627CHAPTER 54 Mechanical Ventilation and Respiratory Care resistance 2 The remaining inspiratory work is stored as potential energy that is used to perform the expiratory work Increased airway resista[.]
CHAPTER 54 Mechanical Ventilation and Respiratory Care 627 100% 63% 86% 95% 98% 99% 37% 5% 14% 0% Inspiration 2% 1% Expiration • Fig 54.2 Schematic representing time constants during inspiration and exhalation resistance.2 The remaining inspiratory work is stored as potential energy that is used to perform the expiratory work Increased airway resistance and decreased chest and lung compliances require a greater pressure to inflate the lung to the same lung volume, imposing a greater workload on the respiratory muscles and increasing the energy cost of breathing When the oxygen supplydemand balance to the respiratory muscles is perturbed, respiratory failure may ensue secondary to muscle fatigue Determinants of Gas Exchange The determinants of systemic arterial oxygenation are multifactorial and include inspired oxygen concentration and tension, lung volume, cardiac output, and ventilation-perfusion (V/Q) matching.3 Alveolar collapse leads to inadequate oxygenation owing to increased physiologic dead space and intrapulmonary shunting resulting from V/Q mismatch During expiration, the presence of alveolar surfactant helps prevent alveolar collapse Surfactant deficiency, loss, or alteration promotes alveolar collapse and increases the critical opening pressure of the alveoli Parenchymal lung disease is also characterized by an increased critical closing pressure and alveolar collapse Alveolar diseases, including pneumonia and acute respiratory distress syndrome (ARDS), affect oxygenation owing to both alveolar collapse and increased resistance to gas diffusion across the alveolar basement membrane Chapters 48, 51, and 52 contain further descriptions of these pathologic states The determinants of systemic partial pressure of arterial carbon dioxide (Paco2) are a balance between CO2 production (metabolic rate) and CO2 elimination (minute ventilation, the product of Vt and respiratory rate).3 Decreased CO2 elimination usually results from decreased central drive, airway obstruction, parenchymal disease, and muscle weakness Sufficient pulmonary blood flow is also necessary for CO2 elimination; states of reduced pulmonary blood flow (such as pulmonary hypertensive crisis or cardiac arrest) will also result in elevated Paco2 Increased metabolic production of CO2 usually is caused by hypermetabolic states or excessive caloric intake, especially high-carbohydrate alimentation Indications for Mechanical Ventilation Respiratory Failure The primary indication for institution of assisted ventilation is respiratory failure, which is generally defined as the presence of inadequate oxygenation, inadequate ventilation, or both Apnea or respiratory arrest is an extreme form of respiratory failure; prolonged apnea is an indication for immediate mechanical ventilation In critically ill children, establishing mechanical ventilation before respiratory failure develops is preferable while the patient maintains some physiologic reserve and is less likely to suffer clinical deterioration—and, potentially, cardiac arrest—during the intubation process Inadequate oxygenation, objectively, is defined as a partial pressure of arterial oxygen (Pao2) less than 60 torr, an arterial hemoglobin oxygen saturation less than 90% in room air, or a Pao2:Fio2 (fraction of inspired oxygen) ratio less than 300.4 Other indices include an alveolar-to-arterial oxygen gradient of greater than 300 torr with Fio2 of 1.0 However, adequacy of oxygenation should be assessed in terms of the patient’s global disease state and oxygen delivery rather than relying solely on an objective cutoff Inadequate oxygenation owing to intrapulmonary shunting can typically be overcome with the addition of increased inspired oxygen concentration provided that the magnitude of the shunt is less than 15%.2 Intrapulmonary shunt can be decreased by reexpanding collapsed alveoli or decreasing the fraction of pulmonary blood flow distributed to the collapsed/ consolidated alveolar segments Inadequate ventilation is objectively defined as a Paco2 greater than 45 torr with an arterial pH less than 7.35 in the absence of chronic hypercapnia Acute-on-chronic ventilatory failure is objectively defined as a change in Paco2 of at least 20 torr with a corresponding decrease in arterial pH Impending respiratory 628 S E C T I O N V Pediatric Critical Care: Pulmonary failure—characterized by rapidly rising Paco2, progressive respiratory distress, progressively poor oxygenation, Paco2 out of proportion to the respiratory effort, or fatigue of respiratory muscles is a relative indication for mechanical ventilation Chronic respiratory failure is defined as the need for mechanical ventilation for more than 28 days Other Indications for Mechanical Ventilation Just as certain disease processes (e.g., shock) may have increased oxygen demands and benefit from early mechanical ventilation, there may be relative indications for mechanical ventilation independent of impending respiratory failure Children may require mechanical ventilation for airway protection, cardiac disease or failure, neurologic impairment or sedation needs, respiratory muscle weakness, and even to reduce caloric needs to optimize nutritional status and allow growth In this manner, clinicians must use their clinical judgment to augment objective definitions of respiratory failure Chapter 44 provides a deeper discussion of the many causes of breathing failure Design and Functional Characteristics of Ventilators All ventilators include input power, a drive system, control system, cycling mechanism, and a system to provide positive endexpiratory pressure (PEEP).5 Common accessories include a heated humidifier and oxygen blender The input power to operate the ventilator is usually electric or pneumatic The drive system provides the pressure gradient between the ventilator and lungs required to generate a gas flow, typically using compressed gases at high pressures from wall outlets, cylinders, or a small compressor designed to be used with individual ventilators In this scheme, the ventilator acts only to modulate the flow of gas and will not function if the external pressurized source of gas fails A ventilator with an internal compressor does not need an external source of gas to inflate the lung A detailed review of the physical characteristics and functional design of ventilators is beyond the scope of this chapter; the reader is referred to several excellent reviews on this subject.5–9 Pressure, volume, flow, and timing are the primary variables that the provider may control with any mechanical breath; these variables will be explored in more detail later It is important to note that these variables are not independent of each other and that each is also dependent on respiratory system compliance and resistance Phases of a Breath A ventilator is simply a machine that performs external work to support a patient’s breathing Energy applied to the device is altered, transmitted, and directed in a predetermined manner to either augment a patient’s breathing efforts or replace the patient’s work of breathing completely Ventilators move gas into the lungs by increasing transpulmonary pressure (PTP) Positive pressure ventilators create PTP by raising the airway pressure (Paw) above the intrapleural pressure (Ppl), whereas negative-pressure ventilators create PTP by decreasing Ppl below Paw During mechanical ventilation, PTP may be generated by the ventilator, respiratory muscles during a spontaneous breath, or a combination of both A breath is defined as one cycle of positive flow (inspiration) and negative flow (expiration) The key components of a mechanical breath are the trigger, flow pattern, limit, cycle, and inspiratory time (Fig 54.3) Trigger is the signal to initiate the mechanical breath, flow pattern describes how the air flows into the patient during inspiration, limit describes the parameter used to terminate lung inflation, and cycle is the parameter that allows lungs to empty While each ventilator may have unique proprietary names for different ventilator modes, defining the trigger, flow pattern, limit, and cycle will allow the bedside provider to understand how each breath is delivered to the patient Total cycle time Peak inspiratory flow End of inspiratory cycle Flow Trigger Inspiratory time Expiratory time Limit Pressure/ volume Time • Fig 54.3 Phases of a breath CHAPTER 54 Mechanical Ventilation and Respiratory Care Inspiratory time is defined as the period from the start of positive flow (into the patient) to the start of negative flow (out of the patient) Expiratory time is defined as the period from the start of negative flow to the start of positive flow Total cycle time is the sum of inspiratory and expiratory times and is equal to the inverse of breathing frequency The inspiratory-expiratory (I : E) ratio is defined as the ratio of inspiratory time to expiratory time Percent inspiratory time (also called the duty cycle) is defined as the ratio of inspiratory time to total cycle time Delivered Vt is the integral of flow with respect to time Initiating Breaths Trigger is the signal that starts inspiration and can be initiated by either the ventilator or the patient (see Fig 54.3) Time, pressure, flow, minimum minute ventilation, apnea interval, and electrical signals (e.g., diaphragmatic activity) can all be used as inspiratory triggers When inspiratory flow starts with a ventilator-generated signal, it is referred to as a ventilator- or time-triggered breath and is independent of a patient-initiated signal Similarly, when inspiration starts on a patient-generated signal (e.g., initiated by a change in flow or pressure), it is referred to as patient triggered and is independent of a ventilator-generated signal Flow or electrical triggers are typically the most sensitive and will allow for improved synchrony, particularly in infants or patients with weak diaphragmatic effort.10,11 Patterns of Gas Flow The pattern of gas flow delivered is determined by an adjustable resistance valve on the driving pressure within the ventilator Airway pressure and lung volume will increase until inspiratory flow is terminated Four distinct inspiratory flow patterns are recognized: (1) constant flow, (2) decelerating flow, (3) accelerating flow, and (4) sinusoidal or sine-wave flow (Fig 54.4) Constant flow will increase pressure and volume linearly, while decelerating flow will result in rapid pressure and volume rise at the beginning of a breath, which then slows toward the end of inspiration (see Fig 54.4) Decelerating flow is most commonly used, as it mimics natural breathing and is likely to meet patient flow demands However, different flow patterns may be beneficial for specific disease processes.6 A sine-wave or sinusoidal inspiratory flow is Constant flow Decelerating flow 629 created when the drive mechanism is a rotary wheel–driven piston Accelerating flow and sine-wave flow patterns are rarely used by ventilators in current practice During exhalation, expiratory flow curves depend on the elastic recoil of the lung in combination with airway resistance as well as the expiratory resistance in the system, including the PEEP valve Limit During assisted breathing, the ventilator will continue to deliver flow into the patient until a predetermined limit is reached The most commonly used limits are pressure and volume In volumelimited ventilation, the ventilator will deliver a set volume of gas and the peak inspiratory pressure required to deliver that volume will be variable In pressure-limited ventilation, the ventilator will provide flow until a set peak inspiratory pressure is reached, with variable tidal volumes A flow-limited breath will terminate flow into the patient at a predetermined peak inspiratory flow Just because the limit is reached and flow into the patient ceases, gas may not necessarily be allowed to leave the patient, as the lungs are held in an inflated state until the beginning of expiration (cycling) Cycling of Breaths Cycling refers to the termination of inspiration and start of expiration (see Fig 54.3) Inspiratory time, pressure, volume, flow, and electrical signals are all used as cycling signals Ventilator cycling refers to ending inspiration based on signals from the ventilator (typically, a provider-set inspiratory time) independent of signals based on patient factors Patient cycling occurs when a patient’s respiratory mechanics reaches a threshold value For example, in flow cycling, which is a form of patient cycling, the ventilator transitions into the expiratory phase once inspiratory flow decays to a threshold value dependent on the patient’s respiratory mechanics (commonly, 10%–25% of the peak expiratory flow for the breath).5 Continuous Positive Airway Pressure and Positive End-Expiratory Pressure In lung diseases characterized by nonuniform or heterogeneous parenchymal involvement, FRC may be reduced below closing volume, resulting in atelectasis These closed alveoli will have higher alveolar surface tension and therefore reduced compliance compared with open alveoli.3 This discrepancy in surface tension Sinusoidal flow Accelerating flow Flow Pressure/ volume Time • Fig 54.4 Flow and pressure scalars for different inspiratory flow patterns 630 S E C T I O N V Pediatric Critical Care: Pulmonary • Fig 54.5 Schematic representing the differential expansion of alveoli in subsegmental atelectasis The partially closed alveolus on the right has a higher surface tension, resulting in the majority of the delivered breath distributed into the more compliant alveolus on the left, causing hyperinflation in this alveolus will have two effects: (1) Vt will preferentially be delivered to open alveoli, potentially causing hyperinflation of normal lung segments and associated volutrauma (Fig 54.5); and (2) recruitment of these collapsed alveoli requires higher airway pressures than those needed to sustain inflation once the alveoli are open, and the reopening of these alveoli may cause shear injury to alveolar cells, a process known as atelectrauma Continuous positive airway pressure (CPAP) refers to the maintenance of positive airway pressure throughout the respiratory cycle with no positive-pressure breaths being delivered to the patient Positive end-expiratory pressure (PEEP) refers to the maintenance of positive airway pressure above atmospheric pressure between breaths The goals of CPAP and PEEP are to (1) increase end-expiratory lung volume above closing volume to prevent alveolar collapse, (2) maintain stability of alveolar segments, (3) improve oxygenation through increasing mean airway pressure and improving V/Q mismatch, and (4) reduce the work of breathing Ventilator Modes and Phase Variables All modern ventilators provide synchronized intermittent mandatory ventilation (SIMV) in that, when possible, the ventilator uses a patient trigger to match the patient’s desire for a breath to an actual delivered breath This synchronization improves patient comfort, reduces air hunger, and allows for optimal lung mechanics In standard ventilator taxonomy, a control mode means that the limit and inspiratory time are predetermined, while a support mode implies that only the limit may have been preselected.6 The set limit will then determine the taxonomy, that is, volume control describes controlled breaths with a volume limit and pressure support describes supported breaths with a pressure limit Control breaths may also be referred to as mandatory breaths, and support breaths may also be referred to as spontaneous breaths, in which the patient determines the timing of breath initiation and cycling A support breath may be assisted (i.e., patient triggered with an additional positive pressure to a set limit) or unassisted (i.e., without any additional ventilator assistance, as might be seen in breathing during CPAP) It is important to understand that these terms—for example, volume control (VC-SIMV), pressure control (PC-SIMV), pressure support (PSV)—are standardized based on general taxonomy and not refer to a specific manufacturer’s branded mode It should be noted that a mixed mode is commonly used for ventilating patients (e.g., volume control/pressure support) In a pure control mode (e.g., assist control), each breath will be a control breath with a predetermined inspiratory time In a mixed mode, the patient will receive a set number of control breaths (determined by the set ventilator rate) with additional patient-triggered breaths above that rate delivered as supported breaths In either of these modes, if the patient goes too long without triggering a breath, the ventilator will deliver a timetriggered control breath The delay until that time-triggered breath occurs is dependent on the set ventilator rate as well as the number of breaths delivered over a set period of time, as determined by the ventilator’s programmed algorithm If the patient does not trigger any breaths, the ventilator will deliver control breaths at the set ventilator rate In a support mode, only spontaneous, patient-triggered support breaths will receive mechanical support However, almost all ventilators are equipped with an apnea setting that will transition to a set rate if the patient is breathing below the alert backup rate Selection of Parameters for Mandatory Breaths Table 54.2 summarizes approximate initial settings for traditional ventilator modes The first parameter typically targeted is the Vt The actual Vt delivered to the patient is known as the effective tidal volume (Vteff), which can be approximated by the following formula: Vteff Vtdel Vtcircuit where Vtdel is Vt delivered by the ventilator and Vtcircuit is the volume of gas that is distributed to the ventilator circuit A desirable Vteff for most mechanically ventilated children is somewhere between and mL/kg, as measured at the endotracheal tube.12,13 Because dead space in the ventilator circuit plays a proportionally larger factor in smaller children, a discrepancy will exist between the targeted ventilator Vtdel and actual Vteff Vt may be directly set in a volume-limited mode or -pressure TABLE Starting Ventilator Settings for Traditional 54.2 Ventilator Modes Tidal volume (Vteff) 5–8 mL/kg ideal body weight 3–6 mL/kg ideal body weight in states of reduced lung compliance Plateau pressure (Pplat) #28 cm H2O #32 cm H2O in children with reduced chest wall compliance PEEP 5–8 cm H2O a Rate (IMV) Infants: 25–30 breaths/min Children 1–6 years: 20–25 breaths/min Children years: 12–20 breaths/min Inspiratory time Infant: 0.4–0.7 s Adolescent: 0.7–1.2 s Allow I:E ratio of at least 1:2 I:E, Inspiratory-expiratory ratio; IMV, intermittent mandatory ventilation CHAPTER 54 Mechanical Ventilation and Respiratory Care (difference between peak inspiratory pressure and PEEP) may be titrated to achieve that volume in a pressure-limited mode To avoid barotrauma, the end-inspiratory alveolar pressure (i.e., plateau pressure or pause pressure) should be targeted no higher than 28 cm H2O (or 32 cm H2O in patients with reduced chest wall compliance).12,13 The plateau pressure can be measured using an inspiratory hold maneuver With an inspiratory hold, both inspiratory and expiratory valves are closed, all flow is stopped, airway resistance is removed from the pressure equation, and the proximal airway pressure equilibrates with the alveolar pressure Large Vt and high peak pressures can each contribute to ventilatorassociated lung injury through volutrauma and barotrauma, respectively To avoid this injury, smaller Vt (3–6 mL/kg) may be targeted in patients with reduced lung compliance.12,13 PEEP is generally the next parameter selected This level will depend on the clinical circumstances The optimal PEEP is the level at which there is an acceptable balance between the desired clinical goals and undesired adverse effects Clinicians should target PEEP to optimal lung expansion and to maintain adequate Pao2 or Sao2 (arterial oxygen saturation) to meet clinical goals with a “nontoxic” inspired oxygen concentration.12,13 Arbitrary limits cannot be placed on the level of PEEP or mean airway pressure required to maintain adequate gas exchange In conjunction with PEEP, Fio2 is adjusted to maintain an adequate Sao2 and Pao2 High concentrations of oxygen can contribute to lung injury from development of free radicals and should be avoided The exact threshold of inspired oxygen that increases the risk of lung injury is not clear, but an Fio2 less than 0.5 is generally considered safe In patients with parenchymal lung disease with significant intrapulmonary shunting, the major determinant of oxygenation is lung volume, which is a function of the mean airway pressure With a shunt fraction of more than 15%, oxygenation may not be substantially improved by higher concentrations of oxygen; lung recruitment with PEEP will be more impactful Ventilator rate is selected based on the age and ventilatory requirements of the patient; subsequently, it may be adjusted Flow according to the Paco2 The inspiratory time is selected to provide an inspiratory-to-expiratory time (I:E) ratio of at least 1 : 2 in most patients Inspiratory time can be set as either a percentage of the total respiratory cycle or as a fixed time in seconds depending on the ventilator Inspiratory time must be selected to allow sufficient time for all lung segments to be inflated In heterogeneous lung disease with varying regional time constants, a short inspiratory time may not be sufficient to inflate all lung segments and may contribute to underventilation and underinflation Similarly, sufficient expiratory time must be provided for all lung segments to empty If inspiration starts before the lung has completely emptied, gas trapping and inadvertent intrinsic PEEP will result In infants with bronchiolitis and children with asthma, the expiratory time commonly must be lengthened to avoid gas trapping (Fig 54.6) This is accomplished most effectively by decreasing the respiratory rate rather than by excessively shortening the inspiratory time Dual-Control Modes Dual-control modes of ventilation adjust the targeted limit on a breath-to-breath basis based on feedback from the patient’s respiratory mechanics In pressure-regulated volume control (PRVC), adaptive pressure ventilation (APV), and variable pressure control, delivered Vt is used as a feedback control for continuously adjusting the pressure limit.14 In these control ventilation modes, the provider selects a target Vt, and the ventilator calculates the necessary pressure based on system compliance on a breath-tobreath basis The ventilator will alarm if the Vt and maximum pressure limit settings are incompatible These modes may be of particular benefit in patients with rapidly changing compliance Volume support and variable pressure support are terms used to describe this same strategy with a support mode of ventilation Modes such as average volume assured pressure support (AVAPS) and pressure augmentation allow the ventilator to switch from a pressure-controlled breath to a volume-controlled breath within the breath These modes can be considered a safety net that always supplies a minimum Vt.15 Once the breath is Expiratory flow has not reached zero prior to next breath Inadequate expiratory time Pressure/ volume Set PEEP 631 Intrinsic PEEP Time • Fig 54.6 Scalars demonstrating gas trapping in a patient with obstructive small airway disease ... breathing Energy applied to the device is altered, transmitted, and directed in a predetermined manner to either augment a patient’s breathing efforts or replace the patient’s work of breathing... characteristics and functional design of ventilators is beyond the scope of this chapter; the reader is referred to several excellent reviews on this subject.5–9 Pressure, volume, flow, and timing are the primary... growth In this manner, clinicians must use their clinical judgment to augment objective definitions of respiratory failure Chapter 44 provides a deeper discussion of the many causes of breathing