Basics of Mechanical VentilationCHAPTER 1

The upper airway in infants is cephalad, funnel shaped with its narrowest area being at the subglottic region (at the level of cricoid ring as compared to the relatively tubular adult airway. Airway resistance increases inversely by 4th power of the radius; i.e. in an already small airway even 1 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 hence the time that apnea can be tolerated by a child.

Respirations are shallow and rapid due to predominant diaphragmatic breathing, and inadequate chest expansion due to the more horizontal alignment of the ribs in infancy giving less play in the bucket handle movement of the ribs during inspiration. Therefore, a child tends to get tachypneic rather than increasing the depth of respiration in response to hypoxemia. Oxygen consumption per 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 interalveolar 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 tracheobronchomalacia as well as lower airways closure.

Therefore, children tend to require smaller tidal volumes, faster respiratory rates, adequate size uncuffed endotracheal tube, adequately suctioned clear airway for proper management of mechanical ventilation. Other important factors for choosing the ventilatory setting 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).

Adequate minute ventilation is essential to keep PaCO₂ within normal limits.

PaO₂= [(Atmospheric pressure – water vapor) × FiO₂] – PaCO₂/RQ

RQ = respiratory quotient

Adequate perfusion to alveoli that are well ventilated improves oxygenation.

Hemoglobin is fully saturated 1/3 of the way through the capillary.

3One can correct hypoventilation by increasing minute ventilation.

One can correct V/Q mismatch by increasing amount of lung that is ventilated or by improving perfusion to those areas that are ventilated.

Compliance = Δ volume/Δ pressure → the change of unit volume over a change of unit pressure applied is the compliance of the lung.

The pressure used for this calculation is the transpulmonary pressure, which is equal to the difference between the alveolar pressure (P_alv) at the end of inflation and the pleural pressure (P_pl or P_es).

Time constant = Compliance × Resistance

One time constant that fills an alveolar unit to 63% of its capacity. It takes three time constants to fill an alveolus to 90% capacity and up to 5 Tc to fill it to 95%.

To calculate a full term neonates Tc who has a compliance of 0.004 L/cm and a R of 30 cm H₂O/L/sec the Tc will be:

–C = 0.004 L/cm H₂O; R = 30 cm H₂O/L/sec

–T = 0.004 × 30 = 0.12 sec.

If we want to set the inspiratory time with this knowledge, then we need 3–5 Tc to fill the alveolus so 0.12 × 4 = 0.48 secs. If we are dealing with RDS/ARDS we could use a lower Ti like 0.4 secs and if we are dealing with an obstructive disease requiring more time we could use a higher Ti like 0.55 secs.

An adult has a Tc of about 0.3 so the Ti is set around 0.9–1 sec commonly.

There are five basic breaths that are used. These are shown below.

Various modes are then permutations and combinations of these breaths. It is essential to be familiar with the appearance of the wave forms that are seen in the flow and pressure scalars of these basic breath types. The volume scalar always looks the same.

By convention, the pressure scalar is always the top one followed by the flow scale (Fig. 1.1).

The patient does not initiate additional breaths above that set on the ventilator (Figs 1.2 and 1.3).

volume control ventilation (VCV): flow-targeted volume-cycled breaths

pressure control ventilation (PCV): pressure-targeted time-cycled breaths

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. Difficult to wean and asynchrony is a major issue. Patients on control modes will need sedation and/or paralysis. This is usually set with some leeway for patient triggering either SIMV + PSV or in the AC or assist control mode so when the patient awakens and breaths there is adequate flow and to avoid flow hunger and asynchrony.

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.

If volume is set, pressure varies; if pressure is set, volume varies according to the compliance.

One must have a set up of high/low pressure alarms in volume cycling and, low expired tidal volume alarm when using pressure cycling.

Caution: Tidal volume changes suddenly as patient's compliance changes.

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.

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 mean airway pressure. 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).

10Ventilators have a negative pressure sensor which can be set at various levels of sensitivity to initiate a breath usually based on patient effort (negative pressure) or elapsed time before the next breath in the event of respiratory depression or apnea. The patient's effort can be “sensed” as a change in pressure or a change in flow in the circuit (flow triggering). A setting of greater than 0 makes it too sensitive (meaning the triggered breath from the ventilator will be too frequent). A negative setting (negative 1 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.

Higher I-times may be needed to improve oxygenation in difficult situations (Inverse ratio ventilation) increasing the risk of air leak.

For ventilation: Respiratory rate, tidal volume (in volume limited) and PIP (In pressure limited mode) can be adjusted.

PEEP 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

Increased CO₂ production: hyperthermia, high carbohydrate diet, shivering.

Inadequate tidal volume delivery (hypoventilation): will occur with endotracheal tube block, malposition, kink, circuit leak, ventilator malfunction.

Common causes include hypoventilation, hypoxemia, tube block/kink/malposition, bronchospasm, pneumothorax, silent aspiration, increased oxygen demand/increased CO₂ production (in sepsis), inadequate sedation.

If patient fighting the ventilator and desaturating: Immediate measures.

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.