A12. Indications and basis of mechanical ventilation

From greek.doctor

Oxygen therapy

Oxygen therapy, also called oxygen supplementation, refers to administration of oxygen to the patient. The most common use case is hypoxaemia (type 1 respiratory failure) for any cause, but it is also indicated in CO intoxication, for example. Multiple devices can be used to administer oxygen, the most commonly used being the nasal cannula. Both the flow (in litres per minute) of the gas delivered and the fraction of inspired oxygen (FiO2) can be adjusted, depending on the device. The FiO2 is the fraction of oxygen gas in the inhaled gas. In room air, the FiO2 is 21%. The devices usually deliver 100% pure oxygen but because the patient also breathes room air, the actual inspired gas is not pure oxygen (100% FiO2).

Excessive oxygen therapy is harmful. For this reason, the oxygen delivered should be titrated to a specific SpO2 range. For healthy people without pulmonary disease, the normal range to aim for is SpO2 94-98%. For people with pulmonary disease, especially COPD or asthma, the range to aim for is 88-92%, to reduce the risk of CO2 narcosis. '

Normal values:

  • O2 pressure in artery (PaO2) ~ 100 mmHg
  • O2 pressure in vein (PvO2) ~ 40 mmHg
  • Alveolo-arterial gradient (PA-aO2) < 20 mmHg

Variable performance oxygen therapy devices

The oxygen delivery of these devices depends on the patient’s own breathing, more specifically their peak inhalatory flow. These devices enrich the inspired air with oxygen during inspiration. There are three types, the nasal cannula, simple face mask, and non-rebreather mask.

Nasal cannula

The nasal cannula lies in the patient's nose. They’re comfortable and cheap, but give a limited amount of oxygen and can dry out the nasal mucosa. By increasing the flow rate of oxygen up to maximum 6 L it can provide a maximum FiO2 of approximately 30%.

Simple face mask

The simple face mask, also called the "50% face mask" can provide an FiO2 of 50% at the maximum. The flow can be adjusted between 5 – 10 L/min of oxygen. If a flow rate og < 5 L/min is used, CO2 may accumulate in the dead space of the mask, causing CO2 retention. The high flow rate can also dry out the oral and nasal mucosa.

Non-rebreather mask

The non-rebreater mask, also called the "100% face mask" could provide an FiO2 of up to 100% in theory, but actually closer to maximum 80% in practice. The flow can be adjusted between 10 – 15 L/min of oxygen.

This type of mask has a reservoir bag which fills with pure oxygen gas between each inhalation, allowing the patient to theoretically breathe 100% pure oxygen (hence the name). A high flow rate is required to fill the reservoir between each breath. The bag has a one-way valve which prevents CO2 from entering the bag and causing CO2 trapping. This type of face mask does not humidify the air either, which is a disadvantage.

Fixed performance oxygen therapy devices

These devices deliver a fixed FiO2 and their efficacy therefore do not depend on the patient’s breathing. More specifically, they provide a higher gas flow than the patient's peak inspiratory flow rate, which is what makes them supply a constant amount of oxygen independent of the patients respiration. They can provide an O2 flow rate of 30 – 60 L/min and for some devices the FiO2 can be adjusted as well. These include the venturi mask and high flow oxygen therapy (HFOT).

These are indicated when a nasal cannula or face mask cannot provide enough oxygen, or when the amount of oxygen should be constant and independent of the patients inspiratory work. This may be useful for COPD, for example.

High flow oxygen therapy

High flow oxygen therapy (HFOT), usually with a high flow nasal cannula (HFNC) and of which Optiflow® is the most commonly known manufacturer, is commonly used. It provides a high flow of oxygen through a nasal cannula or a face mask. The air is humidified and heated by the machine. It can reduce the work of breathing and provide some degree of positive end-expiratory pressure (PEEP), which can keep more alveoli open. It can even wash out the dead space of the airways, decreasing CO2 retention. The oxygen flow can be adjusted between 20-60 L/min and the FiO2 of the inspired gas can be adjusted from 21 - 100%. HFOT is relatively comfortable and the patient can drink and eat.

Venturi mask

A venturi mask is a special type of face mask which uses a special valve that allows the delivery of an exact FiO2. The valve must be exchanged depending on which FiO2 one desires, and the flow rate should be delivered accordingly. For example, if one desires an FiO2 of 60%, 15 L/min should be given and the valve marked "60% 15 L/min" should be used. 60% is the maximum FiO2 a venturi mask can deliver, and 24% is the minimum.

Mechanical ventilation

Mechanical ventilation refers to the use of a machine called a ventilator to assist or replace the patient’s breathing. It is necessary when the O2 uptake or CO2 elimination is insufficient (respiratory failure), when the respiratory muscle power is reduced, and when there is severe circulatory failure. Mechanical ventilation can be non-invasive (NIV) or invasive

Non-invasive ventilation

Non-invasive ventilation (NIV) is a form of mechanical ventilation. It can be used with a nasal mask, face mask, a full face mask, or a helmet. NIV does not protect the airways, and so the patient must be cooperative and protect their own airways. It can also not be used in severe gas exchange disorder. Some patients with chronic disorders may use a non-invasive ventilator continously or intermittently. If non-invasive ventilation is not successful in improving the condition of the patient, invasive ventilation is required.

The most common types or modes of NIV are continous positive airway pressure (CPAP) and bilevel positive airway pressure (BIPAP).

NIV involves the patient breathing "against" some pressure, like breathing against headwind. This can be uncomfortable, and many patients do not tolerate it.

Indications

Patients with OSA and obesity hypoventilation syndrome typically use a CPAP only at night. Those with neuromuscular disorders may use a BIPAP continously or intermittently during the day and night, depending on the severity.

Types

The most common types or modes of NIV are CPAP and BIPAP. Some ventilators can provide both types while some can only provide CPAP.

CPAP (continous positive airway pressure) provides, like the name suggests, a constant positive pressure in the airways throughout the whole respiratory cycle (both inspiration and expiration). This positive pressure is higher than the atmospheric pressure (usually 3 - 8 cmH2O), and it can be adjusted.

BIPAP (bilevel positive airway pressure) provides a positive pressure in the airways throughout the respiratory cycle, like CPAP, but the pressure is not constant throughout the cycle but rather depends on whether the patient is currently expiring or inspiring. The ventilator will detect whether the patient is expiring or inspiring and change the pressure accordingly. During inspiration, the machine will provide a higher airway pressure during inspiration (inspiratory positive airway pressure, IPAP) and a lower pressure during expiration (expiratory positive airway pressure, EPAP). Usual starting setting is IPAP 10 cmH2O and EPAP 5 cmH2O, usually written as 10/5 cmH2O.

Physiology

NIV provides many physiological benefits:

  • Positive airway pressure prevents alveoli from collapsing (atelectasis) at the end of expiration, thereby recruiting more alveoli
  • Increases the diameter of the smaller airways
  • Reduces work of breathing
  • Reduces left ventricular afterload

Contraindications

For a patient to use NIV, they must be awake and able to protect their own airways. If a patient is unconscious, vomiting, have upper airway obstruction, untreated pneumothorax, or is critically ill, NIV is contraindicated.

Complications

The sensation of breathing against pressure is uncomfortable, and so NIV can lead to anxiety and claustrophobia. If hypovolaemic, the patient may become hypotensive.

Invasive ventilation

Invasive ventilation is a form of mechanical ventilation which requires endotracheal intubation or, if ventilation is required long-term, a tracheostomy. It is more effective than NIV, but it’s more invasive and can therefore increase the risk for ventilator associated pneumonia (VAP). It is difficult to wean people off invasive ventilation and back on spontaneous ventilation. It may take weeks.

Intubation and invasive ventilation is usually the last resort of respiratory therapy. It's only used if regular oxygen therapy or non-invasive ventilation is insufficient, although they don't necessarily have to be tried before proceeding to invasive ventilation. The only way to properly protect an airway is by intubation.

Indications

Modes

The ventilator can either be in pressure control mode or volume control mode. In volume control mode, the operator sets a tidal volume for the patient, and the machine selects the pressure required to achieve that tidal volume. Pressure control mode is the opposite. There are advantages and disadvantages of each mode.

A ventilator can also provide positive end-expiratory pressure (PEEP), where it keeps a positive pressure in the airways after the expiration to prevent alveolar collapse (atelectasis) during expiration. A small amount (3 – 5 cmH2O) of PEEP is used in most ventilated patients. This has other advantages as well but also some disadvantages:

  • Advantages
    • Prevents atelectasis
    • Increases functional residual capacity (FRC)
    • Increases gas exchange area
    • Increases compliance
    • Decreases preload
    • Decreases afterload
    • Decreases V/Q mismatch
    • Decreases work of breathing
  • Disadvantages
    • Decreased CO2 elimination
    • May decrease cardiac output in right heart failure or hypovolaemia
    • Increases ICP
    • Increases intrathoracic pressure -> may cause PTX

Complications

Ventilator-induced lung injury (VILI) refers to injury of the lung due to the ventilator. It’s usually avoidable when using appropriate and proper settings and parameters. This can occur in the form of barotrauma (excessive pressure causes rupture of alveoli), volutrauma (excessive volume causes overdistension of alveoli), biotrauma (release of inflammatory mediators), and atelectrauma (repeated opening and closing of alveoli).