at or above the level of the carina. Catheters belo w this level may per- forate the heart and cause cardiac tamponade. Catheters placed via th e subclavian veins or left internal jugular vein must not be allowed to lie with the tip ab utting the wall of the SVC. This may cause pain, perforation and accelerated thrombus forma tion. Either advance the catheter to lie in the long axis of the SVC or pull it back to lie in the brachio- cephalic vein. Bear in mind the limitations of CXR; it is use- ful to con \201 rm central passage and no kinking. The close proximity of the SVC to the pleura, asce nding aorta and other structures means that con \201 rmation of the true intra- venous pos ition cannot be inferred from a plain CXR. \200 \200 \200 \200 \200 106 S BODENHAM & A GRATRIX Common problems with central venous access Cannot \201 nd the vein Check position (ultrasound and/or landmarks) and try again. If un successful do not persist with repeated passages of the needle in the hope of hitting the v ein. You may have misinterpreted the landmarks or the vein may be absent, narrowed or occlu ded (e.g. with thrombus). The management of pneumothorax depends upon the size of the pneumothorax and the patient \200\231s condition, particu- larly whether ventilated or not. A small pneumothorax in an u nventilated patient with good gas exchange may be observed, or aspirated using a small-bore cannula and syringe with a 3-way tap. Larger pneumothoraces, those that fail to resolve or those that cause any impairment of gas exchange and/or haemodynamics require a formal ches t drain. Any significant haemothorax should be formally drained as soon as possible. Once b lood has clot- ted in the chest, drainage is dif \201 cult. Seek cardiothoracic/ surgical opinion. Bleeding around the puncture site can occasionally be a persistent problem. If thi s does not resolve with pressure, use a \201 ne suture (e.g. 5/0 Prolene) to tie a purse string around the puncture site. This usually stops the bleeding. Line colonization with ba cteria and fungi is common, but there is no evidence that changing lines on a regular basis (e.g. every 5 \200\2237 days) is of bene \201 t. Aspirating blood (needle in vein?) but cannot pass wire Check needle position by drawing ba ck on the syringe; good \202 ow is essential. Adjust the angle of incidence of the needle to the vein to improve \202 ow. Tip the patient further head down to expand the vein fur ther. Try rotating the needle through 180 and draw back again. Remember the wire must pas s easily without force. If this doesn \200\231t work, repuncture the vein at a slightly dif ferent angle.