Ventilation: Volume vs Pressure

Let’s start by looking at 3 patient presentations. Please write down the ventilator settings you would use for each patient. The settings should be:

1. Tidal Volume
2. Respiratory Rate
3. FiO2
4. PEEP

Patient 1: A 25 yo, otherwise well, is brought in following an overdose, that has led to decreased conscious state and hypoventilation. His other vitals are normal.

Patient 2: A 65 yo with sepsis and a significant metabolic acidosis, that is tiring and needs to have their airway protected.

Patient 3: A 28 yo with asthma, that has deteriorated rapidly and not responding to all other full treatment and is now being intubated.

Read the rapid review below and then watch the lecture by Dr James Edwards, where the answers are discussed.

Ventilation in the Emergency Department

Ventilators can generally be used in two modes; volume or pressure ventilation.

In volume control ventilation we set the tidal volume and measure the pressure. It assists in the control of ventilation. A set tidal volume is delivered, initially at lower flow rates than with pressure-controlled ventilation. These flow rates are usually constant and limit pressure peaks. It can however, lead to poorer alveolar recruitment in lungs with a low compliance. It results in a lower mean airway pressure, which may not be suitable for patients with severe hypoxia. It can result in barotrauma.

In pressure control ventilation we set the pressure and the ventilator delivers the volume, which is measured. This mode assists in the control of oxygenation. It results in increased duration of alveolar recruitment, may be more comfortable for the patient and is protective against barotrauma, because the pressures are controlled. It gives an increased mean airway pressure, which results in improved oxygenation. However tidal volumes vary and because the volume is not set, it can result in ‘volutrauma’.

In terms of ventilation, I prefer volume cycled ventilation for adults patients, knowing that in severe hypoxia, pressure-controlled may need to be used.

In infants and children, pressure controlled ventilation is preferred as it may result in improved flow and a more rapid improvement in oxygenation and reduces ventilator-induced lung injury.

Lung protective vs Obstructive Lung Strategy

Let’s talk about Lung Protective Strategy vs Obstructive Strategy in the adult patient. We will be using volume assist ventilation. This is based on an excellent reference by Weingart(1).

Lung Protective Strategy aims to minimise barotrauma due to volume and so allows us to set low tidal volumes. It is associated with reduced mortality and is suited to all patients except those with obstructive lung disease such as COPD, or asthma.

Obstructive Lung strategy is used for obstructive lung disease such as asthma or COPD. The main goal is to allow adequate expiration to avoid the ‘air-stacking’ that can occur when breaths are given, without full expiration of the previous breath.

Tidal Volume

The same volume is used for both Lung Protective and Obstructive lung strategy ie.,  6-8mL/kg. This is based on a weight calculation determined by height. Some ventilators will calculate the weight if the height is entered.

In lung protective strategy, low tidal volumes decrease lung damage. The volumes may need to be decreased even further, depending on the alveolar pressures present. The plateau pressure should be measured regularly by pressing the inspiratory hold button. This represents alveolar pressure and should be less than 30 cm H₂O. If the plateau pressure is greater than 30cm H₂O, it may result in alveolar injury. A solution to higher pressures, is to decrease tidal volume by 1mL/kg increments until the plateau pressure falls below 30cm H₂O

In obstructive strategy, the plateau pressure should also be measured. If this is higher than 30cm H₂O, the respiratory rate may be further decreased to allow the pressures to fall. The peak pressures in this mode, which represent the pressure in the larger airways may be permitted to be high, to achieve the full tidal volume.

Respiratory Rate

In lung protective strategy an initial rate of 16 breaths per minute should be used. This can be increased depending on arterial blood gas measurements. The PaCO₂ can be controlled by changing the respiratory rate.

By contrast, in obstructive strategy, a lower respiratory rate of about 8 breaths per minute should be used, to allow full expiration, knowing that hypercapnoea may occur, in the attempt to avoid breath stacking.

FiO₂

FiO2 should be decreased to 30%-40% soon after intubation, aiming for an SpO2 of 88%-95%. Higher FiO2 should not be needed in obstructive disease. In non-obstructive disease, a low SpO₂ can be increased by raising FiO₂ to a certain extent. However physiological shunts will prevent further improvement past a certain point in the FiO2. In these cases, with lung protective strategy, PEEP can be increased.

PEEP

In lung protective strategy, PEEP assists with alveolar recruitment and the combination of PEEP and FiO2 that should be used in a lung protective strategy are set out in the ARDSnet Protocols (see below). In Obstructive strategy PEEP is not required and should be 0.

 

References

1. Weingart SD. Managing initial Mechanical Ventilation in the Emergency Department. Ann Emerg Med 2016: 1-4
2. The Acute Respiratory Distress Syndrome Network. Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. N Engl J Med. 2000;342:1301-1308.