This chapter is intended to get the reader familiar with basic aspects of the ventilator as a machine and its functioning. We feel this has important bearing in the management issues of a critically-ill child requiring mechanical ventilation.
VENTILATOR
A ventilator is an automatic mechanical device designed to move gas into and out of the lungs. The act of moving the air into and out of the lungs is called breathing, or more formally, ventilation.
Simply, compressed air and oxygen from the wall is introduced into a ventilator with a blender, which can deliver a set FiO2. This air oxygen mixture is then humidified and warmed in a humidifier and delivered to the infant by the ventilator via the breathing circuit.
The peak inspiratory pressure (PIP) or tidal volume (Vt), positive end expiratory pressure (PEEP), inspiratory time and respiratory rate are set on the ventilator.
The closing of the exhalation valve initiates a positive pressure mechanical breath. At the end of the preset inspiratory time, the exhalation valve is opened, permitting the infant to exhale. If this end is partly occluded during expiration, a PEEP is generated in the circuit proximal to the occlusion (or CPAP if the infant is breathing spontaneously). Expiration is passive and gas continues to flow delivering the set PEEP.
Parts of a Ventilator
- Compressor: This is required to provide a source of compressed air. An in-built wall source of compressed air, if available, can be used instead. It draws air from the atmosphere and delivers it under pressure (50 PSI) so that the positive pressure breaths can be generated.
The compressor has a filter which should be washed with tap water daily or as directed. If this is not done, it greatly increases the load on the compressor. The indicator on the compressor should always be in the green zone. It should not be placed too close to the wall as it may get overheated. There should be enough space to permit air circulation around it.2
- Control panel: The controls that are found on most pressure-controlled ventilators include the following:
- FiO2
- Peak Inspiratory Pressure: PIP (in pressure controlled ventilators).
- Tidal volume/Minute volume (in volume controlled ventilators).
- Positive End Expiratory Pressure (PEEP).
- Respiratory Rate (RR).
- Inspiratory Time (Ti).
- Flow rate.
The other parameters displayed on the ventilator include mean airway pressure (MAP), I:E ratio (ratio of the inspiratory time to expiratory time). The expired tidal volume will be displayed in all volume controlled ventilators and some pressure controlled ventilators.
Newer ventilator models have digital display controls. Some ventilators also display waveforms, which show the pulmonary function graphically.
- Humidifier: Since the endotracheal tube bypasses the normal humidifying, filtering and warming system of the upper airway, the inspired gases must be warmed and humidified to prevent hypothermia, inspissation of secretions and necrosis of the airway mucosa.Types of humidifiers available:
- Simple humidifier: It heats the humidity in inspired gas to a set temperature, without a servo control. The disadvantage is excessive condensation in the tubings with reduction in the humidity along with cooling of the gases by the time they reach the patient.
- Servo-controlled humidifier with heated wire in the tubings: These prevent accumulation of condensate while ensuring adequate humidification. Optimal temperature of the gases should be 36-37°C and a relative humidity of 70 percent at 37°C. If the baby is nursed in the incubator, temperature monitoring must take place before the gas enters the heated field. At least some condensation must exist in the inspiratory limb which shows that humidification is adequate. The humidifier chamber must be changed daily. It should be adequately sterilized or disposable chambers may be used.
- Breathing circuit: It is preferable to use disposable circuits for every patient. Special pediatric circuits are available in the market with water traps. If reusable circuits are used, they must be changed every 3 days. Reusable circuits are sterilized by gas sterilization or by immersion in 2 percent glutaraldehyde for 6-8 hours and then thoroughly rinsing with sterile water. Disposable circuits may be changed every week.
Terminology
Ventilatory controls that can be altered in mechanical ventilation include the following:
- Inspired oxygen concentration (FiO2).
- Flow rate.
- Positive end-expiratory pressure (PEEP).
- Respiratory rate (RR),or Frequency (f).
- Inspiratory/Expiratory Ratio (I:E Ratio).
- Tidal volume (in volume controlled ventilators).
- Pressure support.
Inspired Oxygen Concentration (FiO2)
An improvement in oxygenation may be accomplished either by increasing the inspired oxygen concentration (FiO2) or by different ventilator settings.
- Increasing peak inspiratory pressure (PIP)
- Increasing inspiratory/expiratory ratio
- Applying a positive pressure before the end of expiration (PEEP).
FiO2 is adjusted to maintain an adequate PaO2. High concentrations of oxygen can produce lung injury and should be avoided. The exact threshold of inspired oxygen that increases the risk of lung injury is not clear. A FiO2 of 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 > 20 percent oxygenation may not be substantially improved by higher concentrations of oxygen.
The administration of oxygen and its toxicity is a clinical problem in the treatment of neonates, especially low birth weight infants.
The developing retina of the eye is highly sensitive to any disturbance in its oxygen supply. Oxygen is certainly a critical factor (hyperoxia, hypoxia), but a number of other factors (immaturity, blood transfusions, PDA, vitamin E deficiency, infections) may interact to produce various degrees of Retinopathy of Prematurity (ROP).
Another complication of oxygen toxicity induced by artificial ventilation in the neonatal period is a chronic pulmonary disease, Bronchopulmonary Dysplasia (BPD), mostly seen in premature infants ventilated over long periods with a high inspiratory peak pressure and high oxygen concentration.
High oxygen concentration may play a role in the pathogenesis of BPD, but recent studies have shown, that the severity of the disease is correlated to the Peak inspiratory pressure (PIP) during artificial ventilation rather than to the doses of supplementary oxygen.
Peak Inspiratory Pressure (PIP)
Peak Inspiratory Pressure is the major factor in determining tidal volume in infants treated with time cycled or pressure cycled ventilators. Most ventilators indicate inspiratory pressure on the front and it can be selected directly.
The starting level of PIP must be considered carefully. Critical factors that must be evaluated are the infant's weight, gestational age (the degree of maturity), the type and severity of the disease and lung mechanicssuch as lung compliance and airway resistance.4
The lowest PIP necessary to ventilate the patient adequately is optimal. In most cases, associated with increased tidal volume, increased CO2 elimination and decreased PaCO2.
Mean airway pressure will rise and thus improve oxygenation.
If PIP is minimized, there is a decreased incidence of barotrauma with resultant air leak (pneumothorax and pneumomediastinum) and BPD.
Hacker et al demonstrated that more rapid ventilator rates and lower PIP are associated with a decreased incidence of air leaksa mode of ventilation which may be recommended in infants with congenital diaphragmatic hernia.
High PIP may also impede venous return and lower cardiac output.
Flow Rate
The flow rate is important determinant during the infant's mechanical ventilation of attaining desired levels of peak inspiratory pressure, wave form, I:E ratio and in some cases, respiratory rate.
In general, a minimum flow at least two times the minute volume ventilation is usually required. Most pressure ventilators operate at flows of 6-10 liters per minute.
If low flow rates are used, there will be a slower inspiratory time (Ti) resulting in a pressure curve of sine wave form and lowering the risk of barotrauma.
Too low flow relative to minute volume, may result in hypercapnia and accumulation of carbon dioxide in the system.
High inspiratory flow rates are needed if square wave forms are desired and also when the inspiratory time is shortened in order to maintain an adequate tidal volume. Carbon dioxide retention in the ventilator tubing will be prevented at high flow rates.
A serious side effect of high flow rate is an increased risk of alveolar rupture, because maldistribution of ventilation results in a rapid pressure increase in the non-obstructed or non-atelectatic alveoli.
Positive End Expiratory Pressure (PEEP)
Positive pressure applied at the end of expiration to prevent a fall in pressure to zero is called Positive End Expiratory Pressure (PEEP).
PEEP stabilizes alveoli, recruits lung volume and improves the lung compliance. The level of PEEP depends on the clinical circumstances. Application of PEEP results in a higher mean airway pressure, and mean lung volumes.
The goals of PEEP are:
- Increasing FRC (Functional Residual Capacity) above closing volume to prevent alveolar collapse
- Maintaining stability of alveolar segments
- Improvement in oxygenation, and
- Reduction in work of breathing.
The optimum PEEP is the level at which there is an acceptable balance between the desired goals and undesired adverse effects. The desired goals are: (1) reduction in inspired oxygen concentrationnontoxic levels (usually less than 50%); (2) maintenance of PaO2 or SaO2 of > 60 mm Hg or > 90 percent respectively, (3) improving lung compliance; and (4) maximizing oxygen delivery.
Arbitrary limits cannot be placed to determine the level of PEEP or mean airway pressure that will be required to maintain adequate gas exchange. When the level of PEEP is high, peak inspiratory pressure may be limited to prevent it from reaching dangerous levels that contribute to air leaks and barotrauma. In children with tracheomalacia or bronchomalacia, PEEP decreases the airway resistance by distending the airways and preventing dynamic compression during expiration.
The compliance may be improved. Improved ventilation may result (improvement in ventilation/perfusion ratio) by preventing alveolar collapse.
Low levels of PEEP (2-3 cm H2O) are often used during weaning from the ventilator in conjunction with low IMV rates only for a short amount of time.
Medium levels of PEEP (4-7 cm H2O) are commonly used in moderately ill patients.
High levels of PEEP (8-15 cm H2O) benefit oxygenation in ARDS (Acute Respiratory Distress Syndrome); tidal volume, and PaO2 increases. HigherPEEP level can also reduce blood pressure and cardiac output explained by a reduced preload. Very high levels of PEEP results in overdistention and alveolar rupture leading to increased incidence of pneumothorax and pneumomediastinum.
Respiratory Rate (RR) or Frequency (f)
Respiratory rate, together with tidal volume, determines the minute ventilation. Depending on the infant's gestational age and the underlying disease, the resulting pulmonary mechanics (resistance, compliance) require the use of slow or rapid ventilatory rates.
Moderately high ventilator rates (60-80 breaths per minute) employ a lower tidal volume and therefore, lower inspiratory pressures (PIP) are used to prevent barotrauma.
High rates may also be required to hyperventilate infants with pulmonary hypertension and right-to-left shunting to achieve an increased pH and reduced PaCO2, thereby reducing pulmonary arterial resistance and shunting associated with increased PaCO2. Respiratory rate is the primary determinant of minute ventilation and hence, CO2 removal from lungs.
Tidal volume RR, increasing the RR lowers the PaCO2 level. A respiratory rate of 40-60 is usually sufficient in most conditions. High rates are necessary in Meconium Aspiration Syndrome (MAS) where CO2 retention is a major problem. It must be recognized that increasing the RR while keeping the IT the same, shortens expiration and may lead to inadequate emptying of lungs and inadvertent PEEP.6
One of the major disadvantages in using the high ventilator rates is an insufficient emptying time during the expiratory phase, resulting in air trapping, increased FRC, and thus decreased lung compliance.
A slow ventilation rate combined with a long inspiratory time, both in animals and infants with RDS resulted in fewer bronchiolar histological lesions, better lung compliance and in infants, a reduction in the incidence of BPD.
Ratio of Inspiratory to Expiratory Time (I:E ratio)
One of the most important ventilator control is the ratio of inspiratory to expiratory time (I:E ratio). This ventilator control has to be adjusted depending on the pathophysiology and the course of the respiratory disease, always with respect to pulmonary mechanics, such as compliance, resistance and time constant.
In infants with, Respiratory Distress syndrome (RDS) with decreased compliance but normal resistance, resulting in shortened time constants inspiratory times I:E with ratios 1:1 are usually used.
Reversed I:E ratios, as high as 4:1 have been shown to result in improvement in oxygenation and in a retrospective study decreased the incidence of BPD. Other investigators also advocated the use of prolonged inspiratory time, since infants in the ‘2:1’ group required less inspired oxygen and a lower expiratory pressure to achieve satisfactory oxygenation. Extreme reversed I:E ratio with a short expiratory time will lead to air trapping and alveolar distention. In addition, prolonged inspiratory time may adversely affect venous return to the heart and decreased pulmonary and systemic blood flow. The concept becomes especially important when higher respiratory rates are used.
If inspiratory time is shorter than three to five time constants, inspiration will not be complete and tidal volume will be lower than expected. If expiratory time is too short, expiration will not be complete which will lead to air trapping.
An IT of 0.3-0.5 sec is sufficient for most disorders. In low compliance condition like RDS use closer to 0.5 sec. In disorders with increased airway resistance like MAS use shorter IT. Once set, IT is usually not changed unless there is persistent hypoxemia unresponsive to changes in PIP and FiO2.
Increasing the IT shortens the expiratory time increasing the I:E ratio. Normal ratio is 1:3. Avoid 1:1 ratio to prevent air trapping.
While ventilating a case of lower airways obstruction (asthma, bronchiolitis) use short IT and slow rate with a longer expiratory time as there is gas trapping and increased risk of air leaks.
Tidal Volume (Vt)
In most volume cycled ventilators, tidal volume of 6 to 8 ml/kg can be set, or a particular flow rate and minute ventilation can be set to get a particular tidal volume. Siemens Servo 300 ventilator measures expired tidal volume and gives a display. If set tidal volume is significantly (the difference 7between set inspired and expired tidal volume is more than 15%) higher than the expired tidal volume, then circuit leak or an endotracheal leak should be looked for and corrected.
Pressure-support
Pressure-support ventilation is a form of assisted ventilation where the ventilator assists a patient's spontaneous effort with a mechanical breath with a preset pressure limit. The patient's spontaneous breath creates a negative pressure, which triggers the ventilator to deliver a breath. The breath delivered is pressure-limited; very high inspiratory flow results in a sharp rise in inspiratory pressure to the preset pressure limit. The inspiratory pressure is held constant by servo-control of the delivered flow and is terminated when a minimal flow is reached (usually < 25% of peak flow), just before spontaneous exhalation begins. Pressure-support ventilation depends entirely on the patient's effort, if the patient becomes apneic, the ventilator will not provide any mechanical breath. Pressure-support ventilation allows better synchrony between the patient and the ventilator than IMV, volume-assisted ventilation, or pressure control ventilation. Pressure-support allows ventilatory muscle loads to be returned gradually during the weaning process like IMV techniques. Since each breath is assisted, it alters the pressure volume relationship of the respiratory muscles in such a way so as to improve its efficiency. With ventilatory muscle fatigue, muscles can be slowly retrained and titrated more efficiently than IMV and thus, promote the weaning process. The emphasis with weaning with pressure support ventilation is endurance training of the respiratory muscles, especially, the diaphragm. The parameters that can be manipulated to titrate the muscle loading are the magnitude of the trigger threshold and the preset pressure limit. PEEP is provided to maintain FRC and prevent alveolar collapse. The amount of pressure-support to be provided depends on the clinical circumstance. A pressure-limit that delivers a VT of 10 to 12 ml/kg has been termed PSV max because at this level respiratory work can be reduced to zero. It is not necessary to provide PSV max at the beginning. The level of pressure support selected should allow for spontaneous respiration without undue exertion and still results in normal minute ventilation. No strict criteria can be established; it has to be applied and titrated on an individual basis. Weaning of pressure-support ventilation is accomplished by reducing the pressure-limit decrementally. Similar to weaning guidelines previously mentioned with each wean, the effect of weaning on muscle loading has to be evaluated clinically. Increase in respiratory rate is an early indication of increasing muscle load. Retraction and use of accessory muscles would indicate a more severe muscle load. If respiratory rate increases during the weaning process, the level of pressure-support should be increased until there is reduction in the respiratory rate. While this method of weaning is attractive theoretically, its benefit in the weaning process is yet to be established in infants and children. A relative contraindication to the use of pressure-support ventilation is a high baseline spontaneous respiratory rate. There is a finite lag time involved from the initiation of a breath to the 8sensing of this effort and from the sensing to the delivery of a mechanical breath. In infants breathing at a relatively fast rate (40 to 50 breaths/minute), this lag time may be too long and result in asynchrony between the patient and the ventilator. Pressure-support has been mainly used to wean adult patients off mechanical ventilation. Its use in pediatrics is gaining popularity. When used at our institution, we tend to keep a base line low SIMV rate (5 to 6 per min) along with pressure support before extubation.