Different ventilator vendors use their own proprietary ventilator mode naming schemes leading to confusion amongst clinicians. Using a standardized approach to naming ventilator modes can, along with a better understanding of control variables, aid neonatal clinicians in assessing patient-ventilator interactions and troubleshoot issues at the bedside.After completing this article, readers should be able to: Recognize the differences between true volume control, set-point pressure control, and adaptive targeted pressure control in neonatal ventilation.Describe how changes in lung mechanics in neonates can affect ventilation in volume control, set-point pressure control, and adaptive targeted pressure control.The history of mechanical ventilation dates to at least the 1500s, when Andreas Vesalius intubated an animal’s trachea to provide positive pressure ventilation.1 The advent of more modern solutions, however, can be traced to Björn Jonson’s work with a fully electronically controlled flow valve.2 This helped transition ventilators from a mechanically compressed bellow or balloon to the ventilators we now recognize in our intensive care units. This revolutionized the way clinicians maintain minute ventilation in patients with critical illness. Since then, ventilators have continued to advance, which has exponentially led to the expansion of available trade-name modes. This has complicated the ability of clinicians to identify and compare these modes. Standard nomenclature agnostic to vendors has been developed to better facilitate learning ventilator behavior and is gaining adoption even in the pediatric realm.3,4An in-depth understanding of how ventilators and their various modes work will allow for better evaluation of patient-ventilator interactions and facilitate bedside troubleshooting. In this review, we focus on the control variables that are used in neonatal ventilation. Different ventilator waveforms created in these different control conditions are shown in visuals and in Video 1. As a representation of this content, we have 3 questions for the reader to consider: Of the following, the statement that best describes how set-point pressure ventilation works is: The ventilator pressurizes the circuit to the set pressure control/peak inflating pressure. Delivered tidal volume depends on lung mechanics and concomitant patient effort.The ventilator adjusts the pressure control/peak inflating pressure using a feedback loop aiming to achieve the tidal volume target.The ventilator injects a known tidal volume into the circuit. The pressure within the circuit depends on lung mechanics and concomitant patient effort.The ventilator injects a known tidal volume into the endotracheal tube. The pressure within the circuit depends on lung mechanics and concomitant patient effort.Of the following, the statement that best describes adaptive targeting (volume guarantee, adaptive pressure ventilation, pressure-regulated volume control) in pressure control ventilation is: The ventilator pressurizes the circuit to the set pressure control/peak inflating pressure. Delivered tidal volume depends on lung mechanics and concomitant patient effort.The ventilator adjusts the pressure control/peak inflating pressure using a feedback loop aiming to achieve the tidal volume target.The ventilator injects a known tidal volume into the circuit. The pressure within the circuit depends on lung mechanics and concomitant patient effort.The ventilator injects a known tidal volume into the endotracheal tube. The pressure within the circuit depends on lung mechanics and concomitant patient effort.All of the following statements provide a rationale for why adaptive targeting of pressure control (volume guarantee, adaptive pressure ventilation, pressure-regulated volume control) is used over volume control in the neonatal population EXCEPT: The flow waveforms employed in volume control are not tolerated by newborns.The use of uncuffed endotracheal tubes and frequent leaks complicates the use of volume control in the neonatal population.Gas compressibility in the circuit leads to a significant portion getting compressed within the circuit rather than being delivered to the patient.Control of volume-control occurs on the ventilator inspiratory side rather than at the Wye/Y of the ventilator circuit.The Hagen-Poiseuille equation describes the constant laminar flow of gas through a cylinder. In its simplest form, the equation can be described as: ΔP=V˙×RΔP represents the pressure drop across the cylinder, V˙ denotes flow, and R indicates the resistance of the system and reflects what is known as a resistive load.A static relationship between tidal volume and pressure within a gaseous chamber with a known amount of gas and no flow can be described by: P=VtCrsIn this equation, P is the pressure within the system, Vt represents the volume, and Crs denotes the compliance of the chamber. Since compliance and elastance (E) are inversely related, this equation can be rearranged to the one following and reflects what is known as an elastic load: P=Vt×EThese 2 equations can be combined into a singular equation: Pressure=Resistive Load+Elastic LoadThis can be expanded into the equation of gas motion: (Pvent−PEEP)+Pmusc=(V˙×Rrs)+(Vt×Ers)The equation of motion describes the relationship between pressure, flow, and tidal volume as air is entering or exiting the respiratory system: Pvent = the pressure within the circuitPEEP = set positive end-expiratory pressurePmusc = patient effort at that point in timeV˙ = flowRrs = the resistance of the respiratory systemVt = the tidal volumeErs = the elastance of the respiratory systemIn volume control (VC) ventilation, the ventilator controls the flow of air into the circuit over a period of time leading to the delivery of a set volume of gas. Alternatively, in pressure control (PC) ventilation, the ventilator pressurizes the circuit to a set pressure.Injecting a known flow over a period of time leads to the delivery of a known amount of gas into a ventilator circuit. This requires intricate adjustments of flow to the circuit through the ventilator’s electronically controlled flow valve within the inspiratory side of the device. As shown in Video 1, flow characteristics can be varied as follows: a constant flow over the set inspiratory time (I-time)a descending ramp or an ascending rampa sinusoidal patterna trapezoidal patternIn volume ventilation, the pressure built up within the circuit is dependent on lung mechanics in combination with patient effort and is not controlled by the ventilator.Most neonatal ventilators disable access to true VC modes due to the relatively small tidal volumes involved the compressibility of gas within the circuits; the higher circuit compliance to patient compliance ratio; and the presence of leaks leading to the loss of most of the tidal volume within the circuit. This is described by the following equation: Vtpatient=Vtinjected into circuit×11+CtotalCpatientVt denotes tidal volume and C represents compliance. Typical conventional ventilator infant circuits have compliances in the order of 1 mL/cm H2O, whereas a premature neonate’s lung compliance may be lower than 0.5 mL/cm H2O.5An alternative to VC is PC, whereby the ventilator’s built-in pressure sensors adjust flow to pressurize the patient circuit to a known value for a set amount of time. This employs a feedback loop to adjust the flow and expiratory resistance to maintain the ventilator circuit at a set pressure over the I-time set in mandatory breaths. The shape of the pressure tracing can be adjusted using a setting referred to as an inspiratory rise time (IRT). A short IRT leads to a squarer-shaped pressure-time tracing with peak inspiratory flow rate occurring early in the breath, whereas a longer IRT leads to a more sine wave–shaped pressure-time tracing with a delayed and reduced peak inspiratory flow rate (PIFR). The resultant flow and tidal volume end up dependent on pulmonary mechanics and patient effort (Video 1).The characteristic exponential decay flow pattern generated in PC has allowed the use of PC in spontaneous breaths in the form of pressure support with a variable I-time. To determine a spontaneous breath’s I-time, a ventilator first registers the PIFR for that breath and monitors for its decay. Once the flow drops to a set percentage of PIFR (referred to as expiratory trigger sensitivity, also known as flow-cycling), the breath will discontinue or cycle off, allowing for exhalation.In set-point PC, the amount of work done by the ventilator does not change as the ventilator circuit pressure does not change. Figure 1 provides an example of an algorithm for providing adaptive targeting of PC. If pulmonary mechanics or patient effort changes, the tidal volume being delivered can vary significantly, leading to either atelectasis, overdistension, or great swings in carbon dioxide levels. To be able to maintain minute ventilation, a ventilator can be allowed to vary the circuit peak inflating pressure based on a running average of the previous breaths. This leads to a more stable average delivery of tidal volume (see Figures 2A and B) along with more consistent gas exchange and can provide other associated benefits such as reduced incidence of bronchopulmonary dysplasia and of death.6–8Adaptive targeting of PC has been referred to under different brand names including pressure-regulated VC (Getinge Servo lines of ventilators), volume guarantee (Draeger/Dräger, CareFusion AVEA line of ventilators), adaptive pressure ventilation (Hamilton line of ventilators), and VC plus (Puritan Bennett line of ventilators), to name a few.In adaptive targeting of PC, the targeted tidal volume should be set depending on the disease state. Neonates with disorders with high elastic loads, such as respiratory distress syndrome, benefit from lower tidal volumes in the “lung-protective” range. In contrast, infants with disorders with a more obstructive load, such as obstructive phenotypes of bronchopulmonary dysplasia, benefit from larger tidal volumes.9A ventilator can be set up to control the integral of flow time in VC or can be set to control the pressure within the circuit in PC. The circuit pressures in VC are dependent on pulmonary mechanics and effort; this facilitates work-shifting from the ventilator to the patient as their disease state improves or their effort increases. It also allows the ventilator to provide more assistance if pulmonary mechanics worsen or if the patient tires out, providing a layer of safety. Due to relatively low pulmonary compliance of infants, especially newborns and premature infants, in comparison with circuit compliance, true VC is often locked out of neonatal ventilator modes.In PC, the flow and tidal volume delivered to the patient are dependent on pulmonary mechanics and infant effort. If the pressure is set, the amount of work the ventilator delivers is held constant. If pulmonary mechanics improve or if the patient effort increases, there is an increase in tidal volume delivery that can predispose patients to volutrauma. If there is worsening of pulmonary mechanics, there will be a decline in flow and tidal volume and the patient may be under-supported, leading to worsening hypercarbia. The use of adaptive targeting of PC allows for more stable tidal volume and minute ventilation delivery, with the ventilator shifting work to and from the patient in response to changing pulmonary mechanics and/or patient effort, providing more stable minute ventilation and decreasing swings in carbon dioxide. The feasibility of adaptive targeting has been well studied in neonatology with some of the earlier reports occurring at the beginning of the current century.6,8,10–13An important issue to recognize is that there are different types of flow sensors employed within ventilators and “proximally” at the patient’s bedside. Differential pressure sensors detect pressure drops across calibrated orifices. Hot-wire anemometers rely on a wire placed in the path of gas flow, which is heated with a known electric current. The rate of cooling is proportional to the flow of gas across it. Ultrasonic sensors use sound waves beyond the hearing threshold along with the doppler effect to measure gas flow in some ventilators.14For internal flow sensors located before the humidifier, the resultant measurements can be used as is, corrected to dry room air (ATPD: Atmospheric Temperature and Pressure Dry) or corrected to body temperature and humidity (BTPS: Body Temperature and Pressure Saturated).15 Some ventilators also feature automatic correction of compressible gas volume, which is known as circuit compliance compensation. In Figure 2, the difference noted in tidal volumes between VC and adaptive targeting of PC is likely due to the lack of circuit compliance compensation and the intrinsic differences in location and type of flow sensor employed between each mode applied.Given that tidal volume is calculated rather than measured directly and is dependent on algorithmic correction for temperature and humidity of flow, variability in these calculations exists between different devices. This variability was demonstrated previously by Krieger et al in 2017, with some neonatal ventilators undershooting the target tidal volume by 14%, whereas others overshot by as much as 26%.16 Waveforms generated by neonatal ventilators can also vary significantly between devices, as demonstrated by Sharma et al in 2007,17 with some ventilators pushing a more square-shaped tracing, whereas others employ more sine-shaped traces; this was associated with variations in peak inflation pressures and mean airway pressures.Clinicians in the neonatal intensive care unit require a good understanding of each type of ventilator mode, the ventilator performance characteristics, and limitations of the specific ventilator used in the unit to provide effective ventilator care. Neonatal clinicians also need to be able to interpret ventilator waveforms to improve bedside troubleshooting leading to better patient respiratory support and patient-ventilator interactions.Answers: 1. A; 2. B; 3. A.American Board of Pediatrics Neonatal-Perinatal Content SpecificationDemonstrate an understanding of the 2 control variables in mechanical ventilation: volume control and pressure control.Identify the limitations of applying volume control in the neonatal population.Recognize how adaptive targeting of pressure control can provide the benefits of volume control while being a pressure-controlled variable.
Conlon et al. (Sun,) studied this question.