Endotracheal intubation involves placing a flexible tube into a patient’s windpipe (trachea) to ensure an open airway and facilitate breathing, typically with a ventilator. Success depends entirely on delivering the tube to a specific, safe distance within the airway. This precise placement allows air to reach both lungs while preventing injury. Medical professionals use anatomical landmarks and measurement techniques to secure the tube at the exact depth required for patient safety and effective ventilation.
The Target Location Inside the Airway
The final resting place for the tip of the endotracheal tube is the mid-trachea, the segment of the windpipe above the split into the lungs. The tube must pass through the vocal cords, which guard the entrance to the trachea, and descend toward the lower airway. The most important anatomical landmark for determining depth is the carina, the point where the trachea divides into the right and left main bronchi.
The goal is to position the tube tip to maximize safety and security despite patient movement. In adults, the tube tip is optimally positioned approximately 4 to 5 centimeters above the carina. This distance provides a safe margin, preventing the tube from migrating too far down into one of the main bronchi or moving too close to the vocal cords. Head and neck movement, such as flexing or extending, can cause the tube to shift by up to two centimeters, emphasizing the need for this safety margin.
Measuring and Securing the Correct Depth
Endotracheal tubes have centimeter markings along their length to manage internal placement from an external perspective. These markings allow the medical team to note the measurement at the patient’s lips or teeth, providing an external reference point for the tube’s depth. Fixing the tube at a standardized external measurement achieves the desired internal position above the carina.
A widely referenced guide for depth in adults is securing the tube at 23 centimeters for males and 21 centimeters for females, measured at the incisor teeth or lips. This general rule accounts for average anatomical differences, though a patient’s height is a stronger predictor of the actual distance from the mouth to the carina. For patients with a shorter airway, a depth of 19 or 20 centimeters may be more appropriate to prevent the tube from going too far.
For pediatric patients, a fixed number is not used because the length of the trachea changes rapidly with growth. Instead, the insertion depth is calculated based on the child’s age or size, aiming to place the tip about 1.5 centimeters above the carina. Once the appropriate depth is achieved based on these external measurements, the tube is secured in place at the lips or teeth with tape or a specialized device to prevent accidental movement.
Immediate Confirmation of Proper Placement
Verification steps must be performed immediately after insertion to ensure the tube is in the correct location and depth. The most reliable method for confirming the tube is in the trachea and not the esophagus is continuous waveform capnography. This device measures the carbon dioxide (CO2) in the air exhaled through the tube; the presence of CO2 confirms the tube has entered the airway leading to the lungs.
A standard technique is auscultation, which involves listening to the patient’s chest and upper abdomen with a stethoscope. The provider checks for equal breath sounds over both the right and left sides of the chest, suggesting air is reaching both lungs. Hearing breath sounds over the stomach area suggests the tube may have entered the esophagus, delivering air to the stomach instead of the lungs.
While capnography and auscultation provide immediate feedback, a chest X-ray is typically performed later to confirm the precise tip-to-carina distance for long-term management. The X-ray allows the medical team to visually confirm the tube tip rests in the mid-trachea, ideally 4 to 5 centimeters above the carina. This secondary confirmation ensures the initial placement is maintained and provides a baseline for monitoring subsequent tube migration.
Risks Associated with Incorrect Positioning
Strict adherence to measurement and confirmation protocols is necessary to avoid two major complications related to incorrect tube depth. The first occurs if the tube is inserted too far, known as mainstem bronchus intubation. If the tube tip passes the carina, it typically enters the right main bronchus due to its straighter alignment with the trachea.
When this happens, only the right lung receives ventilation, and the left lung is denied air, leading to collapse (atelectasis). This complication reduces the oxygen available to the patient and can lead to hypoxemia. The second complication is esophageal intubation, which occurs if the tube bypasses the trachea and enters the esophagus, or food pipe.
In this scenario, air is delivered to the stomach, causing inflation, while the lungs receive no oxygen, leading to rapid oxygen desaturation. The capnography device prevents this by showing a flat line, as the esophagus does not produce carbon dioxide. Both scenarios highlight why the specific distance the tube travels—deep enough to clear the vocal cords but not so deep as to enter a single lung—is carefully monitored.