Respiratory therapy

Respiratory therapy

Goals of oxygen therapy

· increase in alveolar oxygen tension

· reduction of work of breathing necessary to maintain a given arterial oxygen tension

· reduction in myocardial work necessary to maintain a given arterial oxygen tension

Treatment of hypoxemia

When arterial hypoxemia is the result of decreased alveolar oxygen tension, it can be dramatically improved by increasing FI02.

Decreasing work of breathing

Increased ventilatory work is a common response to hypoxemia and /or hypoxia. Enriched inspired 02 atmospheres may allow a more normal alveolar gas exchange to maintain adequate alveolar 02 levels. The result is a decrease in the work of breathing at no expense to the oxygenation status.

Decreasing myocardial work

The cardio vascular system is the primary mechanism for compensation and /or hypoxia therefore oxygen therapy can support effectively many disease states by decreasing and preventing the demand for increased myocardial work

Gas delivery systems

Non_rebreating system

Designed so that exhaled gases have minimal contact with inspiratory gases by venting exhaled gases to the atmosphere via one way valves.

The primary advantage to the non-rebreathing system is that exhaled C02 does not have to be dealt with in the inspiratory gas system. However the gas flow should be sufficient to meet the requirements of the minute volume and the peak flow rate. This is achieved by an inspiratory reservoir, which allows an additional amount of gas to be available during the transient times when inspiratory demands are beyond the capabilities of the uniform flow rates delivered by the apparatus (high flow systems).

Re-breathing systems

In this breathing system a reservoir exists on the expiratory line enabling absorption of the C02 so that exhaled air minus the C02 can re-enter the inspiratory system.

Oxygen delivery devices

The oxygen concentration depends on

· delivery device

· flow rate of oxygen

OXYGEN FLOW RATE. APPROX. Fi02

6l/min 0.50%

8l/min 0.55%

10l/min 0.60%

12l/min 0.65%

Venturi masks

These masks are called fixed performance oxygen delivery systems and are used in chronic respiratory failure where the patient may be sensitive to high oxygen concentrations

OXYGEN FLOW RATE. APPROX. Fi02

2l/min 24%oxygen

4l/min 28%oxygen

8l/min 35%oxygen.

Inspiron nebulizers

This is a cold water nebulizing device for humidification, which can give inspired 0 2 of 35%, 40%, 50%, and 70 %. The oxygen flow must be set at 10l/min. Inspirons are commonly used on self-ventilating tracheostomy patients.

High flow oxygen

Commonly used for patients requiring minute ventilation > 10 l/m. This mode is provided via the Draeger CPAP machine utilising Fisher Paykel humidification and a disposable CPAP circuit.

· slide the humidification chamber onto humidifier base

· hang waterbag at least 50cm above the chamber

· connect the circuit short blue tube from chamber to CPAP port, long blue inspiratory limb to humidification chamber. Remove the white expiratory limb and Y piece and attach aeresol mask to the free end of the long blue inspiratory limb.

· connect the temperature probes

· turn the humidifier on

· select mode for mask patients

Pulse oximerty

Pulse oximetry is a non-invasive technology used to estimate oxyhaemoglobin saturation. It is used for continuous non-invasive measurement of arterial oxygenation saturation. Most of the oxygen transported by the blood is bound to the haemoglobin, and the degree of binding (the saturation) is determined by the percentage of haemoglobin that is loaded with oxygen.

The pulse oximeter detects and calculates the absorption of light by functional haemoglobins to produce a measurement ( Sp02) that is an estimate of arterial oxygen saturation (Sa02). The pulse oximeter probe contains two light emitting diodes on one side, which emit two wavelengths of monochromatic light; - red and infra-red, and a photo detector on the other side. The two diodes are cycling on and off 400 to 480 times per second one at a time. This enables a single detector to be used to sample first one wavelength and then the other.

Venous blood and tissues also absorb both wavelengths of light. The haemoglobin absorption of light is analysed over a full pulse beat to make the saturation measurement independent of these factors. The total absorption of light has a constant component from the tissue and venous blood, and a changing component from arterial pulsation. The constant component is subtracted from the total, so that the net absorption of each wavelength can be attributed to the arterial blood only.

A calculation based on previously determined calibration curves is used to relate this transcutaneous light absorption to directly measured Sa02. To estimate Sp02 with a wide range of pulse amplitudes, the pulse oximeter automatically increases its amplifications as the signal decreases. The saturations that are displayed are not instantaneous but are averages taken over 3 to 10 seconds, to help reduce the effect of pressure wave variations due to motion . Pulse oximetry measures only the percentage of haemoglobin that is carrying oxygen and therefore does not provide

specific information about the patient’s overall level of haemoglobin, adequacy of ventilation, or how well the oxygenated haemoglobin is being delivered to the tissues.

When should pulse oximetry be used?

Any patient who is at risk for hypoxemia should be monitored principally because desaturation is detected earlier with pulse oximetry than with clinical observations.

What is the best location for the pulse oximeter probe?

Choose a site with the best pulsatile vascular bed: the finger, toe, earlobe, and the bridge of the nose have been used. The finger provides the best overall performance, however the earlobe, which is the least vasoactive site and the least susceptible to signal loss, may show faster response and greater accuracy during periods of vasoconstriction and hypotension. When placing the probe on fingers or toes remove nail polish, especially blue, black, green brown, red, or synthetic nails; or place the probe sideways on the finger. Synthetic nails and some colours of nail polish may result in errors of 3% to 6%. There should not be impedance to the blood flow of the extremity where the sensor is placed. The sensor should be placed on the extremity opposite arterial lines and non-invasive blood pressure monitoring devices so that pulsatile flow is not interrupted.

What are the normal parameters and what should be assessed with pulse oximetry?

Optimal SaP02 is greater than 95%, SaP02 less than 90% reflects hypoxia. SaP02 does not adequately monitor ventilation, and therefore in situations of ventilation or acid –base abnormalities arterial blood gases must also be monitored. Remember that measurements during hypoperfusion and vasoconstriction may not be accurate. Nursing care decisions should be based on SaP02 trends rather than isolated values. If SaP02 measurements and other oxygenation data conflict, obtain arterial blood gas to verify oxygenation status. Pulse oximetry should be included as only one part of a total oxygenation and ventilation assessment. Pulse oximetry indicates the amount of haemoglobin that is saturated with oxygen. It does not provide in formation about the amount of haemoglobin present, the adequacy of ventilation or the cardiac output. These must also be assessed on a regular basis.

Considerations in Sa02 measurements

· Sa0 2 measures percentage saturation and is not an absolute measure of oxygen content. A patient with low levels of haemoglobin (anaemia) may be carrying inadequate oxygen in their blood despite the existing levels of haemoglobin being well saturated

· changes in normal physiological temperature, pH, and PaC0 2 will affect oxygen binding to haemoglobin

· haemoglobin may be reversibly bound by inhaled carbon monoxide forming carboxyhaemoglobin. Drugs such as nitrates may create higher than normal levels of methamaglobin. Both carboxyhaemoglobin and methaemoglobin can contribute to Sa02 readings, but may not actually carry oxygen

· the accuracy of the probe measurements may be affected by powerful light source shining above the probe, excessive tremor or shivering, dark skin colouration. and nail polish.A state of shock or hypothermia with significant vasoconstriction will also contribute adversely

Pulse oximetry

Circumstances in which pulse oximetry is recommended.

Patients at risk for hypoxemia.	Desaturation is detected earlier by pulse oximetry than by clinical observation.
During anaesthesia both adults and children.	Standard practice during anaesthesia; its use increases detection of hypoxemia.
After anaesthetic, in both adults and children.	Standard practice following anaesthetic because of hypoxemia due to the effects of anaesthetics, sedatives, relaxants, and opoid drugs is common.
In adults and children who require critical care, especially patients who have marginal or fluctuating oxygenation or high Fi02 or are sedated.	Use of pulse oximetry in these unstable patients enables early detection of and reduction of serious consequences of hypoxemia.
During invasive procedures: placement of central lines, bronchoscopy, endoscopy and cardiac catheterisation.	Risk of hypoxemia increases with the trendelenburg position, use of drapes over the face, length of procedure, and use of sedatives and analgesics during the procedure.
During weaning from mechanical ventilation, use of CPAP and oxygen titration.	Pulse oximetry can be used in addition to analysis of arterial blood gases during weaning from mechanical ventilation.
In patients and children during transport from operating room to recovery and other areas of hospital.	Significant hypoxemia can occur during transport.
In adults and children who are receiving inotropes, vasopressors, vasodilators, sedatives or analgesics.	Vasoactive medications can cause changes in oxygenation that are detected more readily by pulse oximetry than other measures.