Healthcare institutions have often renewed their anesthesia machines and monitors, at least in the course of the last decade and the general anesthetist is sometimes faced with the challenge of using these machines to administer anesthesia to a pediatric patient. In addition, there are different anesthetic circuits in terms of size and length, according to each patient. This article for reflection considers the case of anesthetists that treat children only sporadically, with a view to reducing the level of stress and properly fine-tune the anesthesia machines to the child, providing safety and quality tools for mechanical ventilation that increase the probability of success. We shall then define the term "Modern Anesthesia Machines," their main benefits, the basic mechan ical ventilation concepts applicable to anesthetized children, describing the primary ventilation strategies and, lastly, how to make the best use of graphic monitoring provided by the new anesthesia machines. In the end, we shall be able to answer the question: How do we make the best use of the new anesthesia machines and place their technology at the service of children?

Arbitrarily and from a convenience perspective, this is the name we have given to those anesthesia machines that bear 3 characteristics: complete a previous self-check-up, exhibit ventilation modes other than the typical pressure or volume-controlled ventilation, and have monitoring charts of ventilation mechanics.

Anesthesia machines must be calibrated before their use because of 2 reasons: the first is testing the correct functioning of the machine's sensors and internal devices; the second one is evaluating the anesthetic circuit to be used, measuring compliance, conducting leak tests, and estimating the internal volume. Both processes are intended to accurately ensure a tidal volume (Vt) that can be low enough (up to 5 mL Vt in some brands and up to 20 mL in most brands) and that the percentage of inhaled anesthetic agent delivered is accurate and with the least possible variability, with changes in fresh gas flow (FGF), hence allowing for small volumes, and even low metabolic levels (below lL/minute). These 2 characteristics meant that for many years, the preferred ventilation mode in pediatrics was pressure-controlled, which additionally compensated for small circuit leaks. Furthermore, the variability of the anesthetic agent with FGF changes forced us for many years to avoid the use of metabolic flows-not even low-in the pediatric population. Both of these scenarios are now possible and reliable: low Vt and low FGF.^1,2

The term compliance refers to the change in volume due to pressure changes. In anesthesia circuits, compliance is determined by the ability of the circuit wall used to be compliant, and by the compression of the air molecules inside the circuit when applying pressure, as these molecules are initially compressed and then generate pressure on the walls. Thus, a small, short, and stiffcircuit will be more compliant that a wide, soft, and long circuit. The goal is to have a circuit compliance as close as possible to the pulmonary compliance of the patient, as some machines also make a "dynamic" adjustment during each respiratory cycle, identifying abrupt changes in compliance and correcting them. A large differential between the circuit and the patient's compliance hinders or "fools" the machine sensors to perform this function. With this information, the machine compensates for the volume of air "lost" in the circuit's compliance to make sure that the patient receives the accurate programed volume. ^1,2

Machines usually tolerate and make up for any minor leaks of less than 250mL/minute. During calibration, it is important to establish, together with the manufacturer, if capnography should be installed, as it suctions around 150mL of air per minute, which in some models return back to the respiratory circuit, but in other cases are expelled and hence must be compensated for. This fact is critical in patients less than 20kg, since this non-compensated leak may represent a considerable percent age of the Vt.

This volume refers to the amount of air handled by the anesthesia machine up to the "Y" circuit; in other words, the circuit volume itself, the volume of the canister, the reservoir bag, the internal circuits, and the mechanical ventilator. This volume ranges from 4 to 8 L and certainly varies according to the reservoir bag used (e.g., 3-0.5 L), to the ventilator system (piston or coil, adult, or pediatric) and to the anesthesia circuit itself. The relevance of this volume is that the machine must saturate that volume with anesthetic agent for a reliable and accurate delivery to the patient. The saturation time of this internal volume is called the "K" time constant. The higher the internal volume, the longer it takes for any changes in the dial of the vaporizer to be actually reflected in the patient's airway. Here lies the importance of having the machine estimate this volume and set the sensors to deliver the anesthetic agent during check-up. Moreover, by enhanc ing the precision and reliability of these 2 aspects, the mechanical ventilation setting will be based on the right parameters.

The most important differences and physiological aspects to be considered when ventilating a child are:

Compliance: The child's pulmonary parenchyma has a low compliance that increases with age. The chest wall is quite compliant, mainly as a result of the horizontal cartilaginous nature of the ribs and the meager muscle mass of the child. This results in the need for adequate pressure to open up the alveoli (low pulmonary compliance) with a risk of volutrauma and barotrauma due to the subsequent poor chest wall resistance as a result of its high compliance. ^3,4

Airway resistance: Newborn babies have a very high airway resistance that decreases until it reaches the adult values at around 8 years of age. Inserting a small diameter endotracheal tube further increases this resistance. ⁴

Functional residual capacity: It is the combination of the reserve expiratory volume and the residual volume; in children, it is proportionally less than in the adult and than the total pulmonary capacity. The functional residual capacity is the only oxygen reserve in case of apnea, so the child is overtly vulnerable because of this diminished reserve (Fig. 1). ⁴

Figure 1 Source: Authors

Closing volume: It is the minimum volume that should be left inside the alveoli to prevent their collapse; in the child, this volume is proportionally larger than in the adult. In other words, there must be a larger amount of air left in the lungs to prevent collapse. In adults, this volume approaches the residual volume, whereas in children, it approaches the Vt (Fig. 2), with a marked tendency to develop atelectasia. ⁴

Figure 2 Source: Authors.

Mechanical ventilation: It is hard to believe that although children are ventilated under anesthesia every day, the available literature is poor, with little evidence. Hence, most of the guidelines to ventilate children under anesthesia are derived from adults or from critical children ventilated in the Intensive Care Unit (ICU). ^1-3,5-7 Certainly, these are patients with very different characteristics to those undergoing anesthesia that usually present with healthy lungs. ⁴ Hence, the primary parameters derive from "pulmonary protection ventilation" (PPV) based on 3 aspects applied to any of the ventilation approaches selected:

Low tidal volume: A fair number of trials have shown that both a high Vt (above 10mL/kg), and an extremely low Vt (below mL/kg) results in greater injury; therefore, the recommendation is 6 to 10 mL/kg^1-4,8-11.

One of the key features of the modern anesthesia machines is the broad range of information provided about the ventilation mechanics cycle after cycle; in our opinion, this is the least used characteristic by anesthetists. We will particularly discuss 4 graphs:

(a) Spirometry: There are 3 types of loop type curves (starting and ending at the same point) according to what is being monitored.

Flow-volume loop and flow-pressure loop: These are possibly the least used modalities by anesthetists and the shape of the waves gives us an idea of the endurance of the small airways, or the airflow restrictions, as well as of the high or low airway obstruction, all with a single glance. We shall dwell on the third type of loop that could provide more useful information for the patient's ventilation. ^2,4

Pressure-volume loop: This loop is more widely used by anesthetists and allows for charting pressure on the X-axis and volume on the Y-axis. At a glance, it is possible to identify the pulmonary compliance cycle after cycle and any changes therein. When the curve flattens, that is an indication of a decrease in pulmonary compliance (Fig. 4). The machines allow for storing a "reference" loop from the beginning and to compare against this loop whatever happens during surgery. Furthermore, the lower inflexion point may be established, which is the point at which the slope of the curve changes from flat to ascending during inspiration and is used as a means to determine the ideal PEEP. We may also determine at the end of this curve whether a "duckbill" to the right occurs, indicating alveolar overdistension which should be corrected immediately by adjusting the Vt and/or the PIP parameters, according to the ventila tion mode. The descending expiratory curve follows a different trajectory than the ascending curve; this phenomenon is called hysteresis and is due to different forces acting during inspiration and expiration, because the latter is dominated by a passive process of pulmonary elastic recoil. The loop area reflects the pulmonary volume being used. Based on all of these characteristics, the pressure-volume loop is usually preferred for ventilation monitoring, as most machines only show 1 of the 3 loops described on the screen. ^2,4

Figure 4 Source: Authors.

(b) Pressure-time curve: This curve depicts pressure on the "Y" axis and time on the "X" axis. A pressure mode results in a square wave with the highest plateau representing the maximum peak pressure and the lower level representing the PEEP value. The most relevant information of lung mechanics may be obtained from the volume-controlled mode using a certain amount of inspiratory pause to obtain a shark-fin type graph indicating 3 points and 2 intervals each with specific significance. The maximum pressure point corresponds to PIP, which combines pulmonary resistance and the airway resistance basically deter mined by the endotracheal tube. Then there is a drop in the curve toward an extended plateau, based on how we program the inspiratory pause. This plateau corresponds to P_plat, which is the balance pressure of all the alveoli and hence is a reflection of intra-pulmonary pressure. The third point corresponds to the lower level where the curve drops and coincides with the PEEP level we programed or which the patient is producing and must be adjusted according to each individual patient needs (Fig. 5). There are 2 intervals in between these 3 points. Between PIP and P_plat the airway resistance is determined which in this case is due to the endotracheal tube. This value is around 5 cmH₂O and should not exceed 8 to 10cmH₂O. Beyond this level, it is an indication of a high endotracheal tube resistance issue, for instance, a tube bend, partial obstruction form secretions, or a very small tube for the size of the child, inter alia. A high PIP with a normal interval of resistance involves a high P_plat and this is the result of changes in pulmonary compliance. The second interval is the so-called "pressure drive" which presents between PEEP and P_plat. This should not exceed 14cmH₂O and ideally should be maintained at a value below 10cmH₂O. A big difference involves the need for a large pressure and volume to open and keep the alveoli opened, forcing us to adjust the ventilation parameters, for instance, raising the PEEP level. Both the resistive pressure and the pressure drive above normal have been associated with adverse outcomes in critically ill patients in the ICU, particularly a longer hospital stay and mortality. ⁵ There are no studies relating these findings with the anesthetized child, usually with healthy lungs. However, due to the lack of better evidence, we suggest taking these limits into account for ventilating anesthetized children. ⁴

Figure 5 Source: Authors.

(c) Flow-time curve: This curve represents the flow in liters per minute on the "Y" axis versus time on the "X" axis. If we are dealing with VCV, the result will be a square wave, because the flow will enter and remains constant throughout the inspiratory. In the PCV mode, the flow will reach a maximum peak when it reaches the programed pressure, and from there on, there will be a drop graphically represented as decelerator in the rest of the inspiratory phase until the end of the phase. The expiratory phase is equally traced in both modes. With this type of curve, a single observation is enough to determine the inspiration:expiration ratio (I:E) and if the times are sufficient, for instance, if an inspiratory cycle begins before the expiratory curve reaches the basal level, that would be indicative of a very tight I:E ratio that may result in reinhalation and CO₂ retention. Moreover, this curve may identify inspiratory effort by the patient depicted as wave notches and determine whether these are followed by ventilation support so as to help to adjust the patient-ventilator synchrony. A strategy for improving oxygenation is to shorten the I:E ratio, even inverting it, and a strategy for CO₂ removal is to increase the I:E ratio even beyond 1:3. All of these changes can be confirmed, followed, evaluated, and adjusted through observation and analysis of this curve. ⁴

(d) Capnography: Finally, the graphical representation of CO₂ detection throughout the expiratory cycle is the origin of capnogaphy. Traditionally, medical doctors believe that these graph is only useful to see whether the patient is intubated or not, but these rationale dismisses several other fundamental uses. Capnography in terms of its value and shape helps to assess the ventilation mechanics and pulmonary function, ^1,3 delivering pathognomonic curves, for instance, for bronchospasm (overtly ascending plateau). Furthermore, it should be considered as an indirect measurement but proportional to cardiac output, in as much as CO₂ is determined on the basis of pulmonary blood flow, which is also dependent on cardiac output. Hence, if there are no ventilator changes and CO₂ starts to drop, this maybe the start of a hypotension period of time. ⁴ The applications and complete interpretation of capnography is beyond the scope of this article, but there is a need to stress its forgotten value which is extremely important in pediatric ventilation.

Capnography helps to determine the presence of uncorrected dead space in our ventilation strategy as follows: once the ventilation strategy has been established and a level of capnography stabilized, advance to manual ventilation administering some cycles of hyperventilation (5 cycles are enough); in other words, manually increase PIP and Vt. Under the usual conditions, such a short-lasting hyperventilation should decrease the capnogaphy level or at least maintain it. If the value increases, this is a reflection of a dead space that prevented the Vt to effectively reach the child and with this hyperventilation the spaces are "swept," obtaining the CO₂ that was not previously discharged (Fig. 6). In this case, the Vt must be adjusted. Particularly in small children (less than 10 kg), the programed Vt may be insufficient when adding elements to the circuit after the "Y"-where the dead space begins-without compensating. Remember that the definition for dead space is the segment of the circuit where the flow becomes bidirectional and may be mixed; that means, from the "Y" but not before. For instance, the length of the inspiratory and/or expira tory branches do not affect the dead space (it may affect the circuit compliance, the compression of the air molecules, the internal volume, etc., but not the dead space). Devices such as extensions, filters, elbow joints, capnography sensors, humidifiers, and so on make up a volume that can be as high as 25 mL^1,2. For instance, in a premature 2-kg baby, programing a Vt of 10 mL/kg will result in 20 mL of Vt, which means that the Vt cannot even move from the filter and the child would be absolutely hypoventilated. Under this cir cumstances, the above maneuver is extremely helpful.

Figure 6 Source: Authors.

Based on the above discussion, could we learn how to make the best use of the new anesthesia machines and use that technology for the benefit of children? This can be summarized into the following practical considerations:

Pre-calibrate your machine with the appropriate circuit to be used: This maneuver enables the evaluation of sensors, determines the internal work volume of the machine, establishes and automatically compensates for compliance and minor circuit leaks. In this manner, we will ensure an accurate though small Vt and a reliable delivery of anesthetic gases.

Protection of human and animal subjects. The authors declare that no experiments were performed on humans or animals for this study.

Confidentiality of data. The authors declare that they have followed the protocols of their work center on the publication of patient data.

Right to privacy and informed consent. The authors declare that no patient data appear in this article.