Basic principles of physics and gas flow apply to all age groups; anatomical and physiological differences play a significant role in selecting the type of ventilator as well as the ventilator modes and settings.
Upper airway in children is cephalad, funnel shaped with narrowest area being subglottic (at the level of cricoid ring), as compared to adults where the upper airway is tubular with narrowest part at the vocal cords. Airway resistance increases inversely by 4th power of radius i.e. in an already small airway even one mm of edema or secretions will increase the airway resistance and turbulent flow markedly necessitating treatment of airway edema, suctioning of secretion, measures to control secretions. Low functional residual capacity (FRC: Volume of air in the lungs at end of expiration) reduces the oxygen reserve, and reduces the time that apnea can be allowed in a child.
Respirations are shallow and rapid due to predominant diaphragmatic breathing, and inadequate chest expansion due to inadequate costovertebral bucket handle movement in children. Therefore, a child tends to get tachypnea rather than increasing the depth of respiration in response to hypoxemia. Oxygen consumption/kg body weight is higher; therefore, tolerance to hypoxemia is lower.
Susceptibility to bradycardia in response to hypoxemia is also higher due to high vagal tone. Pores of Kohn and channels of Lambert (bronchoalveolar and intermalleolar collaterals) are inadequately developed, making regional atelectasis more frequent. Closing volumes are lower and airway collapse due to inadequate strength of the cartilage in the airways is common, making a child particularly susceptible to laryngomalacia, and tracheo-bronchomalacia as well as lower airways closure at a higher lung volume.
Therefore, children tend to require smaller tidal volumes, faster respiratory rates, and adequate size endotracheal tubes and adequately suctioned clear airways for proper management of mechanical ventilation. Other important factors for choosing ventilatory settings include the primary pathology i.e. asthma, acute respiratory distress syndrome (ARDS), pneumonia, air leak syndrome, raised intracranial tension, neuromuscular weakness, neonatal hyaline membrane disease, or neonatal persistent pulmonary hypertension (PPHN).
BASIC MECHANICS OF VENTILATION
During spontaneous breathing, pleural pressure is negative. During inspiration active work is done to generate the gradient between the mouth and pleural space as the driving pressure for inspired gases to enter the alveolus, and this gradient is needed to overcome resistance and to maintain the alveolus open, by overcoming elastic recoil forces.
Therefore, a balance between elastic recoil of the chest wall and the lung determines lung volume at any given time. Expiration is passive. During positive pressure ventilation, pressure gradient generated by the ventilator at the mouth (or endotracheal tube) is higher than the pleural pressure which is also positive, however at the end of inspiration, expiration is again passive though it can be manipulated by application of positive pressure to prevent complete deflation at the end of expiration (PEEP: positive end expiratory pressure).
Two main issues are important physiologically during mechanical ventilation: ventilation and oxygenation.
Ventilation washes out carbon dioxide from alveoli keeping arterial PaCO2 between 35 mm of Hg and 45 mm of Hg. Increasing dead space increases the PaCO2.
Alveolar MV = Respiratory rate × Effective tidal volume
Effective TV = TV - Dead space
Dead space = Anatomic (nose, pharynx, trachea, bronchi) + Physiologic (alveoli that are ventilated but not perfused)
Adequate minute ventilation is essential to keep PaCO2 within normal limits.
Partial pressure of oxygen in alveolus (PaO2) is the driving pressure for gas exchange across the alveolar-capillary barrier determining oxygenation.
PaO2 = [(Atmospheric pressure - Water vapor) × FiO2] - PaCO2/RQ
RQ = Respiratory quotient
Adequate perfusion to alveoli that are well ventilated improves oxygenation.
Hypoxemia can occur due to:
- V/Q mismatch (V—ventilation, Q—perfusion)
- Shunt (Perfusion of an unventilated alveolus, atelectasis, fluid in the alveolus)
- Diffusion impairments.
Hypercarbia can occur due to:
- V/Q mismatch
- Dead space ventilation.
Hypoventilation and V/Q mismatch are the most common causes of abnormal gas exchange in the pediatric intensive care unit (PICU).
Hypoventilation can be corrected by increasing minute ventilation.
V/Q mismatch can be corrected by increasing the amount of lung that is ventilated or by improving perfusion to those areas that are ventilated.
Concept of Time Constant
Time constant is the time required to fill an alveolar space (or empty it). It depends on the resistance and compliance. In the pediatric age group one time constant that fills an alveolar unit to 63% of its capacity is 0.15 seconds. It takes three time constants to achieve greater than 90% capacity of the alveolar unit filled.
Time constant = Resistance (pressure × time/volume) × Compliance (volume/pressure)
This signifies that a certain minimum inspiratory time (Ti) is required to fill the alveoli adequately which is generally two to three time constants; i.e. 0.3–0.45 seconds. This is important when selecting the Ti on the conventional ventilator.
INDICATIONS OF MECHANICAL VENTILATION
Indications remain essentially clinical and may not be always substantiated by objective parameters such as blood gas analysis.
Common indications include:
- Respiratory failure:
- Apnea/respiratory arrest
- Inadequate ventilation
- Inadequate oxygenation
- Chronic respiratory insufficiency with failure to thrive
- Neurologic dysfunction:
- Central hypoventilation/frequent apnea
- Patient comatose, Glasgow Coma Score (GCS) <8
- Inability to protect airway
- Postoperative ventilation
- Airway pressures:
- Peak inspiratory pressure (PIP)
- Positive end expiratory pressure (PEEP)Fig. 2: Graph showing factors affecting mean airway pressure (and oxygenation). 1. Inspiratory time, 2. Peak inspiratory pressure, 3. Expiratory time, 4. Positive end expiratory pressure, and 5. Pause time.
- Pressure above peep (PAP or δp)
- Mean airway pressure (MAP)
- Continuous positive airway pressure (CPAP)
- Inspiratory time (Ti)
- I:E ratio: Ratio of Ti and expiratory time in seconds
- Frequency (f): Ventilatory rate (breaths/min)
- Tidal volume (Vt): Amount of gas delivered with each breath
- Expired tidal volume (Ve): Amount of gas measured by the machine at expiration.
MODES OF VENTILATION
In this mode, every breath is fully supported by the ventilator. In classic control modes, patients were unable to breathe except at the controlled set rate. In a conventional controlled mode, weaning is not possible by decreasing rate, the patient may hyperventilate if agitated leading to patient/ventilator asynchrony. Patients on control modes will need sedation and or paralysis with a muscle relaxant in newer control modes, machines may act in assist-control, with a minimum set rate and all triggered breaths above that rate are also fully supported.
Fig. 7: Time-triggered, volume-limited, time-cycled ventilation. (CL: compliance lung; Raw: airway resistance; VT: volume tidal)
Intermittent Mandatory Ventilation Modes
In this mode breaths “above” the set rate are not supported. Most modern ventilators have synchronized intermittent mandatory ventilation (SIMV).
Synchronized Intermittent Mandatory Ventilation
Ventilator synchronizes intermittent mandatory ventilation (IMV) “breath” with patient's effort.
Patient takes “own” breaths in between (with or without pressure support) the set SIMV rate. There is a potential for increased work of breathing and patient/ventilator asynchrony, if the ventilator interferes with the patient's effort to breath or if there is insufficient flow for the spontaneous breaths. Ventilators would have an inbuilt latent period of about 25% of the Ti in which to recognize the patient's effort in order to synchronize the mandatory breath in order to reduce asynchrony. SIMV breath can be pressure limited or volume limited.
Ventilator supplies pressure support (flow) at a preset level but rate is determined by the patient, expiration begins passively when inspiratory flow decreases below a certain level preset in the ventilator (flow cycled). Volume support is also available in Servo 300 ventilators following the principle of pressure support (delivery of the set volume over the patient's natural inspiratory time duration keeping the pressure to a minimum.
Pressure support can decrease work of breathing by providing flow during inspiration for patient triggered breaths. It can be given with spontaneous breaths in IMV modes or as stand-alone mode without set rate as well as for weaning to retrain coordination of respiratory muscles in patients on ventilation for longer than few weeks.
Trigger is defined as the variable that initiates the breath from the ventilator. The trigger variable is usually pressure or flow.
Pressure trigger: With pressure triggering, in order to trigger the ventilator and initiate the inspiratory flow, the patient must decrease the pressure in the ventilator circuit to a preset value, which will then open a demand valve.
Flow trigger: With flow triggering, the patient triggers the ventilator when the respiratory muscles generate a certain preset inspiratory flow. It is generally believed that triggering of the ventilator is better with flow than with pressure.
The real clinical significance is unclear in terms of the work of breathing and patient ventilator interaction. Pressure sensors in current ventilators are much improved, reducing any difference between flow and pressure triggering systems. Recent studies in patients with different diseases show that the difference in the work of breathing between flow and pressure triggering is of minimal clinical significance.
Trigger setting: A pressure trigger setting of greater than 0 (cm of water) makes it too sensitive (meaning the triggered breath from the ventilator will be too frequent). A negative setting (negative1 or negative 2) setting is usually acceptable. Too negative setting will increase the work of the patient (to generate a negative pressure) to trigger a ventilator breath.
BASIC FUNDAMENTALS OF VENTILATION
Ventilators deliver gas to the lungs using positive pressure at a certain rate. The amount of gas delivered can be limited by time, pressure or volume. The duration can be cycled by time, pressure or flow. If volume is set, pressure varies; if pressure is set, volume varies according to the compliance.
Compliance = Δvolume/Δ pressure
Chest must rise, no matter which mode is chosen.
Following are three main expectations from the ventilator:
- Ventilator must recognize patient's respiratory efforts (trigger)
- Ventilator must be able to meet patient's demands (response)
- Ventilator must not interfere with patient's efforts (synchrony)
Whenever a breath is supported by the ventilator, regardless of the mode, the limit of the support is determined by a preset pressure or volume.
Volume limited: Preset tidal volume
Pressure limited: Preset PIP
Pressure versus Volume Control
Goal is to ventilate and oxygenate adequately. Both pressure and volume control modes can achieve it. Important requirements include adequate movement of the chest, smooth gas flow, and minimal barotrauma or volutrauma.
One must have a setup of high/low pressure alarms in volume cycling and, low expired tidal volume alarm when using pressure cycling.
Ventilator stops the inspiratory cycle when set PIP is achieved.
Caution: Tidal volume changes suddenly as patient's compliance changes. Ventilator delivers a decelerating flow pattern (lower PIP for same Vt). This can lead to hypoventilation or overexpansion of the lung. If endotracheal tube is obstructed acutely, delivered tidal volume will decrease. This mode is useful if there is a leak around the endotracheal tube.
For improving oxygenation, one needs to control FiO2 and MAP, (I-time, PIP, PEEP) and to influence ventilation, one needs to control PIP and respiratory rate.
Ventilator stops the inspiratory cycle when set tidal volume has been delivered. One can control minute ventilation by changing the tidal volume and rate. For improving oxygenation primarily FiO2, PEEP, I-time can be manipulated. Increasing tidal volume will also increase the PIP, hence affecting the oxygenation by increasing the MAP. It delivers volume in a square wave flow pattern. Square wave (constant) flow pattern results in higher PIP for same tidal volume as compared to pressure modes.
Caution: There is no limit per se on PIP (so ventilator alarm will have to be set for an upper pressure limit to avoid barotrauma). Volume is lost if there is a circuit leak or significant leak around the endotracheal tube, therefore an expired tidal volume needs to be monitored and set. Some ventilators will alarm automatically if the difference between set inspired tidal volume and expired tidal volume is significant (varies between the ventilators).
Initial Ventilator Settings
One should always have the general idea regarding what initial ventilator settings to choose when initiating the ventilation.
Choose the mode: Control every breath (assist control) if planned for heavy sedation and muscle relaxation or use SIMV when patient likely to breath spontaneously.
General parameters to choose will include:
Rate: Start with a rate that is somewhat normal; i.e. 15 for adolescent/child, 20–30 for infant/small child, 30–40 for a neonate, 40–50 for a premature neonate.
FiO2: 1(100%) and quickly wean down to level <0.5. Depending upon oxygen requirement 0.5 may be a starting point for the FiO2.
PEEP: 3–5 cm of H2O (higher to 6–7 if ARDS, or low compliance disease, lower (2–3 cm) if asthma, or high compliance disease.
Inspiratory time (I-time or I:E ratio): 0.3–0.4 sec for neonates, 0.5–0.6 sec for children, 0.7–0.9 in older children. Normal I:E ratio = 1:2–1:3
Then specifically choose if the modality of delivered breath will be pressure controlled or volume controlled (correct term is pressure limited or volume limited).
Pressure limited: Peak inspiratory pressure is set depending upon lung compliance and pathology
Neonates: Apnea 12–14 cm, hyaline membrane disease 18–22 cm H2O
Children: For normal lung 16–18 cm, for low compliance 18–25 cm H2O, severe ARDS 25–35 cm may be required.
Tidal volume 8–10 mL/kg with a goal to get 6–8 mL/kg expired tidal volume. Initial tidal volume at 10–12 mL/kg may need to be set if leak is present around endotracheal tube; in such patients, pressure limited ventilation may be preferred. Flow in most ventilators is set at 6–10 L for the washout of the CO2 from the internal ventilator circuit, tubing's, etc. Flow less than 4 L/min is not recommended. Following discussion includes cases and principles of ventilation based on disease specific pathophysiology.
Adjustments after Initiation: Usually based on blood gases and oxygen saturations
For oxygenation: FiO2, PEEP, I Time, PIP (tidal volume) can be adjusted (increase MAP)
For ventilation: Respiratory rate, tidal volume (in volume limited) and PIP (in pressure limited mode) can be adjusted.
Positive end expiratory pressure is used to help prevent alveolar collapse at end inspiration; it can also be used to recruit collapsed lung spaces or to stent open floppy airways.
Gas Exchange-related Problems
- Inadequate oxygenation (hypoxemia)
Inadequate oxygenation: Important guidelines
- Do not just increase FiO2
- Increase tidal volume if volume limited mode, PEEP, Ti.
- Increase PIP/PEEP/ Ti if pressure limited mode
- If O2 worse, get chest X-ray to rule out air leak (treat!)/If lung fields show worsening (increase PEEP further)
- Do not forget other measures to improve oxygenation
- Normalize cardiac output (if low output) by fluids and/inotropes
- Maintain normal hemoglobin
- Maintain normothermia
- Deepen sedation/consider neuromuscular block
High PaCO2: Common reasons include hypoventilation, dead space ventilation (too high PEEP, decreased cardiac output, pulmonary vasoconstriction), increased CO2 production, hyperthermia, high carbohydrate diet, and shivering. Inadequate tidal volume delivery (hypoventilation) will occur with endotracheal tube block, malposition, kink, circuit leak, and ventilator malfunction.
Measures for normalizing high PaCO2 guidelines:
- If volume limited: Increase tidal volume (Vt), increase frequency (rate) (f).
- If asthma: Increase expiratory time, may need to decrease ratio to achieve an I:E ratio >1:3.
- If pressure limited: Increase PIP, decrease PEEP, increase frequency (rate).
- Decrease dead space (increase cardiac output, decrease PEEP, vasodilator)
- Decrease CO2 production: Cool, increase sedation, decrease carbohydrate load.
- Change endotracheal tube if blocked, kinked, malplaced or out, check proper placement.
- Fix leaks in the circuit, endotracheal tube cuff, humidifier
Measures to reduce barotrauma and volutrauma: Following concepts are being increasingly followed in most PICUs.
- Permissive hypercapnia: Higher PaCO2s are acceptable in exchange for limiting peak airway pressures: as long as pH>7.2.
- Permissive hypoxemia: PaO2 of 55–65; SaO2 88–90% is acceptable in exchange for limiting FiO2 (<60) and PEEP, as long as there is no metabolic acidosis. Adequate oxygen content can be maintained by keeping hematocrit >30%.
Patient Ventilator Dyssynchrony
In coordination between the patient and the ventilator: Patient fights the ventilator! Common causes include, hypoventilation, hypoxemia, tube block/kink/malposition, bronchospasm, pneumothorax, silent aspiration, increased oxygen demand/increased CO2 production (in sepsis), and inadequate sedation.
If patient fighting the ventilator and desaturating: Immediate measures
USE MNEMONIC: D O P E
D: displacement, O: obstruction, P: pneumothorax, E: equipment failure.
- Check tube placement. When in doubt take the endotracheal tube out, start manual ventilation with 100% oxygen.
- Examine the patient: Is the chest rising? Breath sounds present and equal? Changes in examination? Atelectasis, treat bronchospasm/tube block/malposition/pneumothorax? (Consider needle thoracentesis.)
- Examine circulation: Shock? Sepsis?
- Check arterial blood gas and chest X-ray for worsening lung condition, and for confirming pneumothorax.
- Examine the ventilator, ventilator circuit/humidifier/gas source.
If no other reason for hypoxemia: Increase sedation/muscle relaxation, put back on ventilator.
Sedation and muscle relaxation during ventilation: Most patients can be managed by titration of sedation without muscle relaxation. Midazolam (0.1–0.2 mg/kg/hr) and vecuronium drip (0.1–0.2 mg/kg/hr) is most commonly used. Morphine or fentanyl drip can also be used if painful procedures are anticipated.
Do not use muscle relaxants without adequate sedation.
Routine ventilator management protocol: Following protocol is commonly followed:
- Wean FiO2 for SpO2 above 93–94. In ARDS, 89–92 may be acceptable.
- Arterial blood gas (ABG) one hour after intubation, then am pm schedule (12 hourly), and after major ventilator settings change, and 20 minutes after extubation
- Pulse oximetry on all patients, end tidal carbon dioxide (EtCO2)/graphics monitoring, if available
- Frequent clinical examination for respiratory rate, breath sounds, retractions, color
- Chest X-ray every day/alternate day/as needed.
Respiratory care protocol
- Position changes every 2 hourly → right chest tilt/left chest tilt/supine position and try to maintain 30° head up position.
- Suction 4 hourly and as needed (in line suction to avoid derecruitment/loss of PEEP/desaturation if available)
- Nebulization: In line nebulization is preferred over manual bagging. Metered dose inhalers (MDIs) can also be used
- Disposable circuit change, if visible soiling
- Humidification/in line disposable humidifier
Ventilator care protocols, suctioning, physiotherapy, and positioning should all be under proper protocols for patient safety and to prevent adverse events such as unplanned intubation.
Weaning from Mechanical Ventilation
Process of weaning begins at the time of initiation of ventilation (i.e. minimal ventilatory settings to keep blood gases and clinical parameters within acceptable limits although these settings will be very high).
If such procedure is followed then ventilatory settings would be reduced once the primary pathology/condition that led to ventilation is improving.
How do we know if the condition is improving?
- Improving general condition, fever, etc.
- Decreasing FiO2 requirement
- Improving breath sounds
- Decreasing endotracheal secretions
- Improving chest X-rays
- Decreased chest tube drainage, bleeding/air bubbles(as the case may be)
- Improved fluid and electrolyte status (no overload or dyselectrolytemia)
- Improving hemodynamic status
- Improving neurological status, muscle power, airway reflexes/control. Described weaning criteria such as maximal negative inspiratory force, vital capacity measurement are usually impractical. In pediatrics and neonatal age group, weaning criteria are generally clinical.
There are no set protocols supported by any pediatric studies. Protocol followed at author's institution is as follows:
When FiO2 requirement is down to 0.4, improvement in secretions, and chest X rays, improving clinical condition, muscle relaxant drip is stopped and sedation can be slowly weaned. One should change control mode (or PRVC) to SIMV mode with pressure support. Pressure support can be set at 10–15 cm above PEEP so that the spontaneous breaths can be adequately supported. Trigger sensitivity should be 0 to negative one. Then slowly SIMV rate can be weaned, followed by weaning of pressure support while closely monitoring for signs of respiratory distress, restlessness, nasal flaring, accessory muscle use, tachypnea, desaturations, and hemodynamic instability such as tachycardia, hypertension or hypotension.
Following weaning guidelines can be followed:
- Decrease FiO2 to keep SPO2 >94
- Decrease the SIMV rate to 5 (by 3–4 breath/min)
- Decrease the PIP (to 20 cm H2O, by reducing 2 cm H2O each time/tidal volume, to no less than 5 mL/kg to prevent atelectasis (usually guided by blood gases).
- Ventilator rate and PIP can be changed alternately. If at any point patient's oxygen requirement increases greater than 0.6, or spontaneous ventilation is fast or distressed with accessory muscle use (increased work of breathing), patient gets lethargic, hypercarbia on blood gas, weaning process should be paused and the support level increased. Patient may not be ready. Goal is to decrease what the ventilator does and see if the patient can make up the difference without desaturations/hypercarbia/significant tachypnea, and respiratory distress. (For example, if patient's SIMV was reduced from 20/min to 15/min and the patient's spontaneous rate is increased from 25 to 50, this patient may need more time on the ventilator).
- Spontaneous breathing trials (SBT): A trial for 15–20 minutes may be conducted by connecting the patient to collapsible anesthesia bag (C circuit trial), if no distress, desaturation or excessive tachycardia, sweating or hypertension, consider as readiness of extubation. With or without weaning protocols, most pediatric patients can be extubated successfully. SBT and clinical indicators for extubation readiness may be used in difficult situations of extubation failure; however, none of the pediatric specific weaning protocols and guidelines are able to predict successful extubation.
Most patients can be weaned to SIMV of 5 and extubated, some will need pressure support 5–10 above PEEP with CPAP, while others may need CPAP 5 cm H2O before extubation, with or without SBT with T piece.
Clinical indicators of extubation readiness: Extubation can generally be performed when following criteria are met:
- Control of airway reflexes, minimal secretions
- Patient upper airway (air leak around tube?)
- Good breath sounds
- Minimal oxygen requirement <0.3 with SPO2 >94
- Minimal rate 5/min
- Minimal pressure support (5–10 above PEEP)
- “Awake” patient
- Remember shock and post resuscitation are important indications for ventilation, in addition to respiratory failure and neuromuscular disease.
- If ventilator fails, turn FiO2 to 1 (100%) and take over hand bag tube ventilation: Follow DOPE protocol and correct accordingly.
- If and when in doubt regarding endotracheal tube status, do not waste time: Remove endotracheal tube and try bag mask ventilation.
- Low tidal volume is recommended to prevent lung trauma (permissive hypercapnia and permissive hypoxemia).
- Ventilator care protocols, suctioning, physiotherapy, and positioning should all be under proper protocols for patient safety and to prevent adverse events such as unplanned intubation.
- With or without weaning protocols, most pediatric patients can be extubated successfully.
- Spontaneous breathing trial and clinical indicators for extubation readiness may be used in difficult situations of extubation failure; however, none of the pediatric specific weaning protocols and guidelines are able to predict successful extubation.
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