When intubating children, the question always arises whether to use a cuffed or an uncuffed endotracheal tube (ETT). Historically uncuffed endotracheal tubes have been used when the child is less than about 8 years old. Why is it that we can get away with using an uncuffed tube in a young child, but not an adult? Are there advantages and disadvantages to each? The answers comes from understanding some of the anatomical differences between children and adults.
Obtaining a Seal Around the ETT
Positive pressure ventilation depends on providing enough pressure to inflate the lungs. If there is a break in the pressure seal, such as through a leak around the endotracheal tube, then ventilation may be inadequate. The air takes the path of least resistance back out the mouth, rather than inflating the lungs. In addition, the patient can potentially aspirate around the tube through the gap.
In adults the glottic opening between the vocal cords is the smallest diameter of the larynx. The airway below the cords is bigger than the ETT and the cuff on the ETT seals the gap.
Historically, uncuffed tubes in infants and children younger than about 8 years have been used because a child’s larynx is anatomically different. In a child younger than 8, the smallest diameter of the airway is the cricoid ring. A properly sized round endotracheal tube sealed this round opening, like a round peg in a round hole. Use of the proper size ETT made aspiration unlikely.
In addition, use of cuff were felt to place the youngest children at risk of tracheal mucosal injury and post extubation croup due to pressure from the cuff itself. The optimal size uncuffed tube allows adequate ventilation but leaks at about 20-25 cm of water pressure. This pressure leak is chosen to minimize the risk of too much pressure being applied to the tracheal mucosa causing ischemia.
As the child grows and becomes more adult like at the age of 8 years, the anatomy changes. Now the triangular vocal cord opening becomes the smallest diameter. An ETT cuff is now needed to seal the larger tracheal diameter below the cords in older children.
Newer research, however, has found that the cricoid ring is more elliptical in shape than round (1). This means that a round ETT placed into an elliptical hole can, despite the presence of a leak, put potentially excessive pressure on parts of the tracheal mucosa. This discovery makes use of uncuffed tubes less compelling given the fact that cuffed tubes do offer advantages.
Advantages of Cuffed ETTs:
- protect better against aspiration
- allow higher ventilation pressures in patients with poor pulmonary compliance
- changing an ETT can be avoided when too small a tube is initially inserted
- monitoring ventilation and tidal volume is more accurate.
- during anesthesia, there is less pollution from anesthetic gas leaking around the tube.
Disadvantages of Cuffed ETTs
On the other hand, there are potential disadvantages to using cuffed tubes in small children.
- cuffs take up space and force use of a smaller tube.
- suctioning smaller tubes is more difficult
- decreasing the tube radius a little greatly increases resistance and significantly decreases flow.
The relationship between radius and resistance is described by the Hagen-Poiseuille equation: Q = nPr4/8l
Where Q = Flow in Liters/second
n = Viscosity in Pascals
P = Pressure in Pascals
r = Radius of the tube in meters
l = Length of the tube in question in meters
Let’s look at an example of how radius effects resistance. A cuff takes up space and forces use of a tube one-half to one size (0.5-1mm I.D.) smaller than calculated size. A 4 yo will normally take a 5.0 uncuffed tube, and a 4-4.5 I.D. cuffed tube. A 4.0 cuffed tube has 59% increased resistance to flow compared to the 5.0. Therefore, use of the larger uncuffed tube significantly lowers resistance and thus the amount of force required for breathing.
The second concern about cuffed tubes in small children stems from the risk of damaging mucosa, potential post-extubation croup and the late appearance of tracheal stenosis. Years ago, cuffed tubes were a high pressure-low volume type that exerted more point pressure on a limited area of mucosa, potentially interrupting capillary blood flow. The newer low pressure-high volume cuffs spread that pressure over a larger surface area. Studies using modern low pressure-high volume cuffs have shown no increase in post-extubation croup between cuffed and uncured tubes (2,3,4,5). These studies have also not shown an increase in the rate of complications between cuffed and uncuffed endotracheal tubes in terms of long term complications such as tracheal stenosis.
Choosing the proper Size
The proper size can be estimated by calculating the (age in years divided by 4) plus 4. If you are using a cuffed tube, select a size one-half to one size I.D. smaller than calculated size to allow room for the volume of the uninflated cuff. Check for a leak before inflating. You may find that you don’t need to inflate the cuff to obtain an adequate seal.
Because the tip of the endotracheal tube rises within the trachea with head extension, the cuff can rise into the subglottic areas, risking injury to these areas as well as to the recurrent laryngeal nerves. A cuffed tube may need to be repositioned if the head will be in extension for a prolonged period. The proper depth in cm can be estimated by calculating the (age in years divided by 2) plus 12.
You can use a cuff in infants and children younger than eight if you follow precautions:
- Be meticulous with choice of tube size, tube depth, and stabilization.
- Don’t overinflate the cuff to greater than 20-25 cm H2O to avoid pressure induced injury of the mucosa. Inflate the cuff to minimum seal—the cuff volume where air leak around the tube just stops during constant pressure held at 20-25 cm H2O. Monitor pressure if possible.
- Make sure that the cuff is always below the cricoid cartilage.
- Nitrous oxide used during anesthesia will accumulate in any closed air filled cavity, including ETT cuffs. As nitrous collects, pressure increases over time and distends the cuff. Check the cuff tension periodically — you may have to repeatedly remove a small amount of air to return the cuff to minimum seal.
- During aerial transport of a patient, the air in the cuff will expand at altitude and compress the mucosa. Repeatedly remove air to restore minimal seal. Note that as the aircraft descends, you may have to add a bit of air to the cuff as the cuff loses pressure. Inflating the cuff with saline avoids pressure changes. You will need a lot less saline than air so be careful not to over pressurize.
The Hagen-Poiseuille equation also shows that resistance increases as tube length increases, so a shorter ETT has less resistance to breathing. It’s common practice to cut the length of the endotracheal tube in an infant so that it extends no more than 1-2 cm from where it exits the mouth.
So the jury is still out on cuffed vs. uncured in small children. However, you should consider using a cuffed tube for children at risk for aspiration and children who might need higher inspiratory pressures.
May the force be with you.
King, BR; Baker, MD; Braitman, LE; Seidl-Friedman, J; Schreiner, MS. Endotracheal tube selection in children: a comparison of four methods. Ann. Emerg. Med. 1993; 22(3):530-534.
Gerber, AC. Cuffed tubes for infants and children in anaesthesia and intensive care: Why we should change to cuffed tubes in paediatric airway management (review). Journal of Paediatric Respirology and Critical Care, 2008; 4(4):3-9.
Motoyama, EK. Endotracheal intubation. In: Motoyama, EK; Davis, PJ (eds.). Smith’s anesthesia for infants and children, 7th Ed. St Louis, MO: CV Mosby, Pages 335-337, 1996.
Weiss, M; Knirsch, W; et.al., Tracheal tube-tip displacement in children during head-neck movement–a radiological assessment. Br. J. Anaesth. 2006; 96(4):486-491.