High flow oxygen therapy

There has been growing interest in alternatives to conventional oxygen therapy: CPAP, BiPAP and the heated, humidified high flow nasal cannula oxygen therapy (HFNC), optiflow and airvo.

CPAP Continuous positive airway pressure (CPAP) is a form of positive airway pressure ventilator, which applies mild air pressure on a continuous basis to keep the airways continuously open.

BiPAP BiPAP (also referred to as BPAP) stands for Bilevel Positive Airway Pressure, and is very similar in function and design to a CPAP machine (continuous positive airway pressure).

One of the complaints about CPAP devices is that some patients find the constant singular pressure difficult to exhale against. For patients with higher pressure strengths, exhaling against the incoming air can feel difficult, as if they're having to force their breathing out. BiPAPs can be set to include a breath timing feature that measures the amount of breaths per minute a person should be taking. If the time between breaths exceeds the set limit, the machine can force the person to breath by temporarily  increasing the air pressure. The main difference between BiPAP and CPAP machines is that BiPAP machines have two pressure settings: the prescribed pressure for inhalation (ipap), and a lower pressure for exhalation (epap). The dual settings allow the patient to get more air in and out of their lungs.

AIRVO and OPTIFLOW Are a humidifier with integrated flow generator that delivers high flow warmed and humidified respiratory gases to spontaneously breathing patients through a variety of patient interfaces. (N.P. face mask)

For years, supplemental oxygen administration provided by different devices (such as nasal prongs, nose masks and face masks), has been the first line treatment for hypoxemic respiratory failure. However the oxygen provided by these conventional systems has several limitations. These limitations do not usually have clinical consequences because the delivered oxygen flow is sufficient to correct the hypoxemia.

However, in some patients there can be serious problems However, in some patients there can be serious problems. For example, poor tolerance because of insufficient humidification and heating of the oxygen flow or the fact that the oxygen flow supplied by these devices generally is no more than 15 L/min (the maximum flow delivered by facemasks).

Another drawback of conventional oxygen devices is the difference between the oxygen flow delivered and that the exact amount of the patient's inspiratory flow is not precise; it can vary between 30 and 120 L/min during respiratory failure. This means that the proportion of humidified and oxygenated inspired gas can be very small (below 10%) depending on the extent of oxygen dilution with room air.

One direct consequence is that the fraction of inspired oxygen (FiO2) is not constant during conventional oxygen therapy and it is also unknown.

2 quick need to knows Functional Residual Capacity (FRC) is the volume of air present in the lungs at the end of passive expiration. Positive end-expiratory pressure (PEEP) is the pressure in the lungs (alveolar pressure) above atmospheric pressure (the pressure outside of the body) that exists at the end of expiration.

How it all works- CPAP Positive end expiratory pressure (PEEP) refers to the application of a fixed pressure at the end of a ventilatory cycle. PEEP has been used as a method to improve oxygenation as an adjunct to mechanical ventilatory support for several years. The major benefit PEEP is achieved by its ability to raise functional residual capacity (FRC) above the level at which alveolar closure occurs (closing capacity). The increase FRC achieved by PEEP is accomplished by raising alveolar volume and through the recruitment of alveoli that do not normally contribute to gas exchange. The major effect of increasing FRC is an increase in oxygenation and lung compliance.

Continuous positive airway pressure (CPAP) is very similar to PEEP. CPAP refers to the addition of a fixed amount of positive airway pressure to spontaneous respirations. CPAP does not require the use of a mechanical ventilator as it can also be applied to a non-intubated patient via a face / nasal mask and appropriate respiratory circuit. This means that an increase in FRC and associated improvement in oxygenation and lung compliance can be achieved in non-intubated patients.

While both PEEP and CPAP may improve oxygenation and lung compliance they do not provide or actively assist in ventilation. Ventilation refers to the movement of gas into a person’s lung. In the past, the most common method of providing or augmenting ventilation has been through the use of a mechanical ventilator via an endotracheal or tracheostomy tube. More recently non-invasive ventilation (NIV) has been able to be provided with a BiPAP machine via a face or nasal mask.

While once the domain of intensive care units, both CPAP and NIV are currently being applied within many areas of the hospital and even in the patient’s home or community setting. For example, CPAP may be used in the patient’s home to prevent sleep apnoea, whereas NIV might also be used in the community setting to reduce the work of breathing for patients with chronic airways limitation.

Non-reservoir CPAP can be provided with a high flow system. The gas flow required is very high (up to 120 l/min). The gas flow source may be either a high flow meter or a flow generator (venturi). The system is generally noisy and is difficult to effectively humidify gas because the high flows exceed the humidifier capabilities.

Physiological Responses to CPAP / PEEP Fluid retention and diminished urinary output are commonly observed in patients receiving CPAP/PEEP, particularly in conjunction with mechanical ventilation. Mechanical ventilation and PEEP increase the production of antidiuretic hormone, decrease mean renal artery perfusion pressure, redistribute perfusion from the cortex, reduce urine flow, reduce creatinine clearance and diminish fractional excretion of sodium.

PEEP and CPAP may decrease cardiac output and mean arterial blood pressure through a decrease in venous return and hence ventricular filling. In patients with poor left ventricular function and pulmonary oedema the addition of CPAP or PEEP may improve cardiac output through an improvement of stroke volume.

Non Invasive Ventilation (NIV) or BiPAP Bi Level Positive Airway Pressure (BiPAP) is a form of non-invasive ventilation designed to augment the patient’s tidal volume while reducing the patient’s inspiratory effort, allowing a reduction in workload.

In BiPAP the operator sets an inspiratory and expiratory positive airway pressure. The difference between the inspiratory and expiratory pressure determines the amount of inspiratory assistance that the patient is receiving. The expiratory pressure determines if and how much CPAP the patient is receiving.

The main difference between CPAP and Bi-PAP is that Bi-PAP augments the patient’s tidal volume through the application of an elevated inspiratory pressure. One of the advantages of Bi-PAP is that it can assist the patient’s inspiratory effort and reduce the work of breathing and muscle fatigue.

What Is Humidity? Humidity is water as vapour in a gas mixture. The amount of water present as vapour is dependent upon the gas temperature. Increasing gas temperature increases the ability of the gas to hold water.

Aims of Humidification Maintain mucociliary function Prevent heat gain or loss Prevent airway obstruction

“Inspired humidity can alter lung mechanics by directly affecting airway patency and lung compliance. Extremes of humidity can compromise airway patency by either altering the viscosity of tracheobronchial secretions, delivering excess water, slowing mucociliary clearance, or causing oedema or bronchoconstriction in asthmatic patients.

Physiological effects of high flow therapy (HFNC) -AIRVO pharyngeal dead space washout reduction of nasopharyngeal resistance positive expiratory pressure effect alveolar recruitment greater humidification more comfort and better tolerance by the patient better control of FiO2 and mucociliary clearance

Pharyngeal dead space washout The main effect of delivering high flow oxygen directly into the nasopharynx is to wash CO2 and reduce CO2 rebreathing. This allows the dead space to decrease and increases alveolar ventilation.

Reduction of nasopharyngeal resistance The design of the nasopharynx facilitates humidification and warming of inspired gas by contact with the large surface area. The nasopharynx has a distensibility that contributes to a variable resistance. HFNC minimizes the inspiratory resistance associated with the nasopharynx by providing nasopharyngeal gas flows that match or exceed a patient's peak inspiratory flow. This change in resistance translates into a decrease in the resistive work of breathing.

Positive expiratory pressure (PEEP effect) Physiologically a positive airway pressure effect, generated by high flow oxygen, provides a certain level of pulmonary distending pressure and alveolar recruitment. there is a degree of CPAP generated with the HFNC therapy, which is flow dependent and also dependent on whether the person is breathing with mouth open or closed. Nasal high flow oxygen is not proposed as an alternative to continuous positive airway pressures or noninvasive ventilation, where controlled pressures are indicated, however, the nasal high flow oxygen might provide a bridge to these therapies in selected patients.

Alveolar recruitment effect The positive airway pressure effect provides a certain level of pulmonary distending pressure and alveolar recruitment A part of the improvement in oxygenation observed in patients with acute respiratory failure (ARF) is due to alveolar recruitment.

Humidification and tolerance Some studies have found better comfort, tolerance and oxygenation and lower respiratory rate with HFNC. Active humidification improves mucociliary function, facilitates secretion clearance and decrease atelectasis formation which improves the ventilation-perfusion ratio and oxygenation.

Better control of FiO2 and better mucociliary clearance Included the ability to more accurately control the patient's FiO2. Primary mechanical pulmonary defence mechanisms are sneezing, coughing, gagging and the use of natural filters, i.e., nasal hairs. The second line of defence is the mucociliary transport system which traps and neutralizes inhaled contaminants (in mucus) and transports them up and out of the airway, keeping the lung free from infection-causing pathogens. This critical defence system is very sensitive to humidity. Loss of humidity can be a problem in itself.

Clinical uses of HFNC Acute respiratory failure Postextubation period Preintubation Bronchoscopy and others invasive procedures Palliative care Acute heart failure Chronic airway disorders

HFNC (AIRVO/OPTIFLOW) could be used as an intermediate therapy to improve oxygenation in adult critical care patients. BiPAP and CPAP is used more often as a core therapy in the management of patients with acute and chronic respiratory failure.

Contraindications Heliox therapy in combination with NIV for severe exacerbation of COPD (Heliox is a low density gas mixture of helium and oxygen. Heliox reduces air flow resistances within the bronchial tree and in patients with obstructive lung diseases Heliox may also reduce the work of breathing and improve pulmonary gas exchange efficiency.) Life-threatening hypoxemia (PaO2 <60mmHg on iO2 100%) 3 CPAP in acute lung injury (ALI) Respiratory arrest Untreated pneumothorax Life-threatening dysrhythmias Inability to protect own airway Copious, unmanageable respiratory secretions Facial burns/trauma/recent facial or upper airway surgery

Resources http://www.elsevier.pt/en/revistas/revista-portuguesa-pneumologia-320/artigo/clinical-evidence-on-high-flow-oxygen-therapy-and-90226465 http://www.alaskasleep.com/blog/what-is-bipap-therapy-machine-bilevel-positive-airway-pressure https://en.wikipedia.org/wiki/Continuous_positive_airway_pressure https://www.fphcare.co.nz/getattachment/f4fb5e2f-3c35-4fde-81df-ff9de1d5d3cc/looking-for-the-home-verison/?width=620 http://www.aci.health.nsw.gov.au/__data/assets/pdf_file/0005/220658/mechanical_ventilation_workbook_tweed.pdf