New practical bedside procedures on the intensive care unit

9 New practical bedside procedures on the intensive care unit ANDREW BODENHAM N. R. W E B S T E R

There continues to be an increase in the number of therapeutic and diagnostic interventions that medical staff involved in intensive care have to evaluate and introduce where appropriate. Two such techniques are reviewed here: percutaneous tracheostomy and jugular bulb cannulation, which have been increasingly utilized in the last 5 years. However, their indications and advantages over existing techniques are as yet unproven and both have the potential to produce serious complications. PERCUTANEOUS TRACHEOSTOMY

Historical background A number of techniques, formerly the exclusive preserve of surgeons, may now be performed using percutaneous dilational techniques. Tracheostomy and cricothyroidotomy are two such procedures. The conventional surgical approaches follow the description by Chevalier Jackson (1909). Dilational techniques could be included in the category of minimally invasive therapy (Wickham, 1991); this term describes procedures where surgical intervention requires less invasive techniques, for example percutaneous nephrolithotomy. One dilational tracheostomy kit actually uses modified dilators designed for the latter procedure. Such developments have followed advances in percutaneous guide wire techniques which provide control over instrumentation not previously available. What actually constitutes a percutaneous rather than a surgical technique has not been defined, and the large number of individual approaches and modifications makes comparison of reports difficult. It has been questioned whether percutaneous tracheostomy is a novel technique or technical novelty (Heffner, 1991). The availability of commercial kits has produced a resurgence of interest and a reappraisal of the roles of tracheostomy and cricothyroidotomy in the critically ill. The timing and indications for such techniques are in many ways more controversial than differences in surgical technique, and this is likely to continue to be the case. However, the development of safer and more easily Bailli~re's Clinical Anaesthesiology--

Vol. 6, No. 2, June 1992 ISBN0-7020-1617-9

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performed techniques allows their earlier utilization, with potential benefit for the patient. There is argument whether such procedures are best performed in the intensive therapy unit or the operating theatre, and whether the procedure should be performed by surgeons or physician/anaesthetists. Surgeons would be unhappy to let the latter perform conventional surgical tracheostomy but in the authors' experience do not object to the use of percutaneous guide wire techniques, which have not yet been proved to be any safer. Most clinicians reserve the use of cricothyroidotomy for shorter-term airway access. This practice follows historical teaching that there is a greater risk of later airway stenosis after cricothyroidotomy. Series reporting routine usage of cricothyroidotomy rather than tracheostomy in patients requiring airway access after complications of thoracic/cardiothoracic surgery contradict such beliefs (Brantigan and Grow, 1976). Cricothyroidotomy is generally preferred for true emergency usage as the cricothyroid membrane is more easily palpable and further away from hazardous structures in the neck and thoracic inlet. The timing of tracheostomy in the critically ill is contentious, and fashions change every few years. The larger part of such debates revolves around the incidence of later complications related to endotracheal intubation and tracheostomy, in particular the formation of laryngeal or tracheal stenosis. Such arguments relate to dilational as well as conventional techniques. This subject has been reviewed in depth in a previous issue of this series (Mallet and Browne, 1990). Most evidence relates to historical series, when indications for tracheostomy were likely to differ from those of today. Recent developments such as high-volume, low-pressure tracheal tube cuffs and efficient airway humidification should have lessened the risk of stenosis formation.

Where and by whom should tracheostomy/cricothyroidotomy be performed? Different units vary in the practice of tracheostomy. Best results will follow procedures performed by experienced, interested operators who should be able to match the low complication rates reported in recent series (Stock et al, 1986). A number of reports have demonstrated that tracheostomy may be safely and efficiently performed in the intensive care unit rather than the operating room (Stevens and Howard, 1988; Hawkins et al, 1989). The major advantage of such an approach is the avoidance of problems in transporting patients. There is a reluctance to publish poor results; there have certainly been problems in moving unstable patients to the operating room even if only a short distance away. Nevertheless, many surgeons insist on taking patients to the operating theatre, arguing that lighting is better, patients are better positioned on an operating table and assistance may be more readily available if needed. It should be possible to provide adequate lighting on an intensive care unit, if only for emergency procedures. Intensive care beds are too wide to allow easy patient access by surgeon and assistant. The simplicity and speed of performing dilational tracheostomy

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facilitates procedures in these circumstances. On occasions it will still be necessary to utilize conventional procedures when the patient has abnormal anatomy, for example a goitre.

Dilational tracheostomy Most recent published reports related to subcricoid dilational tracheostomy concern the use of two techniques described by surgeons from the USA and Israel (Ciaglia et al, 1985; Schachner et al, 1989) (Figures 1 and 2). The techniques allow safe and rapid insertion of an 8-ram or 9-ram cuffed plastic tracheostomy tube between tracheal rings with the minimum of tissue trauma. The devices are designed for single use only. The authors, as anaesthetists, have been involved in some hundred of these procedures, without significant problems. Non-surgeons are advised to enlist surgical advice and help for the first few procedures, as inevitably there will be need for surgical assistance on occasions. It is also recommended, in the first instance, that procedures are performed in an operating theatre, if that is the normal practice for the unit. Another percutaneous technique using the Pertrach device was described much earlier (Toye and Weinstein, 1969) (Figure 3). This device is available in adult and paediatric sizes. It differs from the former techniques by having a needle introducer which splits and a flexible plastic introducer/dilator. It has been used in patients aged 8 years upwards for both subcricoid tracheostomy and cricothyroidotomy (Toye and Weinstein, 1986), but has not been used by the authors to date.

Figure 1. The Ciagliadilationaltracheostomykit (CookCriticalCare, Letchworth,UK) with Portex 8-ramtracheostomytube mountedon dilator.

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Figure 2. The Rapitrach percutaneous tracheostomy kit (Surgitech, Sydney, Australia) with Portex 8-mm tracheostomy tube mounted on obturator.

Figure 3. The adult Pertrach device, 5.6 mm internal diameter, with splitting introducingneedle (Pertrach, Bridgeport, WVa, USA).

Anecdotal reports have claimed that, in comparison with conventional techniques, dilational techniques are quicker, easier to perform outside the operating theatre, and cause less trauma to tissues in the neck, reducing the risk of bleeding and infection. The smaller incision is claimed to produce a

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better cosmetic result after decannulation and the lack of tracheal resection may reduce later tracheal stricture formation. Such claims have yet to be verified in controlled clinical trials comparing conventional and dilational techniques (Heffner, 1991). Practical aspects of dilational tracheostomy Tracheostomy may be satisfactorily performed under local or general anaesthesia. General anaesthesia should be provided as for a conventional surgical tracheostomy by a separate anaesthetist, avoiding the need for one person to act as operator and anaesthetist. The authors currently use intravenous opioids and anaesthetic agents to eliminate the requirement for an anaesthetic machine at the bedside. A muscle relaxant is not routinely required. Both techniques can be performed under local anaesthesia utilizing infiltration of tissue layers and injection into the trachea. The Rapitrach technique should be easier to perform under local anaesthesia as it has the advantage of speed and all dilatational forces are confined locally. The Ciaglia dilators causes traction on the trachea and larynx which may not be easily anaesthetized by local techniques. Under general anaesthesia, the oral/nasotracheal tube is withdrawn until the cuff lies in the larynx. This avoids later transection of the tube or cuff with the needle and guide wire. The neck is prepared as for conventional tracheostomy and a local anaesthetic agent (lignocaine 1% and 1/100 000 adrenaline) is infiltrated under the skin. This reduces capillary bleeding from the dilational track. A 1-2 cm horizontal or vertical incision is made in the conventional site, midway between cricoid cartilage and sternal notch. In the patient with chronic lung disease or a short neck this incision may have to be lower. The incision is made just through the skin and deeper layers are then blunt dissected with a pair of forceps. Superficial midline veins (Figure 4) are common and need to be avoided or tied off on occasions. Blunt dissection is continued, pushing the strap muscles aside, and a finger can then be inserted and the tracheal anatomy palpated. A needle and cannula are then inserted into the trachea between the first and second or between the second and third rings, confirmed by free aspiration of air into a saline-filled syringe. A guide wire is then inserted into the trachea. The technique described by Ciaglia et al (1985) uses a series of plastic dilators modified from a percutaneous nephrostomy kit. The largest dilator is 36 French gauge (11 mm outside diameter). Dilators are inserted over a Teflon-coated flexible guide wire (see Figure 1). A tracheostomy tube of appropriate size is then mounted on a smaller dilator and inserted into the trachea. A sterile surgical lubricant is used to aid insertion. Insertion is helped by using a flexible rather than a rigid plastic tracheostomy tube, and ensuring that it fits snugly on the dilator to minimize contours which can catch on tissue layers during insertion. There are developments under way to modify the end contour of tracheostomy tubes to make insertion easier. Air leaks through the dilational tract during the procedure can be controlled by digital pressure. It has been suggested that correct placement of the guide wire should be verified by simultaneous fibre-optic bronchoscopy (Paul et al,

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Figure 4. A n t e r i o r jugular vein in a female neck.

1989; Marelli et al, 1990). The dilation procedure itself takes 5-10 minutes as serial dilators are changed. Experience with this technique is favourable (Hazard et al, 1988; Cook and Callanan, 1989; Bodenham et al, 1991). The incidence of complications may be less than with conventional techniques (Griggs et al, 1991), but to date there has been only one small randomized control trial comparing the two, this study suggested the dilational procedure to be superior (Hazard et al, 1991). A larger trial is needed, given the low incidence of serious complications with both techniques and the poor survival of critically ill patients requiring tracheostomy. Schachner et al (1989) described an alternative technique utilizing a pair of specially designed forceps with grooved jaws, allowing passage over a short guide wire into the trachea. The device, originally designed for emergency usage, gives faster access to the trachea than does the Ciaglia kit. The forceps have a sharp, bevelled tip. After insertion into the trachea the jaws are opened and a tracheostomy tube is inserted through the jaws. The kit is available in sizes designed for the insertion of 6-mm, 6.5-mm, 7-ram, 7.5-mm or 8-mm tracheostomy tubes. The device may be less controllable than the serial dilators of the Ciaglia kit, and its jaws are sharper with the potential for greater damage if misplaced. The authors have opened the jaws in front of the trachea without problems, but there has been a recent case report where the posterior wall of the trachea was split requiring surgical repair (Hutchinson and Mitchell, 1991). We have also had a number of

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ruptured tube cuffs following perforation on the jaws. A combination of an instrument similar to the Rapitrach and the use of the Ciaglia dilators has been described (Grigg et al, 1990). The manufacturer is at present considering alternative safer designs for the jaws of the device. The safety of these dilational techniques is likely to be operatordependent as for any other invasive procedure, but needs to be compared with conventional techniques on any one unit rather than in isolation. Early use of the techniques has been marred in a number of units by complications during insertion. Such problems are likely with any technique as a learning curve is climbed. Caution in the use of such techniques has been rightly advocated in two recent editorials (Mathiesen, 1990; Heffner, 1991). The former editorial also suggested that much of the information regarding complications of conventional tracheostomy is anecdotal and requires reassessment. The incidence of late complications (in particular tracheal stenosis) following dilational techniques is not known, although clinical observations suggest that it is low. Current views on the aetiology of tracheal stenosis suggest that it should not be any more frequent than after conventional techniques. The absence of tracheal resection may in fact reduce the risk. Animal work using the Rapitrach technique has demonstrated minimal trauma and scarring of the trachea after insertion (Schachner et al, 1990). Longer-term controlled human studies are still awaited. The kits for dilational tracheostomy, although expensive (UK price s are cost-effective if the technique is safer, obviates moving patients, and reduces staff commitments and operating theatre time. Early changing of tracheostomy tubes after dilational techniques has not proved a problem in'the authors' experience. The dilated tract is stable due to the lack of dissection of tissue layers, and the tract grips the tube firmly minimizing the risk of dislodgement or bleeding. Such considerations are important before either technique can be recommended for use in patients who rely solely on a tracheostomy for their airway (for example after laryngectomy or known difficulty in orotracheal or nasotracheal intubation). The majority of patients requiring tracheostomy on the intensive care unit can be easily reintubated via the oral or nasal route if necessary. This is usually the preferred technique for airway access in the first instance when problems arise with newly formed tracheostomas. In our experience dilational techniques can safely replace conventional surgical techniques for the provision of elective tracheostomy in the critically ill. An added advantage is that familiarity with such procedures gives confidence and skills which could be useful when the need for true emergency tracheostomy or cricothyroidotomy arises.

Percutaneous cricothyroidotomy Cricothyroidotomy has been rejuvenated by the introduction of minitracheostomy (Matthews and Hopkinson, 1984). This technique was proposed as a method to aid clearance of sputum in patients unable to cough effectively and hence avoid the necessity for orotracheal or nasotracheal

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intubation. There have been no controlled studies, but the largest series to date suggested that the procedure did avoid oral or nasotracheal intubation and assisted ventilation in a significant number of patients (Pedersen et al, 1991). The technique is generally reserved for short-term use only. Other reported indications include emergency usage, tracheal access for highfrequency ventilation, oxygen therapy and relief of sleep apnoea (Ryan, 1990). A large number of complications have been reported after minitracheostomy, leading to the abandonment of the technique in some centres. These complications include life-threatening haemorrhage, pneumothorax, tube inhalation and tube misplacement (Clancy, 1989; Ryan, 1990). The incidence of such problems has prompted the development of the Seldinger guide wire and dilator kits which should reduce complications on insertion (Choudry and Jackson, 1988; Corke and Cranswick, 1988) (Figures 5 and 6). The authors have noted that the thickness and resilience of the cricothyroid membrane may on occasions make insertion of dilators difficult and uncomfortable for a patient who is awake. A significant incidence of misplacements utilizing a guide wire technique has been reported (Jackson et al, 1991). Bleeding is still a potential problem with such techniques and the authors have unknowingly transected veins with needle and guide wire; bleeding then occurs as dilation proceeds. The Mini-Trach kits are based on a 4-mm internal diameter tube which is rather small for suction of thicker secretions or assisted ventilation. There is a 6-mm internal diameter device designed for emergency cricothyroidotomy which overcomes some of these problems (Figure 7). In the authors' experience the successful introduction of percutaneous subcricoid tracheostomy techniques has significantly reduced the number of minitracheostomy procedures performed.

Figure 5. The Mini-TrachSeldinger(Portex, Hythe,UK).

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Figure 6. The Corke-Cranswick cricothyroidotomyset (Cook Critical Care, Bloomingtoa, Ind., USA).

Figure 7. The Melker emergency cricothyroidotomykit (Cook Critical Care, Bloomington, Ind., USA).

MEASUREMENT OF JUGULAR VENOUS OXYGEN SATURATION Over the last twenty years there has been increasing interest in the intensive management of brain-injured patients. The most commonly measured variable is intracranial pressure (ICP). In general intensive care there has recently been a move to monitor oxygen delivery and oxygen consumption of the whole patient and also specific organs or vascular beds. The brain is one such organ that has been studied. It is hoped that by intensive monitoring and manipulation of ICP, cerebral blood volume and cerebral blood flow patients can be offered a better prognosis following brain injury. Like other organs, the brain is dependent on the adequate delivery of

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oxygen to maintain its aerobic oxidative state. Decreased oxygen delivery will result in anaerobic metabolism with the subsequent formation of lactate. The adequacy of oxygen delivery is dependent upon the amount of oxygen present in the blood, the blood flow rate and brain metabolic rate. The brain is dissimilar from other organs in that it is surrounded by a rigid structure--the skull--which in diseased states may impose limitations on blood volume and intracranial blood flow. Rationale

In normal individuals cerebral blood flow (CBF) is closely coupled to and regulated by the cerebral metabolic rate for oxygen (CMRo2). Local CBF may be increased or decreased depending on tissue metabolic requirements. In some altered physiological states, for example seizures, pyrexia and anaesthesia, CBF remains coupled to CMR02. In patients comatose as a result of trauma or metabolic encephalopathy, CMR02 is typically reduced from a normal value of 1.5 txmol/g per minute to 0.6-1.2 txmol/g per minute (Lassen, 1959; Sokoloff, 1971; Robertson et al, 1987). If CBF remains coupled to CMRo2 then CBF will also be reduced. However, normal coupling of CBF is retained in only 45 % of comatose head-injured patients, with CBF either increased or decreased independently of CMRo2 in the remainder. Most of the energy required by the brain is derived from oxidative glycolysis and there are negligible stores of oxygen within the brain. Oxygen concentration within the brain is thus rapidly equilibrated with that of the blood. Therefore, oxygen flux across the brain is a good guide to CMRo2, since according to the Fick principle CMR02 = AVDo2 x CBF, where AVDo2 is the arterio-venous oxygen difference and CBF is the cerebral blood flow. In the normal individual, the arterio-venous oxygen content difference remains constant since CBF and CMRo2 are closely coupled. In the abnormal physiological state where CBF and CMRo2 change independently of each other, then AVDo2 will not remain constant (Figure 8). Few of the factors governing AVDo2 are readily measured; ICP is easily measured, and may affect both CBF and consequently CMR02. In the management of comatose patients much emphasis has hitherto been placed on monitoring and manipulation of ICP with perhaps lesser concern being placed on CBF and CMR02 because the latter two are not easily quantified. There has recently been a resmgence of interest in techniques to examine the oxygen content of 'mixed' venous blood from the brain, and to examine changes in cerebral AVDo2 in a range of pathological states. In animal research the usual sites for sampling cerebral venous blood include the superior sagittal sinus, and the torticular and retroglenoid veins, all requiring surgical exposure. The first safe and practical technique for sampling cerebral venous blood in humans was devised by Mergesson et al (1927). This technique involved puncturing the internal jugular vein near the base of the skull by inserting a needle slightly below and anterior to the tip of the mastoid and advancing it towards the blood vessel. Later a Seldinger technique was devised in a radiological study of the transverse sinus and internal

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jugular vein (Gejrot and Lindbom, 1960). There are now many papers describing the technique of jugular bulb cannulation which have gone on to evaluate the role of AVDoa measurement in comatose patients (Garlick and Bihari, 1987; Goetting and Preston, 1990; Andrews et al, 1991).

Anatomy Cerebral blood drains from the intracranial venous sinuses into the sigmoid sinus and thence into the jugular veins via the jugular foramina. There is a dilatation of the jugular vein just below the base of the skull known as the jugular bulb. The right jugular vein is usually larger than the left and is generally believed to drain most of the blood from the cerebral hemispheres, with blood from the posterior fossa draining into the left jugular vein. The jugular veins then descend in the neck closely applied and lateral to the common carotid arteries. Numerous extracranial veins join the internal jugular vein throughout its course. To measure accurately the venous oxygen content of blood draining from the brain it is, therefore, important that samples are taken high in the jugular bulb. In the normal individual CjBO2is the same from both right and left jugular bulbs (Gibbs et al, 1942). In the head-injured, unconscious patient it has been suggested that blood samples should be taken from the vein ipsilateral to the side of the major pathology (Robertson et al, 1987). Other workers suggest that since the extracranial venous drainage enters the right jugular vein higher than on the left then a more accurate reflection of intracranial oxygen consumption is gained by sampling from the left jugular bulb.

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Another approach in the comatose patient is to determine which jugular vein is predominant by temporary manual occlusion of right and left veins in turn with the patient positioned with 10-15 degrees of head-up tilt. Repeated measurement of ICP, systemic blood pressure and cerebral perfusion pressure are made and the side responsible for maximum change in these parameters is chosen for sampling. Choice of cannula

A Seldinger technique to introduce a catheter into the jugular bulb is now the favoured method. In most patients a 16 s.w.g, or 18 s.w.g, cannula of an appropriate length can be used. For continuous display of jugular bulb oxygen saturation a fibre-optic catheter may be used. It has been the authors' custom to flush the catheters intermittently with heparinized saline (2 ml), but a continuous flush device may be preferable. This catheter has also been reserved for blood sampling; however, other workers suggest that drugs (including inotropes) may be safely administered using this route (Goetting and Preston, 1990). Technique

The patient is positioned in a horizontal or 15 degrees head-up tilt position with the head slightly extended but otherwise neutral. This is a sterile procedure and full aseptic precautions are taken. The neck and upper part of shoulder are prepared with antiseptic solution and sterile towels applied. The site of puncture is next identified being slightly lateral to the carotid impulse at the level of the inferior border of the thyroid cartilage. A local anaesthetic agent is infiltrated into the skin at this site. The internal jugular vein is cannulated by advancing a needle from the insertion point in a cephalad direction with the needle directed posteriorly at an angle of 30 degrees from the skin. It is not uncommon to traverse the jugular vein without realizing because of the compressibility of the vein, particularly in the presence of head-up tilt. If no venous blood enters the syringe it should be withdrawn slowly while maintaining slight suction. When blood flows freely the syringe is disconnected and a J-tipped guide wire is advanced until resistance is met. The needle is then removed and the catheter advanced over the guide wire again until resistance is met. The wire is removed and the catheter is aspirated. The catheter tip readily kinks in the jugular bulb, and if blood does not flow freely the catheter should be withdrawn slightly until there is good flow. The cannula should then be sutured in place and dressed in the usual fashion. The fibre-optic continuous read-out saturation catheter (Oximetric, Opticath) is inserted through a 14 s.w.g, catheter positioned as described earlier. When the tip of the fibre-optic catheter meets the base of the skull a resistance is felt. The introducer 14 s.w.g, catheter is then carefully withdrawn over the fibre-optic catheter. Correct positioning of the catheter within the jugular bulb is confirmed using a lateral skull radiograph (Figure 9).

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Figure 9. Radiograph demonstrating correct positioning of a jugular bulb catheter.

Complications Cannulation of the jugular bulb is somewhat more difficult than conventional caudad cannulation of the internal jugular vein. Goetting and Preston (1990) reported a mean procedure time of 15.6 + 5.0 (SD) minutes with a range of 6-35 minutes. The median number of punctures was two. Hypovolaemia with collapse of the jugular vein seemed to be the main factor that caused either an increased puncture rate or failure of catheterization. Clearly this is made even more of a problem if the patient is positioned with a slight head-up tilt. Under such circumstances it may be safer either to allow slight head-down positioning or to wait until volume resuscitation has dilated the internal jugular vein. Haematoma formation resulting from either repeated venous punctures or carotid artery puncture, particularly in the presence of a coagulopathy, may be a problem. It is possible that compression of the jugular vein with consequent increase in cerebral blood volume and ICP may result. The catheter may be misplaced in a facial vein or may be looped in the jugular vein. The importance of checking the correct position of the catheter by radiography cannot be overemphasized. Other potential complications include phrenic and recurrent laryngeal nerve damage, and Homer's syndrome. Pneumothorax is possible. Other complications common to the Seldinger insertion of cannulae such as infection, perforation of vessel, shearing of wire, etc., can all occur.

Use of jugular blood sampling Cerebral blood flow and cerebral metabolic rate are important determinants of jugular bulb oxygen saturation. Cerebral oxygen delivery (CBF x 02

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content), which is dependent on arterial oxygenation, is also important. Normal cerebral blood flow is 60ml per 100g per minute and normal CMRo2 is 1.5 txmol/g per minute (as determined by positron emission tomography--PET). The normal oxygen extraction rate (OER) of the brain is approximately 40%. By measuring lactate concentrations in the arterial supply and jugular bulb a cerebral metabolic rate for lactate (CMRL) (CBF x AVDlactate) can be determined--normally this is minus 0.02 p~mot/g per minute. It is also possible to calculate the lactate-oxygen index (LOI) (AVDlactate/AVDo2), which is normally less than 0.03. This is useful since no measurement of CBF is required. If CBF is decreased but cerebral oxygen consumption is maintained by an increased OER, then AVDo2 and AVDlactate will be increased and the LOI ratio will be unchanged. If the OER cannot increase sufficiently then cerebral oxygen consumption will decrease, cerebral lactate production will increase and the LOI will be increased. An LOI of 0.08 or more accurately predicts increased lactate production (Robertson et al, 1987). The metabolic changes associated with ischaemia may or may not be reversible. The time required for changes to become irreversible depends on the severity of the reduction in CBF. It has been shown experimentally that a CBF of 0.18 ml/g per minute is tolerated for several hours, while a CBF of less than 0.10 ml/g per minute produces infarction within minutes (Jones et al, 1981). At this point CMRo2 falls but CMRL may remain elevated. Using these principles it has been possible to characterize patterns of cerebral metabolism. 1. 2. 3.

Low CBF, increased AVDo2, normal CMRL--compensated hypoperfusion. Low CBF, increased AVDo2, increased CMRL---early cerebral ischaemia. Low CBF, normal or decreased AVDo2, increased CMRL--ischaemia/ infarction.

If the LOI is less than 0.08 cerebral ischaemia is not present and CBF can be predicted reliably from the AVDo2. This permits identification of patients with compensated hypoperfusion who may benefit from attempts to increase the CBF. When LOI is 0.08 or greater, ischaemia or infarction is present and actual measurement of CBF is required. Under these circumstances, therapeutic manoeuvres that are successful in increasing CMRo2 will cause a decrease in LOI. Perhaps the ideal is a continuous measurement of jugular bulb oxygen saturation and ICP. Periodic measurements of AVDo2 and AVDlactate can be made. A fall in jugular venous oxygen saturation or an increase in the LOI should alert the physician to search for a cause of the decrease in cerebral oxygen delivery. Measuring cerebral blood flow There are currently two techniques used to measure cerebral blood flow: inert gas uptake and Doppler ultrasonography. Since Kety and Schmidt

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(1948) introduced their method of determining cerebral blood flow in humans a variety of elegant modifications have been developed. For a global blood flow measurement probably the original tracer gas nitrous oxide (N20) method is still the best, although it is possible that PET (Powers and Raichle, 1985) and single-photon emission computerized tomography (SPECT) (Lassen and Blasberg, 1988) may also be useful. Radiolabelled tracers such as 133Xe have been widely used to investigate regional cerebral tissue perfusion in humans through an intact skull. The Kety-Schmidt technique uses the wash-in or wash-out curves of an inert substance. The tracers can be either injected into the carotid artery or (perhaps more usefully in the clinical setting) administered intravenously or by inhalation. If 133Xeis used then scintillation detectors are placed over the area of interest. If N20 is used then intra-arterial and jugular bulb blood samples are carefully taken and later analysed for N20 content. For a good review on the subject the reader is referred to Reivich et al (1975). Doppler velocity measurements can be used to give flow in either extracranial (Payen et al, 1982) or intracranial blood vessels (Hopper et al, 1987). First, the diameter of the blood vessel is determined and then the velocity'of the entire arterial blood column is measured. Beat-by-beat cross-sectional blood flow velocity curves can be evaluated which enable peak systolic and end-diastolic velocities to be measured. This system also shows reverse flow in the carotid arteries. There has been a recent report of this technique in the diagnosis of brain death (Payen et al, 1990), which showed that determinations of end-diastolic velocity and end-diastolic blood flow were good discriminators for brain death with no false positive results. In this same study AVDo2 was measured and was also a good discriminator of brain death, with an AVDo2 of less than 2 ml 02 per 100 ml being significant (Black, 1978).

SUMMARY

There is a resurgence of interest in the use of tracheostomy and cricothyroidotomy techniques for airway management in the critically ill. The development of new dilational devices has improved the safety and effectiveness of these techniques, allowing minimal tissue disruption and safe utilization at the bedside. Increasingly such procedures are performed by operators who are not surgeons. Experience reported by the originators of the devices and others has been favourable, although the incidence of late complications is unclear. The place of these techniques in clinical practice is still controversial and needs further evaluation. Measurement of jugular venous oxygen saturation, while at present mainly a research tool, may be clinically very useful. However, clearer studies of clinical outcome are needed before another invasive and potentially dangerous intervention is applied to every brain-injured patient.

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A. BODENHAM A N D N. R. WEBSTER

REFERENCES

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