Critically ill patients who are unable to maintain adequate oxygenation and/or carbon dioxide removal without support require a mode of ventilation that relieves the patient of the majority of the work of breathing while ensuring ventilation and oxygenation. Depending upon the mode selected and level of sedation utilized, the patient may initiate some breaths, and/or breathe spontaneously. Thus, even for the critically ill, there is a continuum of ventilatory support from maximal to minimal support.26 Historically, the disadvantages to maximum ventilatory support included the need for frequent sedation and possibly neuromuscular blockade. In turn, this led to decreased spontaneous respiratory efforts, muscle atrophy, increased atelectasis, and inspissated secretions. These disadvantages were reluctantly accepted as necessary to sustain life. However, technological advances in mechanical ventilation now offer options that allow more limited use of sedation, and particularly paralytics.
For many years, modes of ventilation were broadly categorized into either volume targeted or pressure targeted modes. The volume targeted modes required the clinician to select a tidal volume for the ventilator to deliver (or target) with each mandatory breath, while pressure targeted modes required selection of a fixed ventilating pressure for each mandatory breath. The incorporation of computer technology into the ventilator has allowed increasing sophistication in the delivery of breaths. Computer science, along with evolving clinical knowledge, has paved the way for a third classification of modes known as dual-targeting. In the 1990s, dual-targeting modes permitted the clinician to deliver a breath that combined features from both volume and pressure targeting ventilation.26
Volume-controlled ventilation is probably the simplest and earliest method of positive pressure ventilation. Developed in the 1950s, adults were the primary patient group. In this mode the patient is not allowed, nor required, to initiate a breath, and the work of breathing is primarily provided by the ventilator, as long as the patient's respiratory cycle is synchronized with the mechanically delivered breaths. All breaths are initiated by the ventilator at the rate that has been set by the clinician. The tidal volume is preset for each breath, and the minute ventilation becomes the product of the set rate and the tidal volume. Respiratory muscle efforts and their contribution to oxygen consumption may be eliminated if the patient is chemically paralyzed. Some also believe that subsequent relaxation of the chest wall muscles may enhance recruitment of lung tissue.32 Eliminating patient effort may also relieve patient dyssynchrony, although anxiety can be high. Lung thorax compliance, airway resistance, and auto-PEEP are easily calculated for pressure and flow measurements. The generation of high tidal volumes from volume-controlled ventilation may increase the risk of volutrauma, a major facet of ventilator induced lung injury (VILI)33 (see page 612). It is recommended that tidal volumes and airway pressures be closely monitored to minimize the risk of alveolar over distension and VILI.
Pressure-controlled ventilation applies a pressure that is preset by the clinician, as is the ventilatory rate. The clinician also sets a fixed inspiratory time. In pure control modes, the patient cannot trigger or initiate a breath. Inspiratory effort may appear as dyssynchrony between the patient and the machine, as the patient attempts to draw gas into their lungs, but the machine does not respond. Controlled breaths are delivered at a predictable interval, ie, a rate of 12/min results in a breath every 5 seconds. The inspiratory flow from the ventilator is high initially, the flow decelerates as the alveolar pressure rises with lung inflation. The delivered tidal volume varies depending on the inspiratory time, patient effort, as well as the patient's lung compliance and airway resistance. Minute ventilation is not predetermined.34
Patients receiving ventilatory support frequently require postural drainage with or without manual techniques and suctioning for secretion retention. The physical therapist should be careful not to dislodge or pull on the tracheal tube when turning the patient. With careful positioning, manual techniques and most postural drainage positions, including the prone position, are possible (Fig. 19-4). Obstacles to the prone position include severe kyphosis or a pelvic external fixator. It is possible to place a patient prone while wearing a brace to stabilize spinal fractures, if the patient requires manual techniques, braces such as thoracolumbosacral orthoses (TLSOs) can often be opened once the patient is securely positioned. For patients with cervical bracing such as Halo vests or Yale braces, full prone positioning may be problematic. In these instances, the therapist should try to position the patient as close to one-fourth turn to prone from side-lying as possible. If a patient does not have adequate cervical rotation to lie prone, a towel roll can be placed both under the forehead and under the upper thorax. For patients with a tracheostomy a blanket roll, a sheet roll or wedge is carefully placed under the upper thorax to allow room for the tracheostomy tube and airway suctioning. A roll under the pelvis may also be helpful to allow for a shift in abdominal contents, particularly for patients with a large abdominal girth.
In summary, the disadvantages of control modes of mechanical ventilation include the need for heavy sedation and possibly neuromuscular blockade, decreased spontaneous respiratory efforts, respiratory alkalosis, and progressive atelectasis. Progressive infiltrates and atelectasis develop in dependent lung zones as a result of the gravitational redistribution of fluid, impaired secretion clearance, and poor inflation. The positive pressure breaths of mechanical ventilation result in ventilation along the path of least resistance, hence upper/anterior lung regions (when the patient is supine) are more readily ventilated than the posterior lung, while perfusion is primarily gravity dependent and greater in dependent/posterior regions, creating a V̇/Q̇ mismatch.11 The decrease in spontaneous respiratory effort associated with control modes of ventilation is unfortunate, as it has potential to mitigate negative effects of positive pressure ventilation. Spontaneous breathing has been shown to decrease the development of atelectasis and reduce V̇/Q̇ mismatch, as it improves ventilation of posterior/dependent lung regions based on diaphragmatic mechanics.35–38 Mechanically ventilated patients in the ICU frequently experience atelectasis and consolidation in the dorsal dependent lung regions. Therefore strategies which enhance ventilation and recruitment of these lung regions, such as facilitation of spontaneous breathing, are valuable. Recent studies, primarily in the animal model, have also shown significant decrement in diaphragmatic muscle force39,40 and diaphragm atrophy41 as a result of mechanical ventilation.
Controlled mechanical ventilation results in a greater decrease in diaphragmatic force than assist control ventilation.42 If paralytic agents are required to maintain compliance with a controlled mode of ventilation, inadvertent disconnection from the ventilator can be life-threatening. Despite these disadvantages, controlled mechanical ventilation with low tidal volumes and PEEP remains an option for ventilation of patients with severe respiratory failure and elevated intracranial pressure. Arguably, the simplicity, and immediate responsiveness, makes it an attractive choice for clinicians with less expertise in other modes.
Assist-control (AC) modes may be either volume or pressure targeted, and while similar to their precursors, the control modes, the AC modes are far more commonly used.26 The difference between control and assist-control is the ability of the patient to trigger or request breaths above the set ventilatory rate. All breaths continue to be of the same size and type as the mandatory breaths. The rate set by the clinician becomes the minimum number of breaths a patient will receive, however if the patient initiates additional breaths, the ventilator will reward the patient with a machine breath.
The goals of assist control modes are to allow and improve synchrony between the patient and the ventilator, reduce patient effort, and optimize comfort. However, how a particular mode of ventilation is used may be equally as important as the chosen mode of ventilation. Physical therapists should be familiar with the terms and general principles of AC, synchronized intermittent mandatory ventilation (SIMV), pressure support ventilation (PSV), pressure regulated volume control ventilation (PRVC), airway pressure release ventilation (APRV), and proportional assist ventilation (PAV) when working with mechanically ventilated patients.
Synchronized Intermittent Mandatory Ventilation
Synchronized intermittent mandatory ventilation (SIMV) appears the same as AC ventilation when the patient is receiving only ventilator-assisted breaths (not taking any spontaneous breaths). The patient receives periodic positive-pressure breaths from the ventilator at a preset volume or pressure and rate. With SIMV, the patient can inhale with unassisted spontaneous breaths between mechanically assisted breaths. When a patient is able to breathe spontaneously, spontaneous efforts will be synchronized with the timing of the mandatory ventilator breaths. If spontaneous breaths are taken within the preset triggering period that a mechanical breath is scheduled to be delivered (usually about a 1-second zone), the ventilator will deliver the mandatory breath while synchronizing with the patient's inspiratory effort.19 If the patient makes no effort during the triggering period, the ventilator waits until the end of the triggering period and delivers the targeted volume or pressure. The precursor to SIMV was intermittent mandatory ventilation (IMV), which was developed to facilitate weaning.43,44 SIMV evolved with the goal to avoid problems with dyssynchrony when weaning patients from the ventilator and to gradually decrease the number of mechanically assisted breaths to decrease the duration of mechanical ventilation. However, the use of SIMV to decrease weaning time has not been substantiated in clinical studies (see page 603). SIMV may actually contribute to respiratory muscle fatigue when the patient has a high respiratory rate and increased work of breathing.45,46 SIMV was historically a volume targeted mode until the 1990s with the mainstream introduction of SIMV as a pressure targeted mode as well. In clinical practice SIMV, whether volume or pressure targeted, is routinely used with pressure-support ventilation.
Pressure Regulated Volume Control
Pressure regulated volume control (PRVC) is a combination of volume control and pressure regulation. In this mode, the ventilator initially delivers a volume-controlled breath, while measuring the plateau pressure. The next breath is delivered using the measured pressure of the previous breath. If subsequent breaths increase above the preset volume, the pressure level is incrementally decreased until the preset tidal volume is delivered. If measured tidal volumes fall below the preset volumes, pressure is increased incrementally to reach the preset volume, up to a preset maximum upper pressure limit. In this mode, the ventilator is set to deliver a guaranteed respiratory rate, however breaths may be either ventilator or patient initiated. An alarm sounds if the ventilator is unable to deliver the preset volume within the preset pressure limit. It has been theorized that PRVC may decrease work of breathing while being used in a lung protective strategy, however this has not yet been established.47
Pressure-support ventilation (PSV) is a pressure targeted mode requiring the patient to trigger every breath. The clinician does not set a machine rate, so in the absence of patient effort, the ventilator will not deliver a breath. This, probably more than any other mode, gives the patient more freedom with breathing. With PSV the main setting is the pressure target. When the ventilator senses an inspiratory effort (dependent on the trigger sensitivity that has been set) it responds by delivering a decelerating gas flow which raises the pressure in the airways to the targeted pressure level and holds the pressure constant. Decelerating gas flow is a consistent feature of pressure-targeted modes, as compared to a fixed or constant gas flow, which is a consistent feature of volume targeted modes.26 The pressure is maintained throughout inspiration. The inspiratory period terminates when the gas flow rate decreases to a preset value (typically 5%–25% of the peak inspiratory flow rate). Alternate measures to terminate the inspiratory period are available but are either uncommon or included as a safety feature.48 The patient indirectly controls rate, tidal volume, minute ventilation, and I/E ratio. The two most notable features of PSV are that the patient must trigger every breath, and the clinician does not set a fixed inspiratory period (one of the only modes having this distinction).
In the stable lung, as pressure support increases, respiratory rate decreases and tidal volume increases. In most cases, minute ventilation is not significantly modified. Alveolar ventilation is increased and Paco2 decreases. Conversely, when pressure support is decreased the tidal volume decreases, PSV assists respiratory muscle activity by improving the efficacy of spontaneously initiated breaths, reducing the demand on the inspiratory muscles, and increasing tidal volume, therefore reducing the workload on the respiratory muscles. PSV has been shown to reduce the work of breathing and oxygen consumption of the inspiratory muscles.28,49 Lower levels of PSV counteract the element of work of breathing incurred by the ventilator circuitry and, particularly, the endotracheal tube. However, resistance changes throughout inspiration, being greatest at the beginning of the breath and least at the end of the breath, but the pressure level remains fixed. Therefore, the support to overcome resistance initially under compensates and later overcompensates. Nonetheless, PSV has become a highly useful adjunct particularly in the stable ventilated patient and in the patient being weaned from ventilatory support.26
Physical therapists should be aware that some physicians will use a pressure support of 5 cm of water pressure and a resting respiratory rate of <35 breaths/min as criteria for extubation.50–52
Airway-pressure-release ventilation (APRV) (also referred to as BiVent, BIPAP, DuoPAPBiLevel or Biphasic ventilation, depending upon the manufacturer) is simply a modified form of CPAP (continuous positive airway pressure) which uses two different levels of pressure.53 As the name CPAP suggests, CPAP utilizes a continuous positive airway pressure while the patient breathes spontaneously. APRV is CPAP with a periodic release in the airway pressure to a lower level. Typically, the release is very short (less than 1 second) and the release level is to zero cm H2O. The higher CPAP level of APRV allows the patient to breathe spontaneously, facilitating recruitment, and improving oxygenation. The release phase aids in the removal of CO2. Conceptually, lowering the airway pressure to zero may seem like a bad idea. However, as the release phase is quite short, not all the gas empties from the lungs before the higher airway pressure is reinstituted, thus alveoli tend not to derecruit54–56 (Fig. 19-3). APRV works well with ARDS where lung compliance is low and the respiratory muscles are intact. Proposed advantages of APRV include reducing the risk of VILI (see risks of mechanical ventilation) by limiting peak airway pressures, and a reduction in repetitive recruitment/derecruitment of alveoli, which results in atelectrauma. Studies have also shown a decreased need for patient sedation and neuromuscular blockade,22,57 as well as benefits associated with spontaneous breathing.58 It should be noted that with critically ill patients who are unable to initiate spontaneous breathing, APRV can be used essentially as a “Full Support” mode of ventilation until the patient is able to initiate spontaneous breaths. APRV's standout feature is allowing patients to spontaneously breathe; therefore practices inhibiting breathing, such as heavy sedation and or paralytic use, limit the usefulness of this mode.
Proportional-assist ventilation (PAV) may have promise and replace other ventilatory modes though it has yet to gain widespread acceptance. PAV offers maximal patient autonomy; every breath is initiated and terminated by the patient. The ventilator essentially acts as an accessory muscle imposing no volume or pressure targets; the patient has total control over all aspects of breathing. The operator selects which portion of the work will be performed by the machine. Pressure assistance by the machine is proportional to a variable combination of the inspired volume (elastic assist) and the inspiratory flow rate (the resistive assist). Tidal volume and flow are totally controlled by the patient. When the patient pulls harder, the machine boosts its output, and as the patient relaxes, the machine cuts back.49,59–61 This is different from the patient ventilator interaction observed in conventional modes, where the ventilator-generated pressure is either constant or inversely related to effort. The advantage of PAV is that it yields to the patient's own neuromuscular control mechanisms and is guided by motion of the respiratory system synchronizing the ventilator's output with the patient's continuously changing needs. Because tidal volume and flow rates are controlled by natural breathing with PAV they vary continuously; therefore, PAV requires backup in the event that the patient's ventilatory effort ceases. PAV has the potential for providing appropriate ventilatory support in a variety of clinical settings, ranging from acute lung injury, to weaning from mechanical ventilation, to increasing exercise tolerance in patients with COPD for pulmonary rehabilitation when used noninvasively.62 PAV theoretically improves the physiological relationship between inspiratory effort and ventilatory return that often characterizes respiratory failure. PAV may require lower peak airway pressures than standard volume-targeted modes, improve patient comfort,63 and provide a better synchrony of breathing64, however, the clinical benefit has yet to be clearly established.
In the 1990s, ventilator technology began incorporating a combination of both volume- and pressure-targeted features. In these new “modes”, the clinician sets a volume target for each breath, but unlike historical volume targeting modes, the ventilator utilizes a decelerating gas flow to deliver the breath. This requires the ventilator to initially do a series of test breaths to gauge lung compliance. The ventilator is then able to calculate how to deliver the breath with the lowest possible pressure. Calculations are performed on every breath. As lung compliance improves, the ventilator requires less pressure, as lung compliance worsens, the ventilator will increase pressure. Pressure changes are incremental, usually no more than 3 cms H2O at a time. The clinician sets a pressure limit not to be exceeded and the ventilator alarms as the pressure alarm level are reached. At that time, pressure will not increase, but volume will decrease until the clinician intervenes. Some ventilators have dual targeting designed modes, for example, PRVC, while other ventilators apply the feature of dual targeting to conventional volume modes, for example, SIMV with AutoFlow. It has been theorized that this technology may decrease work of breathing while being used in a lung protective strategy, however this has not yet been established.47
Noninvasive ventilation (NIV) is a term used to describe ventilatory support supplied via nasal prongs or some type of face mask to provide CPAP, BiPap or positive pressure ventilation to the nonintubated patient. Thus, NIV is not a mode of ventilation, rather a technique of delivering a mode. The machine used may be either a standard ventilator or a single purpose unit. Noninvasive positive pressure ventilation has been shown to have particular benefit for patients with COPD in avoiding intubation and failure of extubation, as well as facilitating weaning.65 Noninvasive ventilation may also be used to increase exercise tolerance for patients with COPD62 or for sleep apnea.
Continuous Positive Airway Pressure
Continuous positive airway pressure (CPAP) is a form of ventilatory support that is simply PEEP delivered to a patient who is spontaneously breathing. No machine breaths, that is, positive pressure breaths, are delivered. CPAP is the terminology reserved for patients who are only spontaneously breathing, while PEEP is terminology used when patients are receiving some form of positive pressure breaths. Both CPAP and PEEP increase FRC and help prevent derecruitment of alveoli. In normal subjects, CPAP increases tidal volume by 25% and lowers respiratory rate by over 30%.66 In intubated patients CPAP may decrease the work of breathing by 50%. CPAP is frequently used as a method of weaning the patient from the previously described modes of ventilatory support. CPAP can also prevent the flail action of a paralyzed hemidiaphragm, thereby improving the efficiency of the remaining innervated respiratory muscles, and may also prevent atelectasis.
CPAP can be delivered with a mechanical ventilator or a separate device via a tracheal tube, nasal prongs, or face mask. Nasal CPAP has been shown to reduce the number of apneic episodes, arrhythmias, and hypoxic episodes during sleep and to reduce daytime sleepiness and improve neuropsychiatric function in patients with obstructive sleep apnea, which affects 2% to 4% of the population.67,68 Nasal CPAP provides a pneumatic splint for the airway, preventing airway collapse during sleep (when upper airway dilator muscle activity is low), and increases the airway caliber in the retropalatal and retroglossal regions. Nasal CPAP also increases the lateral dimensions of the airway and thins the lateral pharyngeal walls. Typical settings are 5 to 20 cm of H2O pressure. Poor patient compliance is noted with nasal prongs and face masks, which may be related to facial and skin discomfort, rhinitis, nasal irritation and dryness, difficulty exhaling, and claustrophobia. Full-face face masks have been associated with increased aspiration and are typically reserved for patients with persistent mouth leaks.68
Because CPAP can increase functional residual capacity and shorten inspiratory muscles placing them at a mechanical disadvantage, there is the potential to worsen inspiratory muscle weakness. However, patients with preexisting shortened inspiratory muscles because of COPD may benefit from the ventilatory assistance of CPAP.66–69 Some patients may require CPAP at night or while in bed to maintain adequate oxygenation, yet have adequate oxygenation while they are mobile. After consulting with the physician, the physical therapist can evaluate whether CPAP is required during mobility activities. CPAP can frequently be disconnected for short periods of time during ambulation and wheelchair mobility activities, or extension cords and battery packs can be used to provide adjunctive CPAP during functional tasks. Likewise, CPAP during physical therapy interventions such as aerobic exercise training or functional training may enhance patient tolerance, comfort, and compliance.
Bilevel Positive Airway Pressure
Bilevel positive airway pressure (often referred to as BiPap) may be used with noninvasive ventilation for ventilatory support. As with CPAP, intubation is not required. Sleep apnea is a common indication, along with exacerbations of COPD, congestive heart failure, and cystic fibrosis. BiPap is sometimes referred to as bilevel CPAP because it adds the advantage of an inspiratory positive airway pressure to CPAP. As with CPAP, BiPap may be used exclusively at night or intermittently throughout the day depending on the patient's condition. Successful treatment can be predicted by improvement in pH, Paco2, Pao2, and functional status.18,69
Physical therapy interventions are used to assist in strengthening the respiratory muscles and provide general conditioning to assist in weaning the patient from the ventilator. The physical therapist should closely monitor oxygen saturation, respiratory rate, and the patient's tolerance to activity. The physical therapist should notify the physician when a patient has markedly abnormal signs and symptoms during treatment. It may be necessary to add invasive or noninvasive mechanical ventilation to allow the patient to tolerate physical therapy and nursing interventions.