Assessing Endotracheal Tube Cuff Pressure

Establishing a secure airway via endotracheal intubation is a critical clinical skill and lifesaving technique commonly used in the prehospital setting. The procedure, however, can cause complications even long after the endotracheal tube (ETT) is placed past the vocal cords and secured. Tracheal necrosis,¹ rupture,^{2, 3} stenosis,⁴ laryngeal nerve palsy^5,6 and tracheo-esophageal fistula7 are all potential risks when the pressure in ETT cuffs is excessively high.

It has long been believed, without any evidence-based data, that trained clinicians are capable of determining proper ETT cuff pressures. More specifically, it is presumed that clinicians can detect appropriate inflation pressure and overinflated ETT cuffs by palpation of ETT pilot balloons. In this study, we sought to evaluate newly trained paramedics' ability to inflate ETT cuffs to a safe pressure, as well as to detect overinflated ETT cuffs by palpating the pilot balloon. The phenomenon merits study because overinflation of an ETT cuff may be prevented by using a manometer to directly measure inflation pressure.^1,8

Methods

Study Design

The study was a prospective, observational study of graduating paramedic students' ability to properly inflate and palpate the pilot balloons of ETT cuffs. This study was approved by the Institutional Review Board of St. Luke's-Roosevelt Hospital Center. Informed consent was obtained from all participants prior to the study.

Setting and Population

This cross-sectional study examined a cohort of 23 paramedic students graduating an accredited one-year training program at an accredited U.S. college.

Study Protocol

We sought to determine the ability of the study population to inflate an ETT cuff to a safe pressure using standard syringe technique. A secondary outcome measure was the ability of the study participants to identify overinflated ETT cuffs by palpation.

Safe pressure was defined as less than or equal to 25 cm H2O.9 Below this pressure, capillary perfusion pressure is not typically impaired and there are no expected risks of long-term compression damage to the human airway.

Standard syringe technique consisted of ETT cuff inflation using a syringe to inject air into the cuff and, if participants chose to do so, assessment of cuff pressure by palpation of the external pilot balloon. Specifically, a previously tested simulation model10 with a 7.5 ETT and using a high-volume, low-pressure cuff (Mallinkrodt, St. Louis, MO) placed into a 2.0 cm rigid cylinder. Study participants were then allowed to choose either a 5 mL or 10 mL plastic syringe with standard luer lock (Becton-Dickson, Franklin Lakes, NJ) to inflate the ETT cuffs, while simultaneously manually palpating the pilot balloon as preferred. After the participants completed inflation of the endotracheal tube cuff, the cuff pressures were measured using a highly sensitive and accurate analog manometer (Boerhinger Laboratories, Norristown, PA).

The second part of the study involved participants palpating the pilot balloons of nine ETT cuffs previously inflated to pressures ranging from zero cm H2O to 120 cm H2O and reporting their assessment of pressure of each pilot balloon as appropriate, too low or too high. The pressures, in cm H2O, were: 0, 4, 8, 16, 20, 22, 80, 100, 120.

Over an initial one-month pilot period, measurements of intracuff pressures with participants not included in this study were performed. This was done to ensure investigator skill in making measurements and to assess for flaws in protocol design, implementation or conduct of the study.

Sample Size

Based on a previous study of this phenomenon in emergency medicine residents, a sample size of five was expected to be adequate for statistical analysis, with a 1% significance level () and 80% power (1-). Rather than simply test five study participants, we sampled the entire graduating class of 23.

Data Analysis

Our null study hypothesis was that the maximal ETT cuff pressure generated by study participants would not exceed 25 cm H2O. A secondary hypothesis was that study participants would be able to detect overinflated ETT cuffs by palpation. A student's t-test and Chi-square test were utilized for hypothesis testing where appropriate. Statistical analysis was performed using Intercooled Stata 8.2 statistical software (College Station, TX).

Results

The average ETT cuff pressure produced could not be precisely determined because 70% of participants (n=16) inflated to pressures greater than the upper limit of manometer sensitivity (>120 cm H2O). Using the available censored data, however, the average pressure generated was >98 cm H2O, range 50 - 120 cm H2O.

Using a one-tailed hypothesis test with of 0.01, the null hypothesis that the mean pressure generated by study participants would not exceed 25 cm H2O was rejected. The secondary hypothesis of participants' ability to detect overinflated ETT cuffs by palpation was also rejected. Participants were only 26% sensitive detecting overinflated ETT cuffs. No participant correctly identified all overinflated ETT cuffs.

Survey data revealed that 78% of participants (n=18) inflated ETT cuffs using palpation of the pilot balloon to determine the quantity of air to inject into the balloon when inflating an ETT cuff. The other participants preferred a method of injecting a set volume of air into the cuff, 13% (n=3), or injecting air until they perceived resistance, 9% (n=2).

Discussion

When the pressure in an ETT cuff exceeds the capillary perfusion pressure of the tracheal mucosa, mucosal blood flow becomes obstructed. The precise pressure at which capillary perfusion is impaired certainly will vary from patient to patient, but reports suggest that 25 cm H2O is a "safe" pressure.⁹ The precise pressure at which any individual will experience impaired or obstructed tracheal mucosal blood flow will be dependent upon numerous factors, most importantly their blood pressure. Severe overinflation of the ETT cuff may result in severe, even fatal injury.^3,11 Less severe, but significant, adverse effects are tracheal pain or stridor.^{12, 13}

This study demonstrates that recent paramedic graduates were unable to inflate an ETT cuff to a safe pressure. These study participants were also unable to differentiate overinflated ETT cuffs from appropriately inflated ETT cuffs by palpation of the pilot balloon.

Although these graduating paramedic students grossly overestimated a safe inflation pressure, comparison to a similar study examining the ability of emergency medicine physicians and another study of practicing paramedics showed that these students' abilities were similar to those of the more highly trained or experienced groups.^14,15 This implies that this skill is not acquired over time with increased training or experience.

It has been suggested that using standardized instrumentation to measure cuff pressure would help to decrease the possibility of injury resulting from endotracheal intubation.^1,8,16 Whether this is practical for paramedics to do in the field is unknown. Use of such instruments might interfere with the top priority of securing an airway or rapidly transporting the patient to a hospital environment. Additionally, since there are little data available on short-term complications, it is unclear what difference in outcome would result from measuring inflation pressures in the field relative to measuring such pressures in the hospital. This may be an area for future studies.

Prehospital professionals should consider the available information suggesting that clinicians cannot detect overinflation of ETT cuffs by palpation of the pilot balloon. Emergency department staff should consider taking it upon themselves to measure cuff pressures after endotracheal intubations, whether performed in the field or in the ED, as it is a certain method of detecting overinflated ETT cuffs. Many patients who remain intubated eventually have their ETT cuff pressure checked by respiratory therapists or intensive care unit staff. However, since overinflation of ETT cuffs is recognized as potentially injurious, prehospital personnel and emergency medicine clinicians should consider screening for overinflation of ETT cuffs.

The inability of clinicians to determine endotracheal tube cuff pressure by the traditional standard method of palpation of the pilot balloon has been addressed by other investigators. Anesthesiologists,¹⁷ as well as critical care unit staff,^1,18 have demonstrated a prevalent inability to accurately determine intracuff pressure of the ETT cuff by palpation of the pilot balloon.

The risk of injury resulting from an overinflated ETT cuff warrants evaluation of currently employed endotracheal intubation practices. The practice of inflating ETT cuffs without precisely measuring such pressure, in particular, should be closely scrutinized.

Limitations

As conducted, this study has several potential limitations. The first of these is that the model does not precisely resemble the human airway in shape or compliance. Participants might have performed better and responded differently in live scenarios where real patients were involved. However, ethical considerations preclude being able to conduct such a study on human patients.

Secondly, physiologic variations among human tracheal blood flow might provide for a variation of safe cuff insufflation pressures from person to person, thus limiting the relevance of a study examining a single uniform simulated intubation environment.

A sample size of 23 is considered small for the purposes of statistical analysis. Although the results of this study did not raise any concern that the study lacked power, it would be optimal to have had a larger sample size. Lastly, it is hard to evaluate the uniformity of the group, as students often have not all achieved the same skills set or experience. There is the potential to have variation in outcomes based on individual students' training and experience.

References

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Vivek Parwani, MD, is an assistant clinical professor and EMS physician at the Yale University School of Medicine, Division of EMS, Section of Emergency Medicine.

In-Hei Hahn, MD, is an emergency medicine physician with fellowship training in medical toxicology. She is an assistant clinical professor and assistant director of research in the Dept of Emergency Medicine at the St. Luke's-Roosevelt Hospital Center, New York, NY.

Paul Krieger, MD, Paul Zajac, MD, & Dwight Arakaki, MD, are resident physicians at Beth Israel Medical Center, Department of Emergency Medicine, New York, NY.

Robert J. Hoffman, MD, is a pediatric emergency physician and research director in the Department of Emergency Medicine at the Beth Israel Medical Center, New York, NY.