DESIGN
Queen Elizabeth Hospital, King’s Lynn, UK
Endotracheal tubes
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Endotracheal (ET) tubes have a cuff at the distal end that is inflated to provide a seal in the trachea. Inflation is achieved by injecting air through a one-way valve known as the pilot tube at the other end of the line. Correct inflation pressure is important because over-inflation leads to excess pressure being exerted on the tracheal wall, and under inflation leads to an inadequate seal and an air leak when trying to ventilate the patient. A safe pressure is thought to be 20–30 cmH2O; pressures of more than 40 cmH2O are associated with ischaemia, necrosis and ultimately tracheal stenosis.1,2 Over time, gas diffuses through the wall or escapes through the valve and the cuff needs to be re-inflated. There are three ways of achieving adequate cuff pressure over time.
Guesswork. Air can be introduced into the cuff with a syringe until no leak is heard.
Using a manual cuff inflator device. Pressure and can be left connected to give a continual readout. Examples of products on the market are the Cufflator (JT Posey Company, Arcadia, California, USA), Endotest (Rusch, Duluth, Georgia, USA) or Cuff Pressure Indicator (SIMS-Portex, Keene, New Hampshire, USA). The shortcomings of manual inflator devices are inaccuracy and the need for vigilance on the part of the operator to maintain constant pressure.3 An important problem relates to the compressible volume within the device, which should be kept to a minimum. This causes a fall in cuff pressure as soon as the pilot valve is connected and opens. Unintentional depression of the deflation button on the inflator can also occur. If manual inflators are used, it is necessary for all operators to understand the issues and be proficient in the use of these devices.
Using a mechanical device. This will monitor cuff pressure and adjust it automatically. This is the most appealing option. There are devices commercially available that continually monitor the cuff and add or withdraw air to maintain a constant pressure, for example, the Tracoe Cuff Pressure Control (Tracoe Medical GmbH, Frankfurt, Germany).
Scope of the problem
All commercially available ET tubes have the same female luer slip receptacle to which the tip of any syringe will fit (Figure 1a). Cuff inflators utilise tubing with a luer lock connector at either end to run between the device and the pilot balloon. Intravenous, arterial, intrathecal and epidural cannulae also use the luer lock system and it is therefore possible to connect an air line to any of these and introduce air into the circulation, spinal or epidural space, respectively.
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A simple modification to prevent iatrogenic air embolism in the intensive care unit.
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A typical critically ill patient has three or four lumen central venous cannula, a one or two peripheral venous cannulae, an arterial line and a spinal or epidural catheter with the associated three-way connectors. One patient can have more than 14 luer lock connections around their bedspace in contact with their body. Because of the nature of critical care interventions, lines are frequently disconnected, replaced or swapped. An unfortunately common error is to connect a line containing fluid that can only be given safely through a central vein (for example, concentrated potassium chloride) to a peripheral cannulae, which results in venous necrosis. Although this is painful and distressing for the patient and often leads to disciplinary action against staff, it is unlikely to be fatal. Air embolus is an uncommon iatrogenic event, but it has a high mortality. The volume of air required for a fatal air embolism is relatively small: 0.02 mL/kg. Even this small volume results in 28% mortality in a canine model.4 Because the driving pressure of a cuff inflator is normally greater than the central venous pressure, a mistaken connection of an air line to a central venous catheter would be rapidly fatal. Although it is commonly assumed that the risk of air embolism is apparent only during central venous catheter insertion, the majority of these incidents occur during manipulation, disconnection or removal of the catheter or as a result of a fracture of one of the components.5 With the increasing use of cuff inflator devices, it is unlikely to be long before a critically ill patient suffers a massive iatrogenic air embolus.
Method and materials
It is not feasible to change the pilot tube or other female luer slip connectors because both need to be accessible from a standard syringe tip. Therefore, the new noninterchangeable male connector of the air line has been redesigned to prevent it from fitting a standard luer lock device. Standard luer lock devices have projections from the lip of the female connector (Figure 1b) that allow them to screw into male connectors (Figure 1c). Tracheal tube pilot tubes do not have these projections (Figure 1d), thus there is no need for the corresponding connector to have a thread. The connection is held fast by the taper of the male connector.
In this way a new non-interchangeable connector has been created (patent pending), which has a smaller internal diameter in the cap (Figure 1e) to allow the pilot tube to fit snugly, but prevent a female connector with projections from being entered (Figure 1f). The new connector is used in the intensive care unit of Queen Elizabeth Hospital (King’s Lynn, UK) for all patients whose cuff is connected to a constant pressure device.
Outcome
The connector has been introduced with no complications in 52 patients. Staff were surveyed for their opinions on ease of use. None had noticed that the air line had been changed and no problems with use were reported. This connector has now been introduced as standard care in the hospital’s intensive care unit.
There are no disadvantages in using the new connector and a considerable safety advantage. In a clinical situation where constant pressure devices are being used to inflate tracheal tube cuffs, especially when other luer lock connectors are in place, it is no longer appropriate to rely on nursing vigilance to protect the patient from air embolus. All air lines connecting constant pressure devices to tracheal tubes should have the modified connector to prevent accidental misconnection.
References
1. W.M. Shelly, R.B. Dawson and I.A. May, “Cuffed Tubes As a Cause of Tracheal Stenosis,” J. Thorac. Cardiovasc. Surg. 57, 623–627 (1969).
2. J.D. Cooper and H.C. Grillo, “The Evolution of Tracheal Injury Due to Ventilatory Assistance Through Cuffed Tubes: A Pathologic Study,” Ann. Surg. 169, 3, 334–48 (1969).
3. P.B. Blanch, “Laboratory Evaluation of Four Brands of Endotracheal Tube Cuff Inflator,” Resp. Care, 49, 2, 166–73 (2004).
4. T. Stegmann et al., “Experimental Coronary Air Embolism. Assessment of Time Course of Myocardial Ischemia and the Protective Effect of Cardiopulmonary Bypass,” Thorac. Cardiovasc. Surg. 28, 141–149 (1980).
5. E.W. Ely et al., “Venous Air Embolism From Central Venous Catheterisation. A Need For Increased Physician Awareness,” Crit. Care Med. 27, 2113–2116 (1999).
Luke A. Evans MB BS MRCS is Research Fellow in Critical Care, Queen Elizabeth Hospital, King’s Lynn PE30 4ET, UK, tel. +44 1603 592 244, e-mail: luke.evans@uea.ac.uk
Peter J. Young is a Consultant in Critical Care, Queen Elizabeth Hospital, King’s Lynn PE30 4ET, UK, tel. +44 1553 613 613, e-mail: peter.young@lotrach.com
