NONINVASIVE VENTILATION Noninvasive ventilation was commonly used in the 1950 s during the polio epidemic, mainly with negative pressure ventilators such as the iron lung. Noninvasive ventilation has recently become popular again to treat both chronic and acute respiratory failure, and can refer to CPAP or BiPAP units. Noninvasive positive pressure ventilation (NPPV) has several theoretical advantages over positive pressure ventilation. These include comfort and patient tolerance, speech and swallowing, and maintenance of protective upper airway reflex mechanisms. Limitations to NPPV include the need for patient cooperation and lack of direct access to the airway for delivery of ventilation and removal of secretions. MECHANISM OF ACTION With noninvasive ventilation, there is no direct communication to the lower airway like there is with invasive ventilation. Therefore, the patient must consciously direct the air from his mouth to his lungs, and arrange the soft tissues of his face and upper airway in order to provide an open upper airway and minimize mask leaks. Noninvasive ventilation can improve patient outcomes by providing rest to the respiratory muscles for chronic patients, thereby improving daytime functioning. It can improve gas exchange and decrease the level of carbon dioxide in order to prevent respiratory failure, and improve oxygenation by expanding collapsed alveoli and improving ventilation/perfusion relationships in the lung. It can prevent hypoventilation at night by providing a positive pressure to expand the lungs. INDICATIONS Noninvasive positive pressure ventilation can be useful for acute or chronic respiratory failure. It has been used successfully for COPD exacerbations, congestive heart failure, acute exacerbations of restrictive lung disease, post-extubation respiratory failure, and pneumonia. Many of these patients have avoided intubation and further deterioration of their arterial blood gases. Oxygenation often improves immediately, PaCO 2 may dramatically decrease, and there is often an improvement in the feeling of dyspnea and decreased heart and respiratory rate. NPPV is also used frequently in chronic respiratory failure. It is commonly used in patients with stable or slowly progressive neuromuscular syndromes, such as Muscular Dystrophy, postpolio syndrome, multiple sclerosis, or quadriplegia due to high spinal cord lesions. It is generally not appropriate for patients with rapidly progressive neuromuscular disorders causing airway compromise. It can also be used for patients with thoracic deformities, such as severe kyphoscoliosis or lordosis. The third category of chronic respiratory failure benefiting from NPPV is those with central hypoventilatory syndromes, such as obstructive sleep apnea.
CLINICAL APPLICATIONS Noninvasive positive pressure ventilation can include volume or pressure-cycled ventilators used noninvasively with a nasal or face mask, a nasal CPAP unit, or a BiPAP unit. Oxygen is either internally set or tee d into the circuit, depending on patient needs. Ventilators can be small, portable, lightweight units used for home care, or large intensive care ventilators used in the hospital. Ventilators can be used with nasal or full face masks, and are sometimes advantageous over CPAP or BiPAP units because they have a variety of mode settings. Clinicians disagree on settings to be used with ventilators, but usually a CPAP level is set, along with a peak pressure or pressure support level when used with a pressure-cycled mode. When used with a volume ventilation mode, a tidal volume is usually set that is comfortable for the patient and a low respiratory rate is usually chosen. CPAP units are small, portable machines that deliver one level of positive pressure, usually ranging from 5-17 cm H 2 0 (see fig. 1). CPAP machines are often used by patients at night for obstructive sleep apnea Figure 1 CPAP machine McPherson, Respiratory Therapy Equipment, 4 th Ed., 1990, The C.V. Mosby Company. Some of the newer machines provide a ramp time setting which will slowly increase the pressure over a period of time while the patient falls asleep. CPAP units are commonly used at night for obstructive sleep apnea, to splint open the tissues of the upper airway and prevent obstruction. Nocturnal ventilation has been reported to improve the respiratory status during sleep, improve the quality of sleep, improve the quality of life, and decrease the cost of health care by allowing the patient to be discharged from hospital earlier. They are usually non-humidified, although some include a simple passover humidifier for those patients that need it.
CPAP masks can be nasal or cover the mouth and nose Figure 21 nasal CPAP mask Pilbeam, SP. Mechanical Ventilation 2nd Ed. 1992 Mosby-Year Book, Inc. BiPAP stands for Bilevel Positive Airway Pressure. The BiPAP unit incorporates a ventilation unit and usually a pressure monitoring alarm package (see fig. 3). Units can be used with a nasal or full face mask. They consist of a portable machine capable of delivering a pressure on inspiration (Inspired Positive Airway Pressure or IPAP) and a pressure on expiration (Expired Positive Airway Pressure or EPAP). The EPAP maintains a constant level of CPAP and is adjusted according to oxygenation. The IPAP (similar to a pressure support mode) maintains an inspiratory pressure throughout inspiration. A respiratory rate can be set along with an inspiratory time, or the patient can breathe on a spontaneous mode. BiPAP is often used in congestive heart failure and COPD Figure 3 BiPAP machine Respironics, Inc.
Hazards of noninvasive ventilation include claustrophobia, dryness of the mouth and nose, gastric distention causing a risk of aspiration, nasal congestion and epistaxis, eye irritation, pressure sores, pneumothorax, and decreased cardiac output. Patients should be normotensive, hemodynamically and cardiovascularly stable, be able to clear secretions and protect their upper airway, and mentally competent and cooperative.
WEANING FROM MECHANICAL VENTILATION The process of weaning from mechanical ventilation must be tailored to fit the needs of each individual patient. The ease with which support can be discontinued differs among all patients, depending on the reason for which ventilatory support was initiated in the first place. Other factors include surgeries, complications, length of time on the ventilator, patient condition prior to intubation, age, level of consciousness, other health considerations, and type of artificial airway. Methods of weaning and standards of practice vary between hospitals. There are four standard methods of discontinuing ventilatory support: A. All ventilation is supplied by the ventilator, and the patient is fully supported until all reasons for starting ventilatory support no longer exist, and the patient s physiologic parameters indicated he is ready to breathe spontaneously. This is often reffered to as the sink or swim method, (see fig. 1) and is not the method of choice for patients who have needed long-term ventilation. Shaded areas in the following diagrams represent patient supported ventilation. Figure 1 "sink or swim" method of weaning B. The patient breathes spontaneously on CPAP for short periods of time, progressing to longer periods, and is rested in between on a fully supportive mode such as assist/control (see fig. 2). This method is sometimes used for chronic, difficult-to-wean patients with poor respiratory muscle strength, who tire easily during progressive weaning and need rest periods with full support. Many of these patients start off with several CPAP trials of three to five minutes a day, and increase to longer periods of time each day, resting overnight on assist/control. Figure 2 CPAP trial method of weaning
C. The SIMV method of weaning consists of slowly reducing the IMV rate until the patient is supplying more and more spontaneous breaths on their own, and less mandatory breaths(see fig. 3). This is a traditional method of weaning, and is used frequently in hospitals today. Figure 3 SIMV method of weaning D. The pressure-support wean can be combined with an IMV rate or used on its own to wean patients from mechanical ventilation (see fig. 4). When the patient has an adequate and consistent drive to breath, they are changed to a pressure support mode. The pressure support level is then titrated down slowly if the patient can maintain adequate tidal volumes throughout the wean. Figure 42 pressure support method of weaning Each hospital has it s own criteria patients must meet in order to be considered for pulmonary weaning. Generally, the patients should be awake, alert, and able to follow commands. The should be well rested and not sleep deprived, and be on a normal sleep/wake schedule. Different organ systems such as cardiovascular, renal, gastrointestinal, and hepatic should be functional and stable. Ideally, the patient should be receiving adequate nutritional support.
Table 1 Effects of Malnourishment in Mechanically Ventilated Patients Reduced response to hypoxia and hypercarbia. Muscle atrophy from prolonged bed rest and lack of use: this includes respiratory muscles, especially if the patient is on controlled ventilation. Muscle wasting from lack of nutrition including the respiratory muscles. Respiratory tract infections from impaired cell immunity and reduced or altered macrophage activity. Decreased surfactant production and development of atelectasis. Reduced ability of pulmonary epithelium to replicate, which slows healing of damaged tissue. Lower serum albumin levels which affect colloid oncotic pressures and can contribute to pulmonary edema formation (colloid oncotic pressures < 11 mm Hg with normal left atrial pressure). Pilbeam, SP. Mechanical Ventilation 2nd Ed. 1992 Mosby-Year Book, Inc. Some intensive care units measure the metabolic rate to ensure the patient s metabolic needs are being met. Patients should not be receiving a high carbohydrate diet, as this can increase PCO 2 production. They should have their pain and anxiety under control, and lab data such as hemoglobin, hematocrit, electrolytes, and blood sugars should be within normal ranges. The patient should be afebrile and free from active infections, as well as have their pulmonary secretions under control with a good cough and gag reflex. The table below summarizes some of the problems associated with weaning from a ventilator. Table 2 Factors Associated with Difficulty Weaning from a Ventilator Cardiovascular collapse. Poor muscle strength or atrophy. Increased work of breathing. Excessive secretions. Patient not psychologically or physiologically ready. Primary illness not resolved. Improper weaning procedure or patient cannot be weaned (terminal illness). Pulmonary complications (eg. atelectasis, pulmonary infection, bronchospasm). Poor nutrition. Continued use of sedatives or analgesics. Acid-base imbalance. Electrolyte imbalance. Abdominal distension. Anemia. Fluid overload. Renal failure. Malfunction of equipment. Scanlan et al, Egan's Fundamentals of Respiratory Care, 5th Ed., 1990 The C.V. Mosby Company In addition to ensuring the body is ready for weaning from mechanical ventilation, there are many respiratory criteria that hospitals look at to assess for weanability. These assess for respiratory muscle strength, gas exchange, lung capacities, and lung compliance. Many hospitals use some of the following predictors:
Table 31 Indices for Weaning from Mechanical Ventilation Respiratory rate <25 breaths/min Tidal volume 3-5 ml/lb of body weight or >300 ml Minute volume <20 Lpm Vital capacity >15-20 ml/kg of ideal body weight and 3 x predicted V T Maximum inspiratory pressure >-20 cm H 20 Maximum voluntary ventilation >2 x minute volume Ventilatory pattern synchronous and stable Shunt <20-30% Oxygen status PaO 2/FiO 2 >238 mm Hg Dead space-tidal volume ratio <80% Dynamic compliance >25 ml/cm H 20 Pilbeam, SP. Mechanical Ventilation 2nd Ed. 1992 Mosby-Year Book, Inc.