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Sandra K. Hanneman is associate professor, associate dean for research, and director, Center for Nursing Research, University of Texas, School of Nursing, at Houston.
Use of the AACN research-based practice protocol Weaning From Short-term Mechanical Ventilation is recommended for management of all patients who require mechanical ventilation for 3 days or less. Because many patients are extubated within hours of admission to the critical care unit after cardiac surgery, clinicians may think that these patients do not require assessment of readiness to wean. Although most can be weaned without difficulty, 1 in 5 (20%) has problems with weaning. Thus, every patient should be assessed for readiness to wean before mechanical ventilatory support is reduced substantially. Readiness to wean is a physiological threshold that indicates physiological stability and reserve capacity for the work of spontaneous ventilation.
Q: What are key indicators nurses should use when assessing patients for readiness to wean from mechanical ventilation after cardiac surgery?
A: Readiness to wean is determined by general physiological stability, hemodynamic stability, pulmonary mechanics, adequacy of gas exchange, capability for spontaneous ventilation, and level of consciousness. Each of these determinants is discussed briefly.
General Physiological Stability
Multiple problems of a noncardiopulmonary nature such as hemorrhage, fever, and electrolyte imbalance can strain the physiological reserve and signal an impending deterioration in condition for which treatment with mechanical ventilation will be needed. The potential trauma of reintubation and loss of the ability to provide rapid control of oxygenation and ventilation during resuscitation can compromise a patient’s outcome. Therefore, in order to err on the side of caution, major physiological instability is an indication to postpone weaning.
Hemodynamic Stability
Patients’ responses to both mechanical and spontaneous ventilation vary according to myocardial function and reserve capacity. Cardiac failure, which may not be apparent when a patient is receiving mechanical ventilation, can be exacerbated with transition to spontaneous ventilation. Signs of hemodynamic instability include an acute or gradual decrease in arterial blood pressure, tachycardia, bradycardia, dysrhythmias, weak peripheral pulses, decreased pulse pressure, acute or gradual increase in pulmonary capillary wedge pressure, decreased cardiac output, and decreased mixed venous oxygen saturation. Hemodynamic stability is difficult to evaluate when a patient is receiving vasoactive support, a common situation after cardiac surgery. Therefore, during weaning, hemodynamic parameters should be monitored closely in such patients.
Pulmonary Mechanics
For the purpose of assessing readiness to wean after cardiac surgery, 3 measurements of pulmonary mechanics typically are used: (1) minute volume, with its component parts of spontaneous tidal volume and respiratory rate; (2) vital capacity, expressed as milliliters per kilogram body weight; and (3) negative inspiratory pressure or force. Collectively, these measurements are used to evaluate the strength of respiratory muscles.
Vital capacity of 15 mL/kg or higher and negative inspiratory pressure of –30 cm H2O or less are the most accurate predictors of a patient’s readiness to wean. Vital capacity depends on the effort exerted, and the patient should be coached to use maximal effort. Repeated measurements (eg, 3 measurements) are recommended to capture the patient’s best effort.
The findings of many clinical studies on pulmonary mechanics in a variety of patients in a variety of situations are conflicting so far as the value of measurements of pulmonary mechanics in predicting patients’ readiness to wean after short-term mechanical ventilation. In part, the conflicting findings can be explained by the failure to use standardized procedures to measure pulmonary mechanics across studies. Recommended standardized procedures for measurement of minute volume, vital capacity per kilogram, and negative inspiratory pressure are included in Tables 1
to 3.
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Attention to the breathing pattern and the rate of mechanical ventilation is needed; abnormalities may signal problems with weaning. Signs of an inadequate breathing pattern, in addition to those listed previously, include shallow respiration, irregular respiration, intercostal retraction, suprasternal retraction, tracheal tug (upward and downward movement of the larynx with inspiration and expiration), respiratory alternans, and respiratory muscle paradox. An abnormal respiratory rate in any patient receiving mechanical ventilation should be evaluated with respect to the ventilator mode; however, a total respiratory rate (combined machine- and patient-initiated breaths) that exceeds 30/min may indicate that the patient is not ready to wean.
Adequate oxygenation within a desired range for the patient receiving mechanical ventilation is a criterion of readiness to wean. Signs of inadequate oxygenation may include abnormal arterial oxygen tension, decreased mixed venous oxygen saturation, decreased saturation according to pulse oximetry, decreased PaO2/fraction of inspired oxygen ratio, tachypnea, dyspnea, central cyanosis, restlessness, confusion, agitation, tachycardia, bradycardia, dysrhythmias, intercostal and suprasternal retraction, increasing or decreasing blood pressure, decreasing urine output, and metabolic acidosis. Poor oxygenation can be treated more easily in spontaneously breathing patients than can poor ventilation. Thus, oxygenation status while receiving mechanical ventilation is a relative indicator of readiness to wean.
Capability for Spontaneous Ventilation
Obviously, patients must be capable of spontaneous ventilation in order to wean. Spontaneous ventilation may be evidenced by any contribution of the patient to minute volume, such as the patient’s initiation of ventilator breaths, between or beyond the preset rate. Discontinuing mechanical ventilation, or setting the ventilator mode on flow by, and measuring spontaneous minute volume are often used to assess patients’ ability to breathe spontaneously.
Level of Consciousness
A reduced level of consciousness, including coma, is in and of itself not an indicator of inability to wean. Rather, changes in level of consciousness raise concerns about underlying instability of the patient’s condition from such covert causes as metabolic alterations, cerebral emboli, and residual anesthesia. Nonetheless, sustained alertness after surgery is one criterion for readiness to wean because it indicates reversal of anesthetic agents.
This article is based on the protocol Weaning From Short-term Mechanical Ventilation by Sandra K. Hanneman. It was taken from the Care of the Mechanically Ventilated Patient series of AACN’s Protocols for Practice. Readers are referred to the full protocol for discussion of weaning modes, monitoring of patients, and prevention of complications during weaning. Protocols can be obtained from AACN, 101 Columbia, Aliso Viejo, CA 92656-1491, (800) 899-AACN, (949) 362-2000. $11, AACN members; $14, nonmembers.
Note
This article was first published in Critical Care Nurse October 1999.
To purchase reprints, contact The InnoVision Group, 101 Columbia, Aliso Viejo, CA 92656. Phone, (800) 809-2273 or (949) 362-2050 (ext 532); fax, (949) 362-2049; e-mail, reprints{at}aacn.org.
Today, as in 1999, physiological stability and subjective adequacy of ventilation and oxygenation are the only assessments that have an adequate evidence base for patients who are receiving mechanical ventilation 3 or fewer days. Both research findings and expert consensus support daily screening trials and 2-hour T-piece trials for optimal weaning from mechanical ventilation.
On the basis of the evidence available to date, measurement of pulmonary mechanics, nutritional parameters, gas exchange, hemodynamics, and ventilatory capacity appear to offer little benefit, as independent predictors, to the prediction of readiness to wean. This does not mean that clinicians should abandon such measures; rather, the measures are used to form a clinical judgment of a given patient’s physiological status for assessment of readiness to wean.
The strongest predictor of readiness to wean in multiple populations across multiple settings appears to be the spontaneous breathing trial (SBT). The SBT is recommended for use as follows. A brief (ie, < 30 min) screening trial of SBT is used to assess the patient’s ability to progress to a formal trial of weaning. Thus, the screening trial serves as the predictor of readiness to wean. If the patient tolerates the screening trial, then a formal SBT trial (ie, up to 120 min) can commence. The patient should be monitored continuously for tolerance during both screening and formal SBT trials. Criteria to evaluate patient tolerance of a SBT include respiratory pattern, gas exchange, hemodynamics, and subjective comfort. The Table shows criteria that have been used in clinical trials to assess tolerance of SBT. Clinicians may find these criteria helpful as points of reference, but are cautioned to assess tolerance for each patient as compared with the patient’s status before the SBT.
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Bibliography
Cook D, Meade M, Guyatt G, Griffith L, Booker L. Criteria for Weaning From Mechanical Ventilation. Evidence Report/Technology Assessment No. 23 (Prepared by McMaster University under contract No. 290-97-0017.) AHRQ Publication No. 01-E005. Rockville, Md: Agency for Healthcare Research and Quality; 2000.
Hanneman SK. Weaning from short-term mechanical ventilation. Protocols for Practice. Aliso Viejo, Calif: American Association of Critical-Care Nurses; 1998.
Hanneman SK, Ingersoll G, Knebel A, Shekleton M, Burns S, Clochesy J. Weaning from short-term mechanical ventilation: a review. Am J Crit Care. 1994;3:421–443.
Hanneman SK, Kite-Powell DM. Readiness to wean from mechanical ventilation. In: Melnyk B, Fineout-Overholt E, eds. Evidence-Based Practice in Nursing & Health: A Guide for Translating Research Evidence Into Practice. Hagerstown, Md: Lippincott Williams & Wilkins; In press.
Knebel A, Shekleton M, Burns S, Clochesy J, Hanneman SK, Ingersoll G. Weaning from mechanical ventilation: concept development. Am J Crit Care. 1994;3:416–420.
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MacIntyre NR, Cook DJ, Ely EW, et al. Evidence-based guidelines for weaning and discontinuing ventilatory support: a collective task force facilitated by the American College of Chest Physicians, the American Association for Respiratory Care, and the American College of Critical Care Medicine. Chest. 2001;120(suppl):375S–395S.
Meade M, Guyatt G, Cook D, et al. Predicting success in weaning from mechanical ventilation. Chest. 2001;120:400S–424S.
Meade M, Guyatt G, Griffith L, Booker L, Randall J, Cook D. Section II: systematic reviews of the evidence base for ventilator weaning. Chest. 2001;120:396S–399S.
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