Endotracheal Tube Intubation: Assisting (Neonatal) - CE
ALERT
Don appropriate personal protective equipment (PPE) based on the patient’s signs and symptoms and indications for isolation precautions.
Refer to the American Heart Association (AHA) interim guidance for basic and advanced life support in adults, children, and neonates with suspected or confirmed coronavirus disease (COVID-19) (Box 1)
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Intubation attempts should be limited to 30 seconds.1,2 Stabilize the patient with bag-mask ventilation between attempts to limit complications associated with the procedure.1
Avoid hyperoxygenation, hyperinflation, and hyperventilation.2
Applying cricoid pressure during oral intubation is no longer a recommended practice.7
OVERVIEW
Endotracheal (ET) intubation is a common procedure used to resuscitate patients in the delivery room and the neonatal intensive care unit. To reduce complications, the intubator should perform the procedure expeditiously in an environment that is as controlled as possible. In neonatal emergencies, oral intubation is the preferred route. Nasotracheal intubation is considered to allow more stable tube placement in the trachea and may be used for elective procedures, in patients with copious secretions, or when defects of the oral anatomy are present. Contraindications for nasotracheal intubation include increased risk of bleeding or nasal deformities (e.g., choanal atresia).8
Indications for ET intubation include:1,2,6
- Suctioning bronchopulmonary secretions or obtaining a direct tracheal culture specimen
- Positive-pressure ventilation when bag-mask ventilation is ineffective or prolonged mechanical respiratory support is required
- Administration of exogenous surfactant or medications in the emergency setting before IV access is established
- Critical upper airway tract obstruction (e.g., subglottic stenosis)
- Abnormal blood gas values indicating respiratory failure (high carbon dioxide levels, low pH)
- Frequent, clinically significant apneic episodes that are unresponsive to conventional therapy
- Airway abnormalities that interfere with ventilation (e.g., Pierre Robin syndrome)
- Diaphragmatic hernia
Routine tracheal suctioning of a nonvigorous, meconium-stained neonate is no longer recommended by the AHA.1 Tracheal suction is recommended only if evidence of an obstruction exists.1
Intubation is a two-person procedure. The assistant provides free-flow oxygen, monitors heart rate and peripheral oxygen saturation (SpO2), and suctions if airway secretions interfere with observation.
Proper placement of an ET tube in a neonate is challenging. Proper placement generally is only 1 to 2 cm past the vocal cords.1,2 Inserting the tube too far places the neonate at risk of injury to the carina or improper placement into the right mainstem bronchus. Weight-based estimation for proper insertion depth is no longer considered standard.5 Two methods are recommended for estimating the insertion depth: the nasal-tragus length (NTL) measurement or an estimation based on gestational age (Table 1)
. The NTL method is validated for both preterm and term neonates and uses a calculation based on the distance in centimeters from the neonate’s nasal septum to the ear tragus. The estimated insertion depth is the NTL measurement plus 1 cm.1,2,4
ET intubation may be done via the nasal or oral route. The intubator’s skill and the patient’s clinical condition determine the route. If the nasal route is preferred, such as when facial deformities preclude intubation by the oral route, the process is more difficult and should be attempted only by a skilled intubator. In most cases, the oral route is selected over the nasal route. When the nasal route is used, the depth of insertion can be calculated based on the length of the patient, using the modified Morgan formula (ET tube position at the nares in cm = 0.12 × height [cm] + 5).5
Intubation attempts should take no longer than 30 seconds.1,2 If more than one intubation attempt is required, ventilation using a resuscitation bag or a T-piece resuscitator with a tight-fitting face mask should be performed before each subsequent attempt until the patient recovers. The concentration of oxygen used during resuscitation depends on the patient’s gestational age, chronologic age, and response to resuscitation. Excessive oxygen may cause oxidative stress and further compromise the patient. If resuscitation is initiated without the use of oxygen and no improvement is seen, oxygen should be administered and adjusted to maintain SpO2 readings at desired levels.1,2
Two types of laryngoscope blades exist, straight and curved. The straight blade is preferred for neonatal intubation. The size of the blade is based on the patient’s weight (Table 2)
.
ET tube size reflects the size of the internal diameter of the tube. The most commonly used ET tubes for neonates range from 2.5 mm to 3.5 mm (Table 3)
.1 The ET tube with the largest clinically acceptable internal diameter should be used to minimize airway resistance and air leaks and to facilitate suctioning.
ET tubes used for neonatal intubation are not tapered or cuffed to eliminate pressure on the airway that may predispose the patient to necrosis. The absence of a cuff makes the ET tube prone to dislodgment; thus, securement is vital.
Potential complications of neonatal intubation include hypoxia from prolonged intubation attempts or insufficient supplemental oxygen; apnea and bradycardia caused by hypoxia or vagal stimulation; improper tube positioning (esophageal or right mainstem bronchus intubation); and trauma to the oropharynx, trachea, vocal cords, or esophagus.2
All supplies and equipment necessary for intubation should be checked on a regular basis to ensure that they are in working order before the procedure is initiated. These supplies and equipment should be kept together on either a resuscitation cart or an intubation tray close to the patient’s bedside.
EDUCATION
- Provide developmentally and culturally appropriate education based on the desire for knowledge, readiness to learn, and overall neurologic and psychosocial state.
- Explain the procedure and the reason for intubation, if the clinical situation permits. If time does not permit, explain the procedure and the reason for the intubation after the procedure.
- Explain that because the ET tube passes through the vocal cords, cries from the patient will not be audible while the ET tube is in place.
- Provide information about neonatal procedural pain assessment and management, including the benefits and risks of pharmacologic and nonpharmacologic options.2
- Encourage questions and answer them as they arise.
ASSESSMENT AND PREPARATION
Assessment
- Perform hand hygiene before patient contact. Don appropriate PPE based on patient’s need for isolation precautions or risk of exposure to bodily fluids.
- Introduce yourself to the family.
- Verify the correct patient using two identifiers.
- Assess the family’s understanding of the reasons for and risks and benefits of the procedure.
- Determine the family’s desire to be present during the procedure.
- Assess the patient for facial deformities that preclude oral intubation. If facial deformities exist, assess the patency of the nares for nasal intubation.
Preparation
- Comply with Universal Protocol. Use a standardized list to verify that all required items, including informed consent, are available.
For procedures that must be performed emergently: Comply with Universal Protocol only if it does not delay the procedure.
- Notify the respiratory therapy department, as appropriate, of the impending intubation, so a ventilator can be set up.
- Determine the appropriate-size equipment (blade and tube) based on the patient’s weight and gestational age before initiating the procedure (Table 2)
(Table 3)
.
- Inspect the laryngoscope to ensure that it is functioning properly. Change the batteries or bulb, if needed.
- Ensure that the resuscitation bag or T-piece resuscitator is functioning properly, select an appropriate-size mask for ventilation, and verify an attachment to a 100% oxygen source with the blender set minimally at a flow rate of 5 to 10 L/min.1
- Prepare the suction source set at 60 to 100 mm Hg of negative pressure and suction catheter (sizes 5 to 10 Fr) as appropriate.2
- Have a meconium aspirator present if meconium fluid is present at delivery.2
- Determine the appropriate ET tube insertion depth.1 The tip should be 1 to 2 cm below the vocal cords.1
- For oral intubation, use one of two methods:
- NTL measurement: The depth of the tube at the gums in centimeters should be equal to the measurement from the nasal septum to the ear tragus plus 1 cm.1,4
- Gestational age: This method uses the gestational age to predict the correct insertion depth (Table 1)
.1
- For nasal intubation, use the modified Morgan formula: ET tube position at the nares should be cm = 0.12 × height (cm) + 5.5
PROCEDURE
Oral Intubation
Oral intubation is the preferred route in neonatal emergencies.
- Perform hand hygiene and don gloves. Don additional PPE based on the patient’s need for isolation precautions or the risk of exposure to bodily fluids.
- Verify the correct patient using two identifiers.
- Explain the procedure to the family and ensure that they agree to treatment, if time allows.
- Minimize heat loss by placing the patient in an appropriate environment that provides thermal homeostasis.
- Ensure appropriate monitoring of the patient’s heart rate and SpO2 continuously during the procedure.
Begin SpO2 monitoring as soon as possible. Evidence suggests that preterm neonates are susceptible to oxygen toxicity.1
- If time permits, comply with Universal Protocol. Perform a time-out to verify the correct patient, correct site, and correct procedure.
- Preoxygenate the patient with the appropriate oxygen concentration to achieve SpO2 levels in the desired range. Use a mask, flow-inflating bag device, or free-flow oxygen tubing if ventilations are adequate. If the patient is not adequately ventilating, use a self-inflating or flow-inflating resuscitation bag-mask or T-piece device to ventilate him or her manually.2
- Maintain the respiratory rate between 40 to 60 breaths per minute.6
- Limit the pressure of administered breaths to the minimum required to produce an easy rise and fall of the chest.
- Monitor the patient for an improvement in heart rate, color, SpO2, and tone.6
Rationale: Monitoring helps identify early signs of hypoxemia or hyperoxemia. Gentle breaths reduce the air entering the stomach (leading to gastric distention), decrease airway turbulence, and distribute ventilation in the lungs more evenly.
If the patient is breathing, avoid positive-pressure ventilation because of the risk of gastric distention, aspiration, and vomiting.
Place an 8-Fr orogastric tube to remove air and stomach contents if the patient requires resuscitation with bag-mask ventilation for longer than 2 minutes.1
- Premedicate the patient as prescribed and provide nonpharmacologic interventions.
Rationale: Intubation is a painful procedure associated with unfavorable physiologic effects such as bradycardia, desaturation, increased blood pressure, and increased intracranial pressure. Preintubation medication is associated with a decrease in the potential detrimental effects of intubation.6
- Obtain the appropriate ET tube and laryngoscope blade (Table 2)
(Table 3)
.
- If a stylet is needed, insert it into the ET tube, using aseptic technique;1 ensure that the tip does not extend beyond the end of the ET tube.
Rationale: A stylet inserted into the ET tube allows the intubator to control the direction of the tube. Keeping the stylet from extending past the end of the tube prevents damage to the vocal cords and trachea.
Do not let the stylet tip extend beyond the end of the ET tube.
- Assist with positioning the patient’s head by slightly extending the neck (sniffing position) (Figure 1)
. If necessary, place a small roll under the patient’s shoulders and contain his or her arms as needed.
Rationale: Slight extension of the patient’s neck aligns the mouth, pharynx, and trachea for a better view of the vocal cords.
Avoid hyperextending the neck or rotating the head, which makes the vocal cords difficult to see.2
Applying cricoid pressure during oral intubation is no longer a recommended practice.7
- Hold the equipment so the intubator does not need to look away from anatomic landmarks to suction secretions or grasp the tube for intubation.
- Hand the ET tube to the intubator when requested, maintaining the sterility of the tube. Observe as the intubator places the tube into the airway and visually verifies placement.
Intubation attempts should be limited to 30 seconds.1,2 Stabilize the patient with bag-mask ventilation between attempts to limit complications associated with the procedure.1
- After the tube is correctly placed, read the centimeter mark on the ET tube at the patient’s gums. The estimated depth of insertion is based on the gestational age or the NTL plus 1 cm.2
- Connect the ET tube to a disposable end-tidal carbon dioxide (ETCO2) detector, the oxygen source, a flow- or self-inflating resuscitation bag-mask device, and a T-piece resuscitator or mechanical ventilator to provide positive-pressure ventilation.
- Verify tube placement while a team member ventilates and oxygenates the patient.2
- Immediate methods of verification include:
- Observe the chest for symmetric movement with each breath.
Rationale: Asymmetric movement may indicate right mainstem intubation, esophageal intubation, or a pneumothorax.
- Auscultate bilateral breath sounds near both axillae during positive-pressure ventilation.
- Confirmatory methods, which can be used after a few seconds of manual ventilation, include:
- Check for an increase in heart rate and SpO2 if bradycardia and hypoxia were present before the procedure.
- Observe the ETCO2 detector for a change in color.
Confirming ET tube placement may require eight to 10 positive-pressure breaths.1
The effectiveness of the ETCO2 detector is limited. Proper placement may not produce a color change if heart rate or cardiac output is low or the lungs are not adequately ventilated (i.e., secretions obstructing the ET tube, inadequate ventilator pressure, or large bilateral pneumothoraces).1
- If auscultation reveals unilateral or unequal breath sounds, notify the intubator and observe as he or she withdraws the tube slowly until equal bilateral breath sounds are auscultated.1
- If auscultation detects no breath sounds or detects air entering the stomach and no color change is seen on the ETCO2 detector, notify the intubator and observe as he or she removes the ET tube, discards it, and repeats the procedure after stabilizing the patient with bag-mask ventilation, if necessary.
- After immediate and subsequent confirmation of ET tube position, document the centimeter mark at the gums and secure the tube per the organization’s practice. Place a skin barrier, such as a hydrocolloid dressing, between the adhesive tape and the skin.
Rationale: The lips may swell, leading to variation in the ET tube measurement. The gums are a more reliable landmark.
Maintain the tube in the correct position throughout the procedure.
- Obtain an anteroposterior view chest radiograph with the patient’s head at the midline for definitive confirmation of the ET tube placement. Ensure that the tip of the ET tube is approximately 0.5 to 1 cm above the carina.2
- Cut off any length of tube that extends more than 4 cm beyond the patient’s lip.1,2
Rationale: ET tube length of more than 4 cm beyond the lip increases dead space and may lead to tube kinking.1
- Insert an 8-Fr orogastric tube to remove air and stomach contents if gastric distention is present.6
- Discard supplies, remove PPE, and perform hand hygiene.
- Document the procedure in the patient’s record.
Nasal Intubation
Nasal intubation is performed for elective procedures, in patients with copious secretions, or when the patient has defects of the oral anatomy.
- Perform hand hygiene and don gloves. Don additional PPE based on the patient’s need for isolation precautions or the risk of exposure to bodily fluids.
- Verify the correct patient using two identifiers.
- Explain the procedure to the family and ensure that they agree to treatment, if time allows.
- Minimize heat loss by placing the patient in an appropriate environment that provides thermal homeostasis.
- Ensure appropriate monitoring of the patient’s heart rate and SpO2 continuously during the procedure.
- If time permits, comply with Universal Protocol: Perform a time-out to verify the correct patient, correct site, and correct procedure.
- Preoxygenate the patient with the appropriate concentration of oxygen to achieve SpO2 levels in the desired range. Use a mask, flow-inflating bag device, or free-flow oxygen tubing if ventilations are adequate. If the patient is not adequately ventilating, use a self-inflating or flow-inflating resuscitation bag-mask or T-piece device to ventilate him or her manually.2
- Maintain the respiratory rate between 40 to 60 breaths per minute.6
- Limit the pressure of administered breaths to the minimum required to produce an easy rise and fall of the chest.
Rationale: Gentle breaths reduce the air entering the stomach (leading to gastric distention), decrease airway turbulence, and distribute ventilation in the lungs more evenly.
- Assess the patient for an improvement in heart rate, color, SpO2, and tone.6
If the patient is breathing, avoid positive-pressure ventilation because of the risk of gastric distention, aspiration, and vomiting.
Place an 8-Fr orogastric tube to remove air and stomach contents if the patient requires resuscitation with bag-mask ventilation for longer than 2 minutes.1
- Premedicate the patient as prescribed and provide nonpharmacologic interventions.
Rationale: Intubation is a painful procedure associated with unfavorable physiologic effects such as bradycardia, desaturation, increased blood pressure, and increased intracranial pressure. Preintubation medication is associated with a decrease in the potential detrimental effects of intubation.6
Except for emergent intubation during resuscitation, either in the delivery room or after acute deterioration or critical illness at a later age, consider premedication for all intubations, preferably using medications with a rapid onset and short duration of action.
- Obtain the appropriate ET tube and laryngoscope blade (Table 2)
(Table 3)
.
- If a stylet is needed, insert it into the ET tube using aseptic technique;1 ensure that the tip does not extend beyond the end of the ET tube.
Rationale: A stylet inserted into the ET tube allows the intubator to control the direction of the tube. Keeping the stylet from extending past the end of the tube prevents damage to the vocal cords and trachea. Using aseptic technique preserves the cleanliness of the ET tube.1
Do not let the stylet tip extend beyond the end of the ET tube.
- Lubricate the ET tube with sterile, water-soluble lubricant.
- Assist with positioning the patient’s head by slightly extending the neck (sniffing position) (Figure 1)
. If necessary, place a small roll under the patient’s shoulders and contain his or her arms as needed.
Rationale: Slight extension of the patient’s neck aligns the mouth, pharynx, and trachea for a better view of the vocal cords.
Avoid hyperextending the neck or rotating the head, which makes the vocal cords difficult to see.2
- Hold the equipment so the intubator does not need to look away from anatomic landmarks to suction secretions or grasp the tube for intubation.
- Hand the ET tube to the intubator when requested, maintaining the sterility of the tube. Observe as the intubator passes the ET tube through the naris and into the nasopharynx.
Intubation attempts should be limited to 30 seconds.1,2 Stabilize the patient with bag-mask ventilation between attempts to limit complications associated with the procedure.1
- If requested, hand the intubator the Magill forceps, which are used to direct the tube into position if it does not slide into place easily using the blind technique.
- Connect the ET tube to a disposable ETCO2 detector, the oxygen source, a flow- or self-inflating resuscitation bag-mask device, and a T-piece resuscitator or mechanical ventilator to provide positive-pressure ventilation.
- Verify tube placement while a team member ventilates and oxygenates the patient.2
- Immediate methods include:
- Observe the chest for symmetric movement with each breath.
- Auscultate bilateral breath sounds near both axillae during positive-pressure ventilation.
- Confirmatory methods, which can be used after a few seconds of manual ventilation, include:
- Check for an increase in heart rate and SpO2 if bradycardia and hypoxia were present before the procedure.1,2
- Observe the ETCO2 detector for a change in color.
Confirming ET tube placement may require eight to 10 positive-pressure breaths.1
The effectiveness of the ETCO2 detector is limited. Proper placement may not produce a color change if heart rate or cardiac output is low or the lungs are not adequately ventilated (i.e., secretions obstructing the ET tube, inadequate ventilator pressure, or large bilateral pneumothoraces).1
- If auscultation reveals unilateral or unequal breath sounds, notify the intubator and observe as he or she withdraws the tube slowly until equal bilateral breath sounds are auscultated.1
- If auscultation detects no breath sounds or detects air entering the stomach and no color change is seen on the ETCO2 detector, notify the intubator and observe as he or she removes the ET tube, discards it, and repeats the procedure after stabilizing the patient with bag-mask ventilation, if necessary.
- After immediate and subsequent confirmation of the ET tube position, document the centimeter mark at the naris and secure the tube per the organization’s practice. Place a skin barrier, such as a hydrocolloid dressing, between the adhesive tape and the skin.
Maintain the tube in the correct position throughout the procedure.
- Obtain an anteroposterior view chest radiograph with the patient’s head at the midline for definitive confirmation of ET tube placement. Ensure that the tip of the ET tube is approximately 0.5 to 1 cm above the carina.2
- Cut off any length of tube that extends more than 4 cm beyond the patient’s naris.1,2
Rationale: An ET tube length of more than 4 cm beyond the naris increases dead space and may lead to tube kinking.2
- Insert an 8-Fr orogastric tube to remove air and stomach contents if gastric distention is present.6
- Discard supplies, remove PPE, and perform hand hygiene.
- Document the procedure in the patient’s record.
MONITORING AND CARE
- Monitor the patient’s vital signs frequently before, during, and after the procedure.
Rationale: Any change in vital signs may indicate airway compromise from incorrect ET tube position, dislodgment, or obstruction.
Notify the practitioner of bradycardia, hypertension or hypotension, apnea or abnormal respirations, or decreased SpO2 from the patient’s baseline value.
- Auscultate breath sounds with any change in respiratory status and when providing hands-on care.
Notify the practitioner of absent, decreased, or unequal breath sounds.
- Maintain ET tube stability using a commercially available securement device or adhesive tape per the organization’s practice.
Notify the practitioner of audible crying, unplanned extubation, or tube movement from the original position. Remove the tube and begin bag-mask ventilation as appropriate.
- Monitor and record the position of the ET tube at the patient’s gums or naris on insertion and with hands-on care.
Notify the practitioner of tube movement from the original position.
- Suction the ET tube, as needed.
Notify the practitioner of an inability to pass a suction catheter; copious, frothy, or bloody secretions; or a significant change in amount or character of secretions.
- Inspect the nares or oral cavity every time hands-on care is provided while the patient is intubated.
Notify the practitioner of redness, necrosis, or skin breakdown.
- Perform regular oral care using sterile 0.9% sodium chloride solution or breast milk.
- Assess, treat, and reassess pain.
Notify the practitioner of an inability to manage pain.
EXPECTED OUTCOMES
- Correct placement of patent ET tube
- Properly positioned and secured airway
- Improved oxygenation and ventilation
- Facilitation of secretion clearance
- Stabilization of patient
UNEXPECTED OUTCOMES
- Improper tube placement (intubation of esophagus or right mainstem bronchus)
- Accidental extubation
- Cardiac arrhythmias from hypoxemia or vagal stimulation
- Injury to lips, gums, or naris
- Air leakage from around the ET tube
- Tracheal injury at tip of tube
- Laryngeal edema
- Vocal cord trauma
- Suctioning of gastric contents or food from ET tube
- Obstruction of ET tube
- Ventilator-associated pneumonia
- Pneumothorax
- Laceration of tongue, gums, or airway
- Perforation of trachea and esophagus
DOCUMENTATION
- Education
- Vital signs and SpO2 before, during, and after intubation
- Type of intubation (oral or nasal)
- Time-out procedure, including verification of correct patient, correct procedure, and correct site (if performed)
- Type and size of blade used
- Number of intubation attempts
- Medication administration
- Size of ET tube
- Depth of ET tube insertion (centimeter mark at patient’s gums or naris)
- Assessment of breath sounds
- Confirmation of tube placement with breath sounds and ETCO2 detector
- Confirmation of tube placement by chest radiograph
- Color, amount, and consistency of secretions
- Patient’s response to procedure
- Unexpected outcomes and related interventions
- Pain assessment, interventions, effectiveness of interventions, and adverse reactions
- Aziz, K. and others. (2020). Part 5: Neonatal resuscitation: 2020 American Heart Association guidelines for cardiopulmonary resuscitation and emergency cardiovascular care. Circulation, 142(16 Suppl. 2), S524-S550. doi:10.1161/CIR.0000000000000902 Retrieved September 21, 2022, from https://www.ahajournals.org/doi/10.1161/CIR.0000000000000902 (Level I)
- Bailey, T.B., Maltsberger, H.L. (2021). Chapter 15: Common invasive procedures. In M.T. Verklan, M. Walden, S. Forest (Eds.), Core curriculum for neonatal intensive care nursing (6th ed., pp. 244-269). St. Louis: Elsevier. (Level VII)
- Edelson, D.P. and others. (2020). Interim guidance for basic and advanced life support in adults, children, and neonates with suspected or confirmed COVID-19: From the Emergency Cardiovascular Care Committee and Get With the Guidelines-Resuscitation Adult and Pediatric Task Forces of the American Heart Association. Circulation, 141(25), e933-e943. doi:10.1161/CIRCULATIONAHA.120.047463 Retrieved September 21, 2022, from https://www.ahajournals.org/doi/10.1161/CIRCULATIONAHA.120.047463 (Level VII)
- Gray, M.M. and others. (2017). Accuracy of the nasal-tragus length measurement for correct endotracheal tube placement in a cohort of neonatal resuscitation simulators. Journal of Perinatology, 37(8), 975-978. doi:10.1038/jp.2017.63
- Hunyady, A.I. and others. (2015). Nares-to-carina distance in children: Does a ‘modified Morgan formula’ give useful guidance during nasal intubation? Pediatric Anesthesia, 25(9), 936-942. doi:10.1111/pan.12693 (classic reference)*
- Pappas, B.E., Robey, D.L. (2021). Chapter 5: Neonatal delivery room resuscitation. In M.T. Verklan, M. Walden, S. Forest (Eds.), Core curriculum for neonatal intensive care nursing (6th ed., pp. 69-85). St. Louis: Elsevier. (Level VII)
- Tisherman, S.A., Anders, M.G., Galvagno Jr, S.M. (2019). Is 30 newtons of prevention worth a pound of cure? – Cricoid pressure. JAMA Surgery, 154(1), 18. doi:10.1001/jamasurg.2018.3590
- Watters, K., Mancuso, T. (2019). Chapter 13: Airway management. In B. Walsh (Ed.), Neonatal and pediatric respiratory care (5th ed., pp. 222-243). St. Louis: Elsevier.
*In these skills, a “classic” reference is a widely cited, standard work of established excellence that significantly affects current practice and may also represent the foundational research for practice.
Elsevier Skills Levels of Evidence
- Level I - Systematic review of all relevant randomized controlled trials
- Level II - At least one well-designed randomized controlled trial
- Level III - Well-designed controlled trials without randomization
- Level IV - Well-designed case-controlled or cohort studies
- Level V - Descriptive or qualitative studies
- Level VI - Single descriptive or qualitative study
- Level VII - Authority opinion or expert committee reports