Dalziel, in 1832, reported the use of a bellows-operated box with a seal around the neck or shoulders to provide artificial respiration to a drowned seaman.¹ In 1867, Woillez designed the first workable noninvasive negative-pressure ventilator, which was operated manually.² Doe, in 1989 described a box used for resuscitating newborns.³ Drinker developed the first tank ventilator in 1928 that was subsequently used with great clinical value in patients with poliomyelitis.⁴ Negative-pressure ventilation was introduced for use with neonates in the 1960s.^5,⁶ In 1953, Lassen reported that positive-pressure ventilation administered using an endotracheal tube or a tracheostomy was more successful that negative-pressure ventilation in treating patients with poliomyelitis.⁷ Major technologic advances in the design of positive-pressure ventilators led to the marked decrease in negative-pressure ventilation. Noninvasive positive-pressure ventilation by facemask was first used by Barach et al.⁸ for the treatment of hypoxemic respiratory failure secondary to acute pulmonary edema. Gregory et al.,⁹ in 1971, reported that constant positive airway pressure (CPAP) through nasal prongs was effective in improving oxygenation in neonates with hyaline membrane disease. Vidyasagar and Chernick showed that constant negative pressure applied about the chest and abdomen was as effective as CPAP in improving oxygenation in neonates with hyaline membrane disease.¹⁰ Despite these early reports, noninvasive ventilation did not become popular until more recently. There is a renewed interest in noninvasive ventilation coinciding with technologic advances in the manufacture and design of noninvasive ventilators. NIV through a mask for neuromuscular disorders was pioneered in the 1980s by Rideau et al.¹¹ and subsequently by Bach et al.¹²

Indications

The indications for the use of NIV can be categorized into short-term and long-term NIV. Short-term NIV is indicated where positive pressure support is needed acutely in a hospital setting in acute care or critical care areas for conditions that are reversible within a few days. Long-term NIV is indicated in conditions where respiratory failure is likely to be chronic or progressive.

Short-Term Noninvasive Ventilation

The indications for use of NIV in acute care and critical care situations are given in Box 50-1. The selection guidelines are given in Box 50-2. The first step is to determine whether a patient has a reversible cause of respiratory failure. The second step is to determine whether the patient needs positive pressure support to maintain gas exchange. Before attempting NIV in acute respiratory failure, the clinician must answer the following questions:

1. Does the patient have any contraindications to the use of NIV (see Box 50-2)?

2. Does the patient need to be promptly intubated?

If the answers to both questions are no, then the patient can be considered for NIV. Additional questions that the clinician needs to consider are the following:

1. Does the patient only need increased positive airway pressure to recruit the lung to improve oxygenation?

2. Does the patient need additional ventilatory assistance based on the clinical signs and symptoms or gas exchange derangements?

If the patient only needs increased airway pressure to recruit atelectatic lung and to maintain lung volumes, then the patient can be placed on noninvasive CPAP through an appropriate interface. If the patient needs ventilatory assistance, the patient is a candidate for NIV.

Figure 50-1 shows the algorithm to be followed for initiation of NIV in acute respiratory failure. If NIV is used in acute situations, there should be a determination within 1 to 3 hours whether the patient has improved. Signs of success and failure are given in Box 50-3. A good response to NIV is indicated by: (1) reduction in respiratory rate, (2) reduction in work of breathing, (3) reduction in dyspnea, (4) improvement in pH, (5) improvement in oxygenation, and (6) reduction in PaCO₂. Additionally, there may be hemodynamic effects such as a reduction in heart rate, improved blood pressures, and perfusion. Generally, for short-term use, NIV is used continuously until the patient improves or fails NIV. The goals of short-term NIV are given in Box 50-4.

Figure 50–1 Algorithm for initiation and weaning of short-term continuous positive airway pressure and NIV.

Adult Studies on the Short-Term Use of Noninvasive Ventilation

Adult studies have shown that NIV can be used in hypoxemic respiratory failure (PaO₂/fraction of inspired oxygen [FiO₂] ratio <300) with bilateral parenchymal disease to prevent endotracheal intubation. Ferrer et al. showed that NIV reduced the intubation rate from 52% to 25% with a reduction in intensive care unit (ICU) mortality rate from 39% to 18%.¹³ But, this finding must be tempered by studies that show that NIV may not reduce the need for endotracheal intubation.^14,¹⁵ CPAP, not strictly a form of ventilatory assistance, was described by Barach et al.⁸ for the treatment of acute cardiogenic pulmonary edema. Since then, several studies have demonstrated that both CPAP and NIV can decrease the rate of intubation, improve work of breathing and improve gas exchange.¹⁶^–¹⁸ In adults with acute exacerbations of chronic obstructive pulmonary disease, NIV has been shown to improve both short-term and long-term outcomes.¹⁹^–²¹ It is reasonable to expect a similar response to acute asthma because many of the pathophysiologic features are similar. Uncontrolled studies and one randomized controlled study showed improvement in patients having an acute asthma attack.²²^–²⁴ It is desirable to reduce the probability of nosocomial infections in immunocompromised patients. Therefore, it is appealing to apply NIV to avoid endotracheal intubation in these patients. Adult studies have shown that NIV can reduce the rate of intubation and may improve short term mortality.²⁵^–²⁷

Adult studies have shown that respiratory insufficiency that develops postoperatively in patients can be successfully treated with NIV. NIV reduces the rate of intubation and is more effective than CPAP or chest physiotherapy.^28,²⁹ In patients after lung resection surgery, NIV reduced the rate of intubation and mortality compared with conventional therapy.³⁰ Children with restrictive chest diseases who develop respiratory failure after surgery such as spinal fusion are good candidates for NIV if they develop postoperative respiratory failure provided they do not have any other contraindications.

Endotracheal intubation and invasive mechanical ventilation are associated with complications including ventilator-associated pneumonia, ventilator-induced lung injury, and airway trauma. To minimize these complications, it is desirable to extubate the patient as quickly as possible. Generally, extubation coincides with the end of weaning (i.e., the patient is not placed on any form of positive pressure support postextubation). In some patients, who take a long time to wean, but require fairly low ventilatory assistance, it is possible to extubate them to NIV. Theoretically, it would reduce the incidence of complications associated with endotracheal intubation and invasive mechanical ventilation. Udwadia et al. were among the first to test this paradigm in 22 adult patients who were successfully stabilized on NIV after extubation.³¹ Three adult studies have demonstrated that the use of NIV resulted in a reduction in the duration of mechanical ventilation, a reduction in the incidence of nosocomial pneumonia, and a reduction in the duration of hospital stay.³²^–³⁴ Similar pediatric studies are lacking. NIV to facilitate extubation should be carried out only if the patient meets the following criteria: (1) is awake and alert, (2) has intact bulbar function with a preserved cough reflex, (3) has minimal airway secretion, (4) is able to breathe spontaneously, (5) has stable hemodynamic status, (6) does not have gastric distension, and (7) has a requirement for 40% or less supplemental oxygen. Recently, some adult studies have examined the use of NIV for postextubation respiratory failure (i.e., respiratory failure that develops after extubation on complete spontaneous breathing). The rationale for the use of NIV in this circumstance is the avoidance of endotracheal intubation. In at least one study in adults, this approach was shown to increase the time from respiratory failure to intubation with an increase in mortality rate.³⁵ It is difficult to extrapolate from adult studies to pediatrics. It is possible that some children may benefit from the institution of NIV for postextubation respiratory failure.

Pediatric and Neonatal Studies

In 1981, Sullivan et al.³⁶ first demonstrated that nasal CPAP was effective in reversing obstructive sleep apnea in four adults and one teenager. Guilleminault et al.,³⁷ in 1986, reported that nasal CPAP reversed or reduced obstructive sleep apnea in some children. There were problems related to poor cooperation from parents and difficulty in preventing leaks around the nasal mask. Since the first report, several studies have shown that nasal CPAP was effective in obstructive sleep apnea in infants and children.³⁷^–⁴² In 1995, Guilleminault et al.⁴³ reported that nasal CPAP was successful in 70 of 72 infants with sleep-disordered breathing. In many instances, it provided an interim solution while allowing the patient to grow and postponing surgery.

Patients in a fairly wide age range appear to tolerate NIV therapy in the acute setting, provided they are adequately monitored and sedated. In 1993, Akingbola et al.⁴⁴ published a case report describing effective NPPV therapy in two pediatric patients with acute respiratory distress. Since that time, NIV has been applied to pediatric patients with a variety of respiratory disorders associated with acute hypoxemic respiratory failure, including pneumonia, pulmonary edema, postoperative respiratory decompensation in sleep apnea syndrome, status asthmaticus, spinal muscular atrophy, and end-stage cystic fibrosis.⁴⁵^–⁵¹

Padman et al.⁵² published a preliminary report on 15 patients ages 4 to 21 years who had respiratory failure because of cystic fibrosis (four patients) and neuromuscular disease (11 patients). In 14 patients, the placement of an artificial airway was avoided. They observed significant improvement in hospital days, respiratory rate, heart rate, serum bicarbonate, arterial carbon dioxide, dyspnea, activity tolerance, and quality of sleep. In 1995, Fortenberry et al. reported a retrospective study on the efficacy and complications of biphasic positive airway pressure (Bi-PAP) in 28 children with acute hypoxemic respiratory insufficiency.⁴⁷ Bi-PAP significantly decreased respiratory rate and improved both oxygenation and ventilation. The use of Bi-PAP decreased hospitalization rate and increased patient comfort. Only three of 28 patients required intubation or reintubation. Padman et al. conducted a prospective study in 34 patients with impending failure all of whom required airway or oxygenation/ventilation support and required admission to the pediatric ICU.⁵⁰ A decrease in respiratory rate, heart rate, and dyspnea score, and an improvement in oxygenation were noted in more than 90% of patients studied. The frequency of intubation in these patients was only 8%. Birnkrant et al.⁵¹ reported their experience with Bi-PAP in six patients with spinal muscular atrophy and three patients with other causes of respiratory failure. This uncontrolled study showed that in these patients noninvasive positive-pressure ventilation facilitated extubation. Teague et al.⁵³ reported that NIV treatment acutely improved oxygenation and reduced cardiorespiratory distress in 19 of 26 patients. This group of “NIV responders” had significantly shorter lengths of stay in both the ICU and hospital. However, the seven nonresponders required endotracheal intubation as respiratory distress progressed despite sedation. Teague et al.⁵³ also showed that NIV improves gas exchange in children with upper airway obstruction In a prospective, randomized, controlled trial of NIV in children with acute respiratory failure, Yan et al.⁵⁴ showed that NIV improved hypoxemia and the signs and symptoms of acute respiratory failure and reduced the rate of endotracheal intubation. Thill et al.,⁵⁵ in a randomized, crossover study in children with lower airway obstruction, showed that NIV decreased signs of work of breathing such as respiratory rate, accessory muscle use, and dyspnea as compared with standard therapy with no serious associated morbidity. Pancera et al.⁵⁶ recently published their experience with NIV in immunocompromised pediatric patients. NIV as the first-line therapy was compared to invasive mechanical ventilation. Seventy-five percent of the patients treated with NIV did not require endotracheal intubation showing that NIV is a viable first-line therapy for respiratory failure in immunocompromised patients for preventing endotracheal intubation.⁵⁶ More recently, NIV has been suggested as an alternative to endotracheal intubation for the perioperative management of scoliosis correction in patients with muscular dystrophy.⁵⁷

Interfaces

A properly fitting facial appliance is essential for the optimal application of NIV. There are currently several devices in use: (1) oral-nasal mask, (2) nasal mask, (3) total face mask, (4) Adam’s circuit, (5) head hood, (6) nasal prongs, and (7) mouthpieces. A correctly sized interface minimizes leaks, improves effectiveness of positive pressure support, and improves comfort.

Oral-Nasal Masks

This is the most common interface used to provide NIV. The ideal face-mask should (1) be made of a clear material to allow visual inspection, (2) conform to the contours of the patient’s face, (3) be easily moldable from its factory shape, (4) be soft and not apply excess pressure on the skin of the face, and (5) maintain its deformed shape (has “memory”) when it is removed. The mask should extend from the bridge of the nose to just below the lower lip. The mask is secured using an anchoring system that goes around the head.

Nasal Masks

A nasal mask rests between the bridge of the nose and above the upper lip. As a rule, the smaller the mask, the better the fit. Patients who are unable to keep their mouth closed may use chin straps to close the mouth.

Total Face Mask

This mask covers the whole face including the eyes. The advantage of a total face mask is that it does not have to conform to the shape of the face and therefore does not have to be molded to fit every patient. The disadvantage is that it has an increased deadspace and therefore, there may be difficulty in eliminating CO₂.

Adam’s Circuit

This device uses a “nasal pillow” that attaches to a manifold that is placed over the top of the head. Some patients prefer this to the nasal mask. “Nasal pillows” are available in various sizes.

Head Hood

Also called a helmet, the head hood has been used successfully in a number of European ICUs. It appears best suited for application of CPAP. Dead space is a major concern and therefore should be reserved for patients in the ICU.

Equipment

A detailed description of the devices available for NIV at home is beyond the scope of this chapter and the reader is referred to several excellent reviews.^58,⁵⁹ This discussion focuses on the characteristics of positive-pressure ventilators that can be used for providing NIV. Negative-pressure ventilators are discussed elsewhere in the chapter. The ventilators that are used for NIV can be classified into three categories: (1) conventional ICU ventilators with a double limb circuit without leak compensation, (2) devices with a single limb circuit with leak compensation, and (3) devices that combine the above two categories to include both leak compensation and having a double-limb circuit. Category 1 devices can only be used in the hospital. Category 2 and 3 devices can be used both in the hospital and at home. The examples of these devices would be any of the conventional ICU ventilators that can provide pressure support with positive end-expiratory pressure (category 1), Respironics Bi-PAP (category 2), and the Pulmometrics LTV ventilator (category 3). The performances of these ventilators vary widely in delivered tidal volumes, air-leak compensation, response to simulated effort, inspiratory trigger, expiratory cycling, rebreathing, response to high ventilatory demands, and patient-ventilator synchrony.⁶⁰ Category 1 or ICU ventilators operate with high-pressure gas sources with an oxygen blending system. Category 2 and 3 devices can operate without a high-pressure gas source. Bilevel ventilators do not have a blender; therefore the delivered FiO₂ is unpredictable depending on the oxygen flow rate, ventilator settings, amount of leak, site of oxygen enrichment, and the type of exhalation port.⁶¹

Optimizing Patient-Ventilator Interaction

Optimal patient-ventilator interaction requires that the ventilator be able to detect the patient’s inspiratory efforts as quickly as possible and terminate inspiration (expiratory cycling) as close to the beginning of the patient’s expiration as possible. Inspiratory trigger function differs significantly among the ventilators.⁶²^–⁶⁴ Factors affecting the inspiratory trigger function include trigger response to the inspiratory flow, leak-induced auto-triggering, and pressure-time and flow-time waveform heterogeneity.⁶²^–⁶⁴ Strategies to optimize expiratory cycling include setting a suitable threshold for the inspiratory time and adjusting the expiratory cycling flow threshold. Patients with obstructive lung disease tend to do better with high inspiratory flow, whereas patients with neuromuscular disease seem to do better with low inspiratory flows.⁶⁵ An adjustable backup rate is available in most modern ventilators used for NIV. This is particularly useful when sedation is used to improve patient compliance with NIV. Leaks around the interface reduce its effectiveness. Some amount of leak around the interface is expected with NIV. Complete elimination of air leak is not desirable because it comes at the expense of a very tight-fitting mask that may lead to patient discomfort and skin breakdown. Ventilators differ in their capacity to compensate for leaks. One of the drawbacks of a large airleak is auto-triggering. Humidification is important to prevent mucosal drying. Humidification can be provided using a heated humidifier, HME, or a pass-over humidifier.

Ventilator Settings

With CPAP, the pressure level is selected based on clinical need. There should be relief of symptoms within 1 to 2 hours of application of CPAP. With short-term NIV, two strategies have been employed. One approach is to start with a high inspiratory pressure (about 20 to 25 cm H₂O). The goal of such an approach is rapid relief of symptoms. If successful, a lower level that also is tolerated is selected. This approach is called the high-low approach. The other approach starts with a lower inspiratory pressure (about 8 to 10 cm H₂O) and gradually increased to produce relief of symptoms. The aim of this approach is to maximize patient comfort and tolerance. This is called the low-high approach. Within the first 1 to 3 hours, subsequent adjustments may be necessary depending on the patient response. Expiratory pressure is used routinely during short-term NIV. Maximal FIo₂ with bilevel ventilators is usually about 45% to 50%. If a higher FIO₂ is required, and endotracheal intubation is not indicated, then an ICU-type ventilator may be used.

Complications and Concerns During Short-Term Noninvasive Ventilation

Aerophagia and gastric distension can occur with NIV. The higher the pressure used, the greater the chance of gastric distension. Regurgitation of gastric contents and aspiration into the lungs are major concerns with the use of an oronasal total face mask and a helmet. Close monitoring is needed to prevent aspiration. A nasogastric tube to keep the stomach decompressed is necessary in these acutely ill patients. Pressure sores related to the masks are the other major concern. It should be possible to pass one or two fingers between the headgear and the face. Care should be taken to avoid fitting the interface too tightly. A spacer with soft padding can reduce the incidence of pressure sores on the face.

Circuit and Carbon Dioxide Rebreathing

Single limb circuits that are employed with bilevel ventilators can result in significant CO₂ rebreathing. All single-limb circuits have an exhalation port that is placed close to the patient interface through which exhaled gas will escape. If the exhaled gas flow exceeds the rate of escape through the exhalation port, exhaled air is rebreathed with the next inspiration. This results in significant CO₂ rebreathing and can cause hypercarbia. Rebreathing also depends on the level of expiratory positive airway pressure (EPAP), the location of the exhalation port, and the design of the exhalation port. Earlier devices had significant CO₂ rebreathing of the exhaled air, especially if the expiratory positive airway pressure levels were set too low. In the newer designs, this problem has been reduced. The closer the exhalation port to the patient interface, the less the rebreathing of CO₂. In fact, if the exhalation port is located in the mask itself, it results in the lowest amount of CO₂ rebreathing. Dual-limb circuits eliminate the risk of rebreathing.

Long-Term Noninvasive Ventilation

Long-term NIV is indicated in conditions that result in chronic respiratory failure. This can be either progressive or nonprogressive. Generally, long-term NIV is intermittent with periods off NIV. The most common form of long-term NIV is nocturnal NIV with off-periods during the daytime. The goals of long-term NIV are given in Box 50-4. The indications for long-term NIV are given in Box 50-5.

Physiologic Effects and Outcomes of Long-Term Noninvasive Ventilation

Intermittent NIV consistently improves daytime gas exchange.^66,⁶⁷ Patients with neuromuscular and chest wall disorders often have sleep-related disturbances.⁶⁸ NIV ameliorates hypoventilation, intermittent sleep apneas, and other sleep-disordered breathing.⁶⁹ In children with neuromuscular disorder, symptoms of daytime sleepiness, headache, and sleep quality improved after initiation of NIV.⁷⁰ Hospitalization rates and health care costs decreased. Quality of life remained stable after NIV, despite disease progression.⁷⁰ NIV, applied appropriately, improves the quality of life with restrictive thoracic disorders.⁷¹ Children with spinal muscular atrophy (types 1 and 2) often develop thoracoabdominal asynchrony, especially during sleep. NIV improved sleep breathing parameters and thoracoabdominal coordination during sleep in spinal muscular atrophy Types 1 and 2.⁷² Long-term NIV combined with mechanically assisted coughing has also been shown to increase survival in patients with Duchenne muscular dystrophy.⁷³

Ventilator Settings

As with short-term NIV, the ventilator settings must relieve patient symptoms. The approach that is generally recommended for long-term NIV is the low-high approach with gradual increase in support. A backup rate sufficiently high to control breathing nocturnally can rest the respiratory muscles and prevent apnea, especially in patients with neuromuscular disease. NIV can be provided both intermittently and continuously. There has been an increase in the continuous use of NIV, especially in patients with neuromuscular disorders. This has resulted in prolonging survival in these patients.

Monitoring of Patients with Long-Term Noninvasive Ventilation

Monitoring patients who are on long-term NIV requires an assessment of their respiratory function. Pulse oximeters are useful in detecting hypoxemia especially during sleep. End-tidal CO₂ monitors are only used in the clinic or hospital. Treatment of sleep-disordered breathing improves the quality of life and may prolong survival. Polysomnography may be required to document sleep disorders but is more expensive than nocturnal pulse oximetry. If a patient has bulbar weakness or severe obesity, there may be obstruction of the upper airway or obstructive sleep apnea in addition to sleep hypoventilation. Both problems may be treated with nocturnal ventilation.

Negative-Pressure Ventilation

Design and Modes of Negative-Pressure Ventilators

All negative-pressure ventilators have a chamber in which subatmospheric pressure is generated and a pump which generates this pressure. The chamber may cover only the chest and the upper abdomen (cuirass) or all of the extracranial portions of the body (tank respirator, isolette, or a body suit). The tank respirator has both the chamber and the pump in one unit. In all other cases the two units are separate. The cuirass can be prefabricated or custom designed to fit the contours of the chest. Custom-designed cuirasses are especially useful in patients with skeletal or spinal deformities. All body suits fit over a hard shell similar to the cuirass placed over the chest and the upper abdomen. Most negative pressure pumps in use today are pressure cycled. Some volume-cycled pumps have been used but their use has been limited because the pump cannot compensate for the variable amounts of air leak. Additionally, some ventilators provide an inspiratory assist mode. At the present time, there are four modes of negative pressure application: (1) cyclical negative pressure, (2) negative/positive pressure, (3) continuous negative extrathoracic pressure (CNEP), and (4) negative pressure/CNEP. Cyclical negative-pressure ventilation refers to a mode where the ventilator generates the preset subatmospheric pressure during inspiration while expiration is passive. Negative/positive-pressure ventilation refers to a mode which is a combination of negative pressure during inspiration with positive pressure during expiration. CNEP refers to a mode in which a constant subatmospheric pressure is provided throughout the respiratory cycle and the patient breathes spontaneously. Negative pressure/CNEP refers to a mode where negative pressure inspiratory cycles are superimposed on CNEP.

Clinical Applications

Box 50-6 shows the indications, advantages, disadvantages, contraindications, and clinical side effects of negative-pressure ventilation. In 1965, Stahlman et al.⁷⁴ first reported on the use of a tank respirator to treat neonates with respiratory distress syndrome. Vidyasagar and Chernick,¹⁰ in 1971, were the first to show that CNEP improved oxygenation in neonatal respiratory distress syndrome. In the 1970s, several controlled and uncontrolled studies showed that negative-pressure ventilation was effective in the management of neonatal respiratory syndrome.⁷⁵^–⁷⁷ However, the utility of this technique was limited because of problems such as upper airway obstruction, difficulties in achieving access to patients, difficulties in achieving an adequate seal, sores from the neck seal, and inability to maintain an adequate neutral thermal environment. Negative-pressure ventilation has been reintroduced in the treatment of neonatal respiratory distress syndrome. A recent randomized control study showed that application of CNEP was associated with fewer intubations and decreased the total duration of oxygen therapy.⁷⁸ However, CNEP was associated with a slightly higher (not statistically significant) mortality rate, cranial ultrasound abnormalities, and pneumothoraces. Further studies are warranted to assess the utility and safety of negative-pressure ventilation in neonatal respiratory distress syndrome.

Box 50–6 Indications, Advantages, Disadvantages, and Contraindications of Negative Pressure Ventilation

Advantages

1. Avoids intubation or tracheostomy

2. Preserves physiologic functions such as speech, cough, swallowing, and feeding

3. Allows fiberoptic bronchoscopy to be performed without disconnection from the ventilator

4. Promotes venous return by creating negative intrathoracic pressure.

Contraindications

1. Gastrointestinal bleeding

2. Rib fracture

3. Recent abdominal surgery

4. Uncooperative patients

5. Sleep apnea syndrome

6. Neurologic disorders with bulbar syndrome

Clinical Side Effects

1. Tiredness

2. Musculoskeletal pain or tightness

3. Esophagitis

4. Rib fractures and pneumothorax

5. Impaired sleep quality

6. Poor compliance

Respiratory Dysfunction and Failure

During the polio epidemics in the 1930s and 1940s, negative-pressure tank ventilators reduced mortality in patients with spinal polio.⁷⁹^–⁸¹ There has been a resurgent interest in negative-pressure ventilation in patients with neuromuscular diseases and skeletal deformities. In adults with kyphoscoliosis and neuromuscular disorders with chronic hypoventilation, intermittent use of negative-pressure ventilation, mostly nocturnal, decreased or reversed day time symptoms of hypoventilation and gas exchange abnormalities.^82,⁸³ These studies show that negative-pressure ventilation is useful in acute or chronic respiratory failure associated with skeletal or neuromuscular disorders. Isolated case reports in adults have been published about using CNEP in adult respiratory distress syndrome and in persistent flail chest deformity.⁸⁴^–⁸⁷ CNEP has been used to assist ventilation in children with diffuse alveolar disease and progressive respiratory failure.⁸⁸^–⁹⁰ In 1975, Sanyal et al.⁸⁹ published a case series of five children with progressive respiratory failure from Pneumocystis carinii pneumonitis. Three patients improved with CNP and survived, whereas the other two continued to worsen and died. Sanyal et al.,⁹⁰ in 1977, published a larger series where CNEP was used to assist ventilation in 14 children who had progressive respiratory insufficiency caused by diffuse bilateral alveolar disease. Within 6 hours after therapy was started, oxygenation improved. The improvement was sustained and within 24 hours permitted a decrease in fractional concentration inspired oxygen. There was a concomitant decrease in intrapulmonary right-to-left shunt and a decrease in respiratory rate. Four of the 14 patients developed pneumothorax that was successfully decompressed. Ten patients survived. These observations establish CNEP therapy as an effective means of improving arterial oxygenation in spontaneously breathing older children, avoiding endotracheal intubation and prolonged use of muscle relaxants and sedatives.

Cardiovascular Disorders

Raine et al.⁹¹ in 1992 published a pilot study in 10 children with respiratory failure and found the CNEP, introduced either in conjunction with intermittent positive-pressure ventilation or alone, did not produce large changes in cardiac output. In patients with Fontan-type operations, pulmonary blood flow and cardiac output increases with spontaneous inspiration and positive-pressure ventilation decreases antegrade pulmonary blood flow and may increase pulmonary valvular incompetence in patients with restrictive right heart physiology after repair of tetralogy of Fallot.⁹² In children after repair of total cavopulmonary connection and tetralogy of Fallot and after Fontan-type procedures, negative-pressure ventilation provided using a Hayek external high-frequency oscillator improved cardiac output by 42% to 46% almost entirely by an increase in stroke volume with improvement in mixed venous oxygen saturation.⁹³ Systemic and pulmonary vascular resistances decreased significantly as well. A similar finding was observed in children after transcatheter occlusion of an asymptomatic patent ductus arteriosus and after open heart surgery.⁹⁴ Raine et al.95 in 1992 reported in an uncontrolled study that negative-pressure ventilation using either CNEP or intermittent negative-pressure ventilation was a viable alternative to positive-pressure ventilation in patients with phrenic nerve palsy after pediatric cardiac surgery, reduced the need for diaphragmatic plication, and facilitated weaning from positive-pressure ventilation. These studies show that negative-pressure ventilation is a useful technique in selected patients after cardiac surgery where positive-pressure ventilation is not desirable or results in unwanted hemodynamic effects.