WEANING THE PATIENT FROM THE VENTILATOR
Respiratory weaning, the process of withdrawing the patientfrom dependence on the ventilator, takes place in three stages: the patient is gradually removed from the ventilator, then from the tube, and finally from oxygen. Weaning from mechanical venti-lation is performed at the earliest possible time consistent with patient safety. The decision must be made from a physiologic rather than from a mechanical viewpoint. A thorough under-standing of the patient’s clinical status is required in making this decision. Weaning is started when the patient is recovering from the acute stage of medical and surgical problems and when the cause of respiratory failure is sufficiently reversed.
Successful weaning involves collaboration among the physi-cian, respiratory therapist, and nurse. Each health care provider must understand the scope and function of other team members in relation to patient weaning to conserve the patient’s strength, use resources efficiently, and maximize successful outcomes.
Careful assessment is required to determine whether the patient is ready to be removed from mechanical ventilation. If the patient is stable and showing signs of improvement or reversal of the dis-ease or condition that caused the need for mechanical ventilation, weaning indices should be assessed. These indices include:
· Vital capacity: the amount of air expired after maximum in-spiration. Used to assess the patient’s ability to take deep breaths. Vital capacity should be 10 to 15 mL/kg to meet the criteria for weaning.
· Maximum inspiratory pressure (MIP): used to assess the pa-tient’s respiratory muscle strength. It is also known as nega-tive inspiratory pressure and should be at least −20 cm H2O.
· Tidal volume: volume of air that is inhaled or exhaled from the lungs during an effortless breath. It is normally 7 to 9 mL/kg.
· Minute ventilation: equal to the respiratory rate multiplied by tidal volume. Normal is about 6 L/min.
· Rapid/shallow breathing index: used to assess the breathing pattern and is calculated by dividing the respiratory rate by tidal volume. Patients with indices below 100 breaths/min/L are more likely to be successful at weaning.
Other measurements used to assess readiness for weaning include a PaO2 of greater than 60 mm Hg with an FiO2 of less than 40%. Stable vital signs and arterial blood gases are also important pre-dictors of successful weaning. Once readiness has been determined, the nurse records baseline measurements of weaning indices to monitor progress (Cull & Inwood, 1999).
To maximize the chances of success of weaning, the nurse must consider the patient as a whole, taking into account factors that impair the delivery of oxygen and elimination of carbon dioxide as well as those that increase oxygen demand (sepsis, seizures, thy-roid imbalances) or decrease the patient’s overall strength (nutri-tion, neuromuscular disease). Adequate psychological preparation is necessary before and during the weaning process. Patients need to know what is expected of them during the procedure. They are often frightened by having to breathe on their own again and need reassurance that they are improving and are well enough to handle spontaneous breathing. The nurse explains what will hap-pen during weaning and what role the patient will play in the pro-cedure. The nurse emphasizes that someone will be with or near the patient at all times, and answers any questions simply and concisely. Proper preparation of the patient can reduce the wean-ing time.
Considerable effort has been devoted to finding the best method of weaning from mechanical ventilation, but research has not es-tablished which method is best (Tasota & Dobbin, 2000). Suc-cess depends on the combination of adequate patient preparation, available equipment, and an interdisciplinary approach to solv-ing patient problems (Chart 25-15). The most common weaning methods in use today are described below.
Assist–control may be used as the resting mode for patients undergoing weaning trials. This mode provides full ventilatory support by delivering a preset tidal volume and respiratory rate; if the patient takes a breath, the ventilator delivers the preset vol-ume. The cycle does not adapt to the patient’s spontaneous ef-forts. The nurse assesses patients being weaned on this mode for the following signs of distress: rapid shallow breathing, use of ac-cessory muscles, reduced level of consciousness, increase in carbon dioxide levels, decrease in oxygen saturations, and tachycardia.
The patient on intermittent mandatory ventilation (IMV) can increase the respiratory rate, but each spontaneous breath receives only the tidal volume the patient generates. Mechanical breaths are delivered at preset intervals and a preselected tidal volume, re-gardless of the patient’s efforts. IMV allows patients to use their own muscles of ventilation to help prevent muscle atrophy. IMV lowers mean airway pressure, which can assist in preventing baro-trauma.
Synchronized intermittent mandatory ventilation (SIMV) de-livers a preset tidal volume and number of breaths per minute. Between ventilator-delivered breaths, the patient can breathe spontaneously with no assistance from the ventilator on those extra breaths. As the patient’s ability to breathe spontaneously in-creases, the preset number of ventilator breaths is decreased and the patient does more of the work of breathing. SIMV is indi-cated if the patient satisfies all the criteria for weaning but cannot sustain adequate spontaneous ventilation for long periods.
IMV and SIMV can be used to provide full or partial ventila-tory support. Nursing interventions for both of these include monitoring progress by recording respiratory rate, minute vol-ume, spontaneous and machine-generated tidal volume, FiO2, and arterial blood gas levels.
The pressure support ventilation (PSV) mode assists SIMV by applying a pressure plateau to the airway throughout the patient-triggered inspiration to decrease resistance by the tracheal tube and ventilator tubing. Pressure support is reduced gradually as the patient’s strength increases. A SIMV backup rate may be added for extra support. The nurse must closely observe the patient’s respiratory rate and tidal volumes on initiation of PSV. It may be necessary to adjust the pressure support to avoid tachypnea or large tidal volumes.
The proportional assist ventilation (PAV) mode of partial ven-tilatory support allows the ventilator to generate pressure in pro-portion to the patient’s efforts. With every breath, the ventilator synchronizes with the patient’s ventilatory efforts (Giannouli,Webster, Roberts & Younes, 1999). Nursing assessment should include careful monitoring of the patient’s respiratory rate, arte-rial blood gases, tidal volume, minute ventilation, and breathing pattern.
The continuous positive airway pressure (CPAP) mode allows the patient to breathe spontaneously, while applying positive pressure throughout the respiratory cycle to keep the alveoli open and promote oxygenation. Providing CPAP during spontaneous breathing also offers the advantage of an alarm system and may reduce patient anxiety if the patient has been taught that the ma-chine is keeping track of breathing. It also maintains lung vol-umes and improves the patient’s oxygenation status. CPAP is often used in conjunction with PSV. Nurses should carefully as-sess for tachypnea, tachycardia, reduced tidal volumes, decreas-ing oxygen saturations, and increasing carbon dioxide levels.
Weaning trials using a T-piece or tracheostomy mask (see Fig. 25-2) are normally conducted with the patient disconnected from the ventilator, receiving humidified oxygen only, and performing all work of breathing. Patients who do not have to overcome the resistance of the ventilator may find this mode more comfortable, or they may become anxious as they breathe with no support from the ventilator. During T-piece trials, the nurse monitors the pa-tient closely and provides encouragement. This method of wean-ing is usually used when the patient is awake and alert, is breathing without difficulty, has good gag and cough reflexes, and is hemo-dynamically stable. During the weaning process, the patient is maintained on the same or a higher oxygen concentration than when on the ventilator. While on the T-piece, the patient should be observed for signs and symptoms of hypoxia, increasing respi-ratory muscle fatigue, or systemic fatigue. These include restless-ness, increased respiratory rate greater than 35 breaths/min, use of accessory muscles, tachycardia with premature ventricular con-tractions, and paradoxical chest movement (asynchronous breath-ing, chest contraction during inspiration and expansion during expiration). Fatigue or exhaustion is initially manifested by an in-creased respiratory rate associated with a gradual reduction in tidal volume; later there is a slowing of the respiratory rate.
If the patient appears to be tolerating the T-piece trial, a sec-ond set of arterial blood gas measurements is drawn 20 minutes after the patient has been on spontaneous ventilation at a con-stant FiO2 pressure support ventilation. (Alveolar–arterial equili-bration takes 15 to 20 minutes to occur.)
Signs of exhaustion and hypoxia correlated with deterioration in the blood gas measurements indicate the need for ventilatory support. The patient is placed back on the ventilator each time signs of fatigue or deterioration develop.
If clinically stable, the patient usually can be extubated within 2 or 3 hours of weaning and allowed spontaneous ventilation by means of a mask with humidified oxygen. Patients who have had prolonged ventilatory assistance usually require more gradual weaning; it may take days or even weeks. They are weaned pri-marily during the day and placed back on the ventilator at night to rest.
Because patients respond in different manners to the various weaning methods, there is no definitive way to assess which method is best. With all of the methods, ongoing assessment of res-piratory status is essential to monitor patient progress (Woodruff, 1999).
Successful weaning from the ventilator is supplemented by in-tensive pulmonary care. The following are continued:
· Oxygen therapy
· Arterial blood gas evaluation
· Pulse oximetry
· Bronchodilator therapy
· Chest physiotherapy
· Adequate nutrition, hydration, and humidification
· Incentive spirometry
These patients still have borderline pulmonary function and need vigorous supportive therapy before their respiratory status returns to a level that supports activities of daily living.
Weaning from the tube is considered when the patient can breathe spontaneously, maintain an adequate airway by effectively cough-ing up secretions, swallow, and move the jaw. If frequent suc-tioning is needed to clear secretions, tube weaning may be unsuccessful (Ecklund, 1999). Secretion clearance and aspiration risks are assessed to determine if active pharyngeal and laryngeal reflexes are intact.
Once the patient can clear secretions adequately, a trial period of mouth breathing or nose breathing is conducted. This can be accomplished by several methods. The first method requires changing to a smaller size tube to increase the resistance to airflow and simultaneously plugging the tracheostomy tube (deflating the cuff ). The smaller tube is sometimes replaced by a cuffless tra-cheostomy tube, which allows the tube to be plugged at length-ening intervals to monitor patient progress. A second method involves changing to a fenestrated tube (a tube with an opening or window in its bend). This permits air to flow around and through the tube to the upper airway and enables talking. A third method involves switching to a smaller tracheostomy button (stoma button). A tracheostomy button is a plastic tube approx-imately 1 inch long that helps to keep the windpipe open after the larger tracheostomy tube has been removed. Finally, when the patient demonstrates the ability to maintain a patent airway with-out a tracheostomy tube, the tube can be removed. An occlusive dressing is placed over the stoma, which usually heals anywhere from several days to many weeks (Ecklund, 1999).
The patient who has been successfully weaned from the ventila-tor, cuff, and tube and has adequate respiratory function is then weaned from oxygen. The FiO2 is gradually reduced until the PaO2 is in the range of 70 to 100 mm Hg while the patient is breathing room air. If the PaO2 is less than 70 mm Hg on room air, supplemental oxygen is recommended. The Centers for Medicare and Medicaid Services, formerly the Health Care Fi-nancing Administration (HCFA), requires that the patient’s PaO2 on room air be less than 55 mm Hg for the patient to be eligible for financial reimbursement for in-home oxygen.
Success in weaning the long-term ventilator-dependent patient requires early and aggressive but judicious nutritional support. The respiratory muscles (diaphragm and especially intercostals) become weak or atrophied after just a few days of mechanical ven-tilation, especially if nutrition is inadequate. Fat kilocalories pro-duce less carbon dioxide than carbohydrate kilocalories. For this reason, a high-fat diet may assist patients with respiratory failure who are being weaned from mechanical ventilation. Research is being conducted on the role of fatty acids in lung disease (Schwartz,2000). A high-fat diet may provide as much as 50% of the total daily kilocalories. Adequate protein intake is important in in-creasing respiratory muscle strength. Protein intake should be ap-proximately 25% of total daily kilocalories, or 1.2 to 1.5 g/kg/day. Because a high-carbohydrate diet can lead to increased carbon diox-ide production and retention, total carbohydrate intake should not exceed 25% of total daily kilocalories, or 2 g/kg/day in pa-tients being weaned from mechanical ventilation. Care must be taken not to overfeed patients because excessive intake can raise the demand for oxygen and the production of carbon dioxide. Total daily kilocalories should be closely monitored (Lutz & Prytulski, 2001).
Soon after the patient is admitted, a consultation with a di-etitian or nutrition support team should be arranged to plan the best form of nutritional replacement. Adequate nutrition may de-crease the duration of mechanical ventilation and prevent other complications, especially sepsis. Sepsis can occur if bacteria enter the bloodstream and release toxins that, in turn, cause vasodila-tion and hypotension, fever, tachycardia, increased respiratory rate, and coma. Aggressive treatment of sepsis is essential to re-verse this threat to survival and to promote weaning from the ven-tilator when the patient’s condition improves. Optimal nutritional intake is an essential part of the treatment of sepsis.