Mechanical Ventilation

 

Newborn


1 year


7 years


Adult


Compliance (ml/cm H2O)


5


15


50


60–100


Resistance (cm H2O/l/s)


40


15


4


2



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Fig. 67.1

Ventilation per minute and physiological dead space



Lung compliance (C) is defined as the change of volume in relation to the change of airway pressure (Fig. 67.2), meaning ∆V/∆P, and it is determined by the elastic forces inside the lung in conjunction with the surface tension generated by the air–tissue interface inside the alveolus. The described curve is a sigmoid curve with a decrease of the slopes in the zones of low and high lung volume. Compliance can then be divided into dynamic compliance and static compliance. Static compliance provides an estimate of the total compliance of the lung system; it is calculated dividing the tidal volume by the difference between plateau pressure or static inflation pressure (Pplat) and PEEP. Dynamic compliance, on the other hand, includes and reflects the contribution of the airway resistance to the airflow, and it is calculated dividing the tidal volume by the difference between maximal inspiratory pressure (MIP) and PEEP.

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Fig. 67.2

Lung compliance and hysteresis curve


The airway resistance is the pressure difference between the mouth and the alveolus needed to move air through the airway in a constant flow. It is determined by the flow rate, the length of the airway, the physical properties of the inhaled gas, and the radius of the airway, the latter being the most important variable.


The time constant (TC) corresponds to the measure of how fast an alveolar unit reaches a pressure equilibrium with the proximal airway, both in the filling as in the emptying phase. In operational terms, it is the product of C and R. In one CT, 63% of the equilibrium is reached: 85.5% in two, and 95% in 3-time constants (Fig. 67.3). Owing to this, according to age and the CT, inspiratory times that vary from 3 CT to a maximum of 5 CT are recommended, and it is important that the expiratory time has to have at least the same duration of inspiration. In a newborn, the CT can carry between 0.1 and 0.15 seconds, with an acceptable average inspiratory time (IT) of 3 CT. It must be noted that the CT in an older child is raised to 0.2 or 0.25, and thus the IT can reach 0.75.

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Fig. 67.3

Relationship between pressure equilibrium in the airway and the time constant


All ventilation methods require that a pressure gradient is established between the alveoli and the airway (or atmospheric pressure) to produce a gas movement. Thus, adequate pressure must be generated to open the collapsed airway, so that any method that does not reach that critical pressure point during inspiration will determine the production of atelectasis and hypoxemia. The maximum pressure generated during the inspiratory phase of the mechanical ventilation that allows the airflow to overcome the airway resistance is known as maximum inspiratory pressure (MIP). This is proportional to the resistance and the tidal volume or mobilized volume during inspiration, and it is inversely proportional to lung compliance.


If one occludes the expiratory gate just before the expiration and pauses, a static inflation pressure or plateau pressure (Pplat) will be obtained, which, in practice, is considered as approximating the pressure that is reached in the distal alveoli. On the other hand, apart from the pressure generated by inspiration, an adequate level of pressure has to be maintained during expiration, in order to not reach a critical point in which the airway closes, generating atelectasis and hypoxemia again. This continuous positive pressure of the airway that avoids collapsing at the end of expiration is known as PEEP (Fig. 67.4).

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Fig. 67.4

Pressure/volume relationship in volume-control mode. MIP Maximal inspiratory pressure, Pplat plateau pressure, PEEP positive end expiratory pressure, It Inspiratory time, Et Expiratory time


Basic Elements of the Cardiopulmonary Interactions


During mechanical ventilation, a series of interactions are caused between the blood flow that enters and leaves the heart and the moments in which the lung is in inspiration or expiration as a consequence of the intrathoracic pressure variation. This intrathoracic pressure varies with the ventilatory maneuvers, and it affects the pressure gradient between the blood that returns to the heart (venous return) and the blood that leaves the thorax (stroke volume of the left ventricle).


In positive pressure ventilation, the increase of lung volume produces an increase in intrathoracic pressure, which is transmitted to all structures inside the thorax, affecting atria filling and ventricular ejection. During inspiration of positive pressure ventilation, the increase of intrathoracic pressure will determine an increase of pressure in the right atrium and a decrease of the venal return due to the decrease of the pressure gradient between the big veins and the atrium, thus determining a decrease of the preload of the right ventricle.


On the other hand, the positive pressure ventilation determined an increase of alveolar pressure and, consequently, an increase of the pressure of the alveolar vessels, determining a subsequent increase of the lung vascular resistance, meaning an increase in the afterload of the right ventricle and in consequence, a decrease of the lung blood flow. Despite an initial increase of the left atrium filling, after two to three cycles, a diminishing of the venous return appears, and, as a consequence, the preload of the left ventricle is reduced.


Finally, the afterload off the left ventricle that is related to the resistance to the left ventricle exit flow depends of the transmural pressure that exists in it; in other words, in the difference between the internal pressure of the left ventricle and the intrathoracic pressure. During positive pressure ventilation, the increase of intrathoracic pressure decreases the transmural pressure of the left ventricle, which determines a decrease of the afterload of the left ventricle and, in turn, an increase of its stroke volume (Fig. 67.5).

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Fig. 67.5

Cardiopulmonary interaction in the left ventricle in positive pressure ventilation. PTVi left ventricular transmural pressure, PVi left ventricle peak pressure, PIT intrathoracic pressure, SV stroke volume


The understanding of cardiopulmonary interactions is essential when considering the mechanical ventilator as another therapeutic tool, for example, in heart failure, the use of invasive mechanical ventilation can be essential when diminishing the left ventricle afterload and improving cardiac output. The same considerations must be taken in patients with severe air trapping or a significant restrictive illness that could imply the use of mPaw that will deteriorate the output and worsen the prognosis of the patient.


Mechanical Ventilation Indications


The moment to start mechanical ventilation will depend of the desired clinical objective for the patient that needs connection. Before connecting the patient, the pediatrician must consider why the patient requires it: are they a patient with a severe lung disease? Is the lung disease obstructive, restrictive or mixed? Does the patient present neurological compromise? Does the patient have a serious ECT or signs of endocranial hypertension? Is the patient in septic shock or cardiogenic shock? Among other questions. All the previous questions allow defining which condition determines the indication of ventilating the patient invasively (Table 67.2).


Table 67.2

Indications to initiate mechanical ventilation






























Alveolar hypoventilation: PaCO2 > 60


Failure in arterial oxygenation (PaO2 < 70 con FiO2 ≥ 60)


Severe obstructive symptoms


Apnea or respiratory arrest


Neuromuscular condition


Decrease of metabolic consumption: shock


Cardiogenic shock


Severe ECT


Complicated polytrauma


Substitution of breathing work


Thoracic wall stabilization


Surgery, ICU procedures


The most common cause of mechanical ventilation corresponds to the maintenance of gas exchange in the patient with respiratory failure, because an adequate arterial oxygenation was not achieved (PaO2 < 70 with FiO2 > 60) or an adequate alveolar ventilation was not achieved (PaCO2 > 55–60 in the absence of chronic lung disease). Another indication of mechanical ventilation is in those situations that require a decrease or substitution of the breathing work, either because spontaneous breathing work is ineffective on its own, or because the respiratory system is incapable of performing its function due to a muscular or skeletal failure: scoliosis, trauma, thoracic cavity malformations, neuromuscular diseases, etc.


Decrease of the oxygen consumption (VO2) constitutes another general indication of mechanical ventilation, because in pathological circumstances, oxygen consumption by the respiratory musculature can reach up to 50% of the total consumption. Thus, mechanical ventilation allows an oxygen reserve to be used by other tissues, which can be crucial in certain pathologies, such as septic shock, cardiogenic shock, endocranial hypertension, among others.


Finally, other indications of mechanical ventilation are the stabilization of the thoracic cavity in cases of polytrauma, flail thorax, thoracic surgery, or allowing sedation, analgesia, or muscle relaxation during the postoperative care of complex surgery, or in invasive procedures in critical pediatric care.


Most Used Ventilatory Modes and Initial Parameters of Ventilation in Pediatric Procedures


The ventilation offered by the mechanical ventilator is determined by an air flow provided to the patient whose only objective is usually to offer a determined volume or pressure (Fig. 67.6). The end of the inspiratory phase or cycle is reached at the moment that the objective of a determined volume, pressure, flow or time is reached according to the programming of the ventilator. The most commonly used modes will be detailed next (Fig. 67.7).

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Nov 7, 2020 | Posted by in Uncategorized | Comments Off on Mechanical Ventilation

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