How to Set a Mechanical Ventilator?
A CPAP (continuous positive airway pressure) maskClick thumbnail to view full-size
Mechanical Ventilator-The machine which intimidates !!
Mechanical Ventilators are commonly used in intensive care units. It is the machine which gives breath when the patient cannot breathe on his/her own. But mechanical ventilators can be very intimidating to professionals who do not know fully about them. I would also like to mention here that it is a very vast topic with so many details if you want to learn about mechanical ventilators and its functioning. This hub is a quick review and describes briefly about the important points to remember if you are asked to take care of a patient on ventilator.
Indications for Mechanical Ventilation
What are the indications of Mechanical Ventilation?
* Respiratory failure: PaO2<50, PaCO2>50
* Respiratory Fatigue
What are the different Types of Mechanical Ventilation?
Positive-pressure ventilation- positive pressure is applied to the airways using endotracheal tube or face mask to inflate the lungs directly.
Negative-pressure ventilation- Air is drawn into the lungs by applying negative pressure to the abdomen and thorax (e.g.,Iron lung used in polio epidemics during the early 20th century). Negative pressure ventilation is NOT routinely used in hospitals now.
Understanding Ventilator Settings
What are the different Modes of Ventilation?
Controlled Mandatory Ventilaation
Assist (volume) control
Synchronous Intermittent mandatory ventilation
Assist control mode of ventilation ensures that the patient have a guaranteed minute ventilation (tidal volume X respiratory rate) with the minimum work of breathing,
Some other useful links on Mechanical Ventilation
How are the Initial Settings of a Mechanical ventilator determined?
The initial settings like the mode of ventilation, tidal volume, respiratory rate, FiO2 and the level of positive end expiratory volume (PEEP) are determined by the baseline condition of the patient. The patients at the time of intubation will usually have hypoxemia, hypoventilation, or increased work of breathing
Mode: Usually volume control mode is chosen initially. Assist (volume) control mode ensures certain amount of minute ventilation and the greatest reduction in work of breathing.
Tidal Volume: usually 8ml/kg. Higher tidal volumes are shown to cause lung injury especially in acute respiratory distress syndrome.
Respiratory rate: 12/mt A normal pH is maintained by adjusting the tidal volume and respiratory rate.
FiO2: 100% (FiO2-1)
PEEP: 5cm of H2O. Low level of PEEP (5 cm of H2O) is kept for a patient who otherwise maintains oxygenation on ventilator to prevent alveolar collapse. Higher PEEP is required in conditions like pulmonary edema and ARDS to recruit alveoli that are collapsed. In such patients a higher PEEP is a better option than higher FiO2 as higher FiO2 may cause further lung injury.
Hypoxemia is assessed by oxygen saturation (pulse oximetry). FiO2 and PEEP are adjusted to prevent hypoxemia.
Sedation using midazolam is indicated if the patient fights on ventilator.
Chest X ray has to be taken after intubation to check the position of the endotracheal tube. An arterial blood gas has to be done after half an hour of intubation. The peak airway pressure should be maintained below 35 to avoid lung injury.
Weaning from Mechanical Ventilator
Which of the following mode is not used for weaning off patient from Mechanical Ventilator?
What is the difference between Assist-control Mode of Ventilation and SIMV Mode of Ventilation?
Minimum minute ventilation (tidal volume X respiratory rate) is assured in both the modes by delivering a preset tidal volume with a preset respiratory rate. Beyond the preset respiratory rate if the patient initiates respirations they are managed differently. In SIMV, the additional patient initiated respirations are generated by the patient whereas in assist control, every additional patient-initiated respiration receives the preset tidal volume.
PEEP Valve for Ventilation
Accurately setting PEEP with transpulmonary pressure
Important points on Mechanical Ventilation
Tidal volume and respiratory rate are adjusted to maintain arterial pH in the normal range
Hypoxemia in a ventilator patient is prevented by adjusting FiO2 (fraction of inspired oxygen) and PEEP (positive end expiratory pressure).
Hypoxemia, is assessed by arterial O2 concentration (PaO2) or saturation of arterial oxygen (SaO2).
What is the difference between Pressure Control Mode and Pressure Support Mode?
- Pressure Control mode: is designed to limit high pressures inside the lungs. A set pressure is selected at the endotracheal tube (ET tube). Machine delivers a breath at set rate and tidal volume decided by pressure inside the lungs
Pressure Support: A set pressure is selected at the ET tube (e.g., 15 cm H2O), Patient’s effort determines breath rate and tidal volume. Pressure support mode works only in a conscious patient with an intact respiratory drive.
The patient generates inspiratory gas flow according to the inspiratory pressure set on the ventilator and when a certain threshold flow has been reached the cycle goes in for expiration. Since the tidal volume and the respiratory rate is determined by the patient’s effort, there can be variations in the minute volume which may predispose the patient for hypoventilation in severe underlying diseaseconditions.
Pressure support is often combined with SIMV because in SIMV additional breaths above the preset respiratory rate depend on the patient’s effort and a reduction in the work of breathing can be achieved by combining pressure support with SIMV. Increased work of breathing is imposed on the patient if he/she is with a small diameter endotracheal tube
Why do Patients Fight on Ventilator?
Machine disconnection- low o2, low tidal volume, auto PEEP
Reasons of the Lung-mucous, spasm, pneumothorax, pulmonary edema, decreased PaO2, increased PaCO2, collapse, embolism
Metabolic Reasons- Acidosis, sepsis, pain, out of sedation, alcohol, drug withdrawal.
If the patient fights on ventilator, correct the reason. Other options are to give bronchodilators, to try a bigger ET tube, to add external PEEP and to sedate judiciously.
What is Positive End Expiratory Pressure (PEEP)?; What are the benefits and risks of PEEP?
Normally airway pressure fails to atmospheric level in expiration. Adding positive pressure inside expiratory circuit of machine at the end expiratory phase prevents collapse of the alveoli and it is called PEEP.
Benefits of PEEP:
Prevent collapse of alveoli
Increased functional residual capacity
Increased ventilation perfusion
Risks of PEEP:
- Hypotension- Because of increased intrathoracic pressure, decreased venous return and the resultant decreased cardiac output there can be hypotension. Monitor BP, heart rate and urine output.
- Auto PEEP: In auto PEEP there is incomplete exhalation before delivery of a positive-pressure breath. It is common in patients with acute respiratory distress syndrome or asthma. Prolonging the expiratory time may help to some extent in reducing auto PEEP.
Do you feel intimidated now?
If you can keep in mind the points which we have discussed above you don't need to feel low. Having said that I would like to emphasize that if you are a health professional taking care of a patient on ventilator, you must know what we have discussed here and a little more...