RDS Therapies

Mechanical ventilators work by generating a pressure gradient within the airways opening the lungs.
A simple model is one of a circuit of continuous gas flow. If this flow is interrupted the pressure rises in the circuit. This is transmitted to the lungs and the increase in pressure results in lung expansion. When the interruption is removed the pressure in the circuit drops and the baby is allowed to breath out.

The cycle of flow interruption is called intermittent, positive pressure ventilation (IPPV).

The circuit can be designed to either deliver a set pressure or a set tidal volume. Currently pressure controlled ventilators are the most frequently used but tidal volume control is becoming increasingly popular.

During inspiration air flows rapidly into the lungs. The time constant is the time taken for 63% of the delivered air to flow – it takes three time constants to deliver 95% and five to reach 99% of equilibrium.
On ventilators the inspiratory and expiratory times can be set. A very short inspiratory time can lead to inadequate tidal volume whilst too short an expiratory time leads to gas trapping.

Damage to the larynx and trachea rarely occurs during intubation. Ventilation can also injure the larynx, trachea and lungs if the need is prolonged or a high pressure is required.

Features of ventilators

• Gas blender - Allows adjustment of the inspired oxygen concentration between 21% and 100%.
• Time adjustment - Allows different inspiratory and expiratory times.
• Pressure gauge - Measures applied airway pressure.
• Expiratory relief valve - A safety device which limits the peak inspiratory pressure.
• Humidifier - Saturates inspired gases with water and delivers it to the lungs at 37°C.
• PEEP - Sets level of PEEP – used to maintain functional residual capacity (FRC).
• Alarms - Warns of inadvertent disconnections, pressure loss, and incorrect cycle timing.

Aims of ventilation

CO2 elimination
CO2 elimination depends largely on the volume of gas that passes in and out of the alveoli – proportional to minute volume.
CO2 elimination can be increased by either an increase in tidal volume or frequency.
Tidal volume can be increased by increasing peak inspiratory pressure or decreasing PEEP. Tidal volume can also be influenced by inspiratory time and flow rate of the ventilator. Hypercapnia (too much CO2) can result from hypoventilation. Therefore adjustments to ventilation can result in dramatic improvements.

Oxygenation
Oxygenation (when ventilated) is dependent on:

• Oxygen concentration
• Mean airway pressure (average pressure delivered through the respiratory cycle)
• Peak Inspiratory Pressure (PIP) is one of the determinants of MAP both these therefore should perhaps be mentioned. Other factors of MAP are inspiration time, frequency and PEEP.

Mean airway pressure adjustments can optimise functional residual capacity (air left in the lungs at end expiration) – this is a key element in gaseous exchange throughout the respiratory cycle.

Typical settings for conventional ventilation

• Oxygen 40-50%
• Inspiratory time 0.3-0.5 secs
• Expiratory time 0.3-0.5 secs
• I:E ratio 1:1
• Rate 40-90/minute. Here large variations exist. Few centers use 90 with conventional ventilators and some start out with 30
• Inspiratory pressure (PIP) 16-20cm H2O
• Positive End Expiratory pressure (PEEP) 4-5 cm H2O
• Gas flow 5-8 litres/minute

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