Overview of Assisted Ventilation

Effective mechanical ventilation of newborn infants is a relatively late phenomenon in the care of newborn infants, having evolved within the lifetime of many practitioners active today. The death in 1963 of the late preterm son of a president of the United States from respiratory failure is a stark reminder of how inadequate respiratory care was only 50 years ago. Today, few babies die as a result of primary respiratory failure; mortality is more often from complications of extreme prematurity and infection. However, while mechanical ventilation has greatly reduced mortality from pulmonary causes, morbidity, including bronchopulmonary dysplasia, remains high.

As discussed in Chapter 17 , avoidance of mechanical ventilation may be the best way of avoiding ventilator-induced lung injury. With increased use of antenatal steroids and improved delivery room stabilization approaches, most moderately preterm and many very preterm infants are able to be supported noninvasively. In contrast, a substantial proportion of extremely low gestational-age neonates (ELGANs) continue to require mechanical ventilation. Almost 90% of extremely low birth-weight infants (ELBW) cared for in the Neonatal Research Network centers in 2005 were treated with mechanical ventilation during the first day of life, and 95% of survivors were invasively ventilated at some point during their hospital stay. In the Surfactant, Positive Pressure, and Oxygenation Randomized Trial (SUPPORT), 83% of the ELBW infants initially assigned to noninvasive support received endotracheal intubation and mechanical ventilation at some point during their neonatal intensive care unit (NICU) stay. Thus invasive ventilation is largely reserved for the relatively small number of the most immature or very sick infants. Because fewer infants now receive mechanical ventilation, there is a decreased level of experience for trainees and practitioners. Infants who receive mechanical ventilation today tend to be smaller and more immature than those ventilated in an earlier era and may remain ventilator-dependent for extended periods, sometimes for reasons not related to their lung disease. The spectrum of lung disease that neonatologists treat has expanded into more chronic conditions that we are less accustomed to treating. Furthermore, today’s patients may be uniquely susceptible to lung injury because of the very early stages of lung development at which they are born.

All these issues make it important to optimize the way mechanical ventilation is managed so that preventable mortality and morbidity can be avoided. Some degree of lung injury is probably inevitable in mechanically ventilated ELGANs even with optimal respiratory support. However, the wide range of the incidence of bronchopulmonary dysplasia in the individual NICUs within the U.S. Neonatal Research and Vermont–Oxford Networks suggests that mechanical ventilation may be a potentially modifiable risk factor. Although the evidence to guide respiratory support strategies remains incomplete, the potentially best practices and the rationale for them will be outlined in this and subsequent chapters.

Unique Challenges in Mechanical Ventilation of Newborn Infants

Individuals involved in the care of critically ill newborn infants should be keenly aware that newborns are not simply small children, any more than children are simply small adults. Sophisticated microprocessor-based ventilators with advanced features enabling effective synchronized ventilation are now widely available. However, it is essential to recognize that better technology alone will not improve outcomes. Unless used with care and with optimal ventilation strategies that are appropriate for the specific condition being treated, these machines cannot materially influence outcomes. To optimally utilize the complex devices at our disposal, we need to be aware of the many unique aspects of a newborn infant’s respiratory physiology. These are reviewed in detail in Chapter 2 , but key aspects that directly affect the provision of invasive mechanical ventilation are summarized below.

Lung Mechanics

Small infants with poorly compliant lungs have very short time constants and normally have rapid respiratory rates with very short inspiratory times to match their lung mechanics. They have limited muscle strength and a very compliant chest wall so they struggle to develop adequate inspiratory flow or pressure. This situation imposes great technological challenges on device design, especially in terms of triggering ventilator inflations in synchrony with the onset of inspiratory effort, inflation termination, and tidal volume measurement. Suboptimal ventilator design for neonatal applications may lead to excessive trigger delay with asynchrony, failure to trigger or terminate inflation, and errors in tidal volume measurement or delivery. These technological challenges have largely been overcome in modern ventilators but remain a problem in some older devices still in use.

Uncuffed Endotracheal Tubes

Uncuffed endotracheal tubes (ETTs) have traditionally been used in newborn infants, because of concern about pressure necrosis of the tracheal mucosa. The small size of the tubes also makes inflatable cuffs difficult to incorporate without compromising lumen size. For this reason, some degree of gas leak around the ETT is present in most infants. Despite the lack of supporting evidence, some practitioners believe that it is important to have an audible leak around the tube to ensure the fit is not too tight. Unfortunately, substantial leak makes tidal volume estimation increasingly inaccurate, an issue that has become more relevant with increasing use of volume-targeted ventilation. Increasing leak around the ETT develops over time in infants who require prolonged ventilation, because the larynx and trachea progressively dilate from the cyclic stretch of many thousands of inflations per day. Leak is greater during inflation, because the pressure gradient driving the leak is greater and because the airways distend with the higher inflation pressure. Therefore, it is important to measure both inspiratory and expiratory tidal volume (V T ), with the latter more closely approximating the volume of gas that had entered the lungs. The leak varies from moment to moment because the ETT is inserted only a short distance beyond the larynx and thus the leak will change with any change in the infant’s head position and movement of the ETT up and down in the trachea. Because of these difficulties, a reconsideration of the prohibition of cuffed tubes has been proposed.

Measurement of Tidal Volume

The importance of very accurate V T measurement in any sort of volume-controlled/volume-targeted ventilation of extremely small infants is obvious, considering that infants weighing 400 to 1000 g require V T in the range of 2 to 5 mL. Unfortunately, most so-called universal ventilators designed primarily for adult patients, but capable of supporting the full range of ages, measure flow and calculate V T at the output of the flow control valve within the ventilator rather than at the input to the patient (i.e., the airway opening). This approach is convenient and avoids extra wires and the added instrumental dead space of a flow sensor. However, in neonates, this remote placement of flow measurement introduces a high degree of inaccuracy of V T data. When the V T is measured at the ventilator end of the circuit, the value does not account for compression of gas in the circuit, distention of the circuit, or leak around the ETT and is subject to inaccuracies related to approximate corrections for heat and humidification of the cold dry air from the control valve. The loss of delivered V T in the circuit is proportional to the compliance of the ventilator circuit and humidifier (and the compressibility of the volume of gas they contain), relative to the compliance of the patient’s lungs. In large patients with a cuffed ETT, the volume measured at the ventilator correlates reasonably well (using appropriate corrections) with the actual V T entering the lungs. In tiny infants, whose lungs are very small and noncompliant, the loss of volume to the circuit is proportionally much larger and not easily corrected, especially in the presence of significant ETT leak.

Principles of Ventilator Design, Function, and Nomenclature

A mechanical ventilator is simply a device designed to augment or replace a patient’s own respiratory effort and ensure adequate entry of fresh gas into the lungs to satisfy the body’s respiratory needs. There is a wide variety of ventilator designs, but they all offer most of the basic modes of ventilation. To understand how ventilators work, the clinician should focus on how the specific modes work and on understanding the sometimes complex interactions between an awake, breathing infant and the particular mode on the specific device. This is important, because with today’s confusing terminology, ventilation modes with identical names may function differently on different devices. Thus the question of interest is, “How does this specific mode work on this device and how can I best use this tool to support my patient?”

A mode of ventilation is defined as a predetermined pattern of interaction between a patient and a ventilator. Unfortunately, there is no standardization in the industry regarding either naming modes or explaining their operation. Because different manufacturers employ different nomenclature to describe often closely related modes of ventilation, communication among users of different devices has become increasingly difficult.

For example, one ventilator commonly used for infants (the CareFusion Avea) offers the clinician the choice of 44 different modes. To better understand ventilator function and improve communication among clinicians, we need a ventilator mode taxonomy, or classification system. This taxonomy is not as intimidating as it may sound. It is based on a small set of specifically defined terms (known as a standardized vocabulary) and is organized as a hierarchical structure, similar to the order, family, genus, and species outline used in biology.

To better appreciate the need for a systematic approach, consider that at last count we have identified 290 names of modes on 33 ventilators in the United States alone. Can you imagine having to use that many drugs by trade name only without any generic or chemical names and no classification system? That is the situation with ventilator modes today. Additionally, many ventilators used around the world today are designed to span the entire age range from preterm newborn to adult and have a variety of modes that have never been evaluated in newborn infants. The importance of having a classification system is to be able to identify, on different ventilators, which modes are the same and which are different. Then we can identify the technological capabilities of different ventilators for both purchasing decisions and clinical application. Indeed, having identified the modes themselves, we can then compare and contrast their specific features to determine which modes best meet the clinical goals of mechanical ventilation for a particular patient at a particular time. Whereas there are many indications for initiating mechanical ventilation, we must provide mechanical ventilation with three key goals in mind : (1) safety—optimize gas exchange with a minimum of hemodynamic adverse effects and minimize ventilator-associated lung and brain injury, (2) comfort—minimize asynchrony between the ventilator and spontaneous breathing, and (3) liberation—minimize the duration of ventilation and incidence of adverse events. Thus, three basic skills (mode classification, goal selection, and mode selection) make up a rational framework for mastering the art and science of mechanical ventilation.

The following paragraphs will focus on conventional mechanical ventilation. Classification of high-frequency ventilation modes and indications for high-frequency ventilation use are addressed in detail in Chapter 22 , Chapter 23 .

Ten Maxims for Understanding Modes of Conventional Ventilation

In this section some basic terms related to mechanical ventilation will be defined in general, with specific applications to neonatal ventilation highlighted along the way. To understand these terms in context, 10 basic technological concepts (maxims) that underlie all modes of ventilation will be described. These concepts are each fairly simple and intuitively obvious. But taken together, they result in a classification system applicable to any mode on any ventilator.

Defining a Ventilator Cycle

A ventilator cycle is defined as one cycle of positive flow (inflation) and negative flow (expiration) defined in terms of the flow–time curve ( Fig. 15-1 ). As pointed out in an editorial, ventilators do not breathe, and so they do not deliver breaths, they deliver inflations. Only living people and animals breathe. We will therefore use terminology that avoids confusion between patient breaths and ventilator inflations.

FIG 15-1

A ventilator cycle is defined in terms of the flow–time curve. Important timing parameters related to ventilator settings are labeled.

(Modified from Mandu Press Ltd., with permission.)

Defining the Assisted Breath

A breath is assisted if the ventilator inflation provides some or all of the work of breathing. Graphically, this corresponds to airway pressure increasing above baseline during inspiration.

Assistance with Volume or Pressure Control

A ventilator assists breathing using either “pressure control” or “volume control” based on the equation of motion for the respiratory system:

<SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='P(t)=EV(t)+RV.(t)’>P(t)=EV(t)+RV.(t)P(t)=EV(t)+RV.(t)
P ( t ) = EV ( t ) + R V . ( t )

This equation relates pressure ( P ), V T ( V ), and flow in the ETT <SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='(V)˙’>(V)˙(V)˙
( V ) ˙
as continuous functions of time ( t ) with the parameters of elastance ( E ) and resistance ( R ). If any one of the functions ( P , V , or <SPAN role=presentation tabIndex=0 id=MathJax-Element-3-Frame class=MathJax style="POSITION: relative" data-mathml='V.’>V.V.
V .
) are predetermined, the other two may be derived.

The term control variable refers to the function that is controlled (predetermined or preset) during a ventilator cycle. This form of the equation assumes that the patient makes no inspiratory effort and that expiration is complete (no auto-positive end-expiratory pressure [PEEP]) at the end of each cycle.

Volume control (VC) means that both V T and ETT flow (variables on the right-hand side of the equation) are preset. In journal articles on neonatal and pediatric mechanical ventilation, you will often see the following terms used interchangeably to mean VC: volume targeted, volume limited, and volume preset. Pressure control (PC) means that inflation pressure (the variable on the left-hand side of the equation) is preset. In practice, this means one of two things: (1) the peak inflation pressure is preset (i.e., airway pressure rises to some target value and remains there until inflation time is complete; this is the traditional approach in neonates) or (2) inflation pressure is controlled by the ventilator so that it is proportional to the patient’s inspiratory effort; this is a newer method represented by the modes called proportional assist ventilation and neurally adjusted ventilatory assist. In journal articles on neonatal and pediatric mechanical ventilation, you will often see the following terms used interchangeably to mean PC: pressure controlled, pressure limited, and pressure preset.

Time control (TC) is a general category of ventilator modes for which flow, volume, and pressure are all dependent on respiratory system mechanics. As no parameters of the pressure, volume, or flow waveforms are preset, the only control of the cycle is the timing. Examples of this category are high-frequency oscillatory ventilation (CareFusion 3100 ventilator) and volumetric diffusive respiration (Percussionaire ventilator). Characteristic waveforms for VC and PC are shown in Figure 15-2 .

FIG 15-2

Characteristic waveforms for pressure control and volume control. Note that mean airway pressure <SPAN role=presentation tabIndex=0 id=MathJax-Element-4-Frame class=MathJax style="POSITION: relative" data-mathml='(P¯aw)’>(Paw)(P¯aw)
( P ¯ aw )
is less for volume control than for pressure control given the same tidal volume and inspiratory time.

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Jan 30, 2019 | Posted by in PEDIATRICS | Comments Off on Overview of Assisted Ventilation
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