Ventilation is the movement of air into and out of the lungs. Ventilation carries oxygen to provide tissue perfusion and removes carbon dioxide which accumulates from aerobic metabolism. Sometimes, especially within the critical care setting self-ventilating becomes difficult or impossible for the patient. This is where mechanical ventilation is introduced – to provide artificial control or support during each breathing cycle through the use of a machine, namely a ventilator.
A ventilator typically has the following colour-coded pipes:
- white pipe – oxygen at 100%
- white and black pipe – compressed air at 21%
- yellow pipe – suction
These pipes have the following features:
- uncrushable – cannot be crushed if stepped over etc
- pin indexed – one pipe cannot be inserted by mistake into a different socket by mistake
- tug test – may tug oxygen hose after the probe is plugged into wall or cylinder socket to ensure it is firmly attached
- internal battery
- electricity supply
Mechanical Ventilation Indications
Before mechanical ventilation takes place, clinical judgement has to ascertain that such an intervention would be providing improved quality of life whilst lowering the mortality risk of the patient. Thus, mechanical ventilation should be opted for…
- when the patient’s own ventilation mechanism is unable to sustain life
- when the critically ill patient needs ventilation control
- when there is the risk of impending collapse of physiological functions, in which case mechanical ventilation is opted for as a prophylactic measure
(Byrd et al., 2006)
Mechanical Ventilation helps the critically ill patient by:
- improving alveolar ventilation and oxygenation
- decreasing the required breathing effort and oxygen consumption
- reversing hypoxaemia (low level of partial pressure oxygen in the blood)
- reversing acute respiratory acidosis (too much carbon dioxide leading to an acidic base)
- enabling sedation and muscle relaxation eg. prior to surgery
Specific indications for mechanical ventilation include:
- apnoea
- impending respiratory arrest
- heart failure
- pulmonary oedema
- pneumonia
- sepsis
- chest trauma
- surgery complications
- ARDS (Acute Respiratory Distress Syndrome – a life-threatening condition with which the lungs are unable to provide the body’s vital organs with enough oxygen)
- COAD acute exacerbation (Chronic Obstructive Airway Disease – a long term lung disease a.k.a. chronic bronchitis or emphysema, or the latest term COPD)
- neuromuscular disorder
- acute brain injury – mechanical ventilation almost always required since it controls the level of air that is exchange; the more carbon dioxide, the more vasodilation and the more blood pooling in the brain
- coma
Mechanical Ventilation aims to prolong life, not prolong death…
Negative Pressure Ventilation
Air moves from one area to the other due to the difference in pressure a.k.a. pressure gradient. Spontaneous breathing happens through the generation of negative pressure. Negative pressure ventilation increases the normal physiological breathing pattern by producing a negative pressure outside the chest wall, which then causes the air to be automatically inhaled when the patient opens the airway.
The Iron Lung vs Today’s Negative Pressure Ventilation Equipment
The Iron Lung, which was used extensively for patients up to the mid 1950’s, was a large airtight metal cylinder which enclosed patients fully, exposing only their head and neck. It worked through an electric pump which generated negative pressure, causing the patient’s chest to rise.
Modern Negative Pressure Ventilation equipment comprises of airtight jackets or flexible canopies a.k.a. cuirass, which cover the chest area only. They are available in oscillatory mode so as to assist with secretion clearing.
Patients who benefit from such equipment include patients receiving home care who suffer from respiratory muscle group weakness, skeletal problems which restrict thoracic function, and patients with central hypoventilation syndrome.
Negative Pressure Ventilation Limitations
Within the critical care setting, it is difficult to achieve accurate pressure, volume and gas flow due to abnormal lung compliance and impaired airway control. Additionally, the seal required around the patient’s chest wall may lead to pressure sores. With regards to nursing care, invasive procedures and chest examinations become difficult to perform. And while a Negative Pressure Ventilator provides ventilation, it does not provide oxygenation.
(Ashurst, 1997)
Positive Pressure Ventilation
Through positive pressure ventilation, the normal pressure gradient is reversed as oxygenated air is forced into the patient’s lung by the ventilator, and as airway pressure drops, recoil of the chest causes passive exhalation by pushing out the tidal volume.
Routes for Positive Pressure Ventilation Delivery
INVASIVE ROUTES:
- oral or nasal endotracheal tube
- tracheostomy
NON-INVASIVE ROUTE:
This is achieved through the use of a NIV – non-invasive ventilator. Whilst this type of ventilation does not require sedation and it reduces the risk of nosocomial pneumonia, a NIV requires that the patient is conscious, breathing spontaneously and is compliant. It also requires the use of a tight-fitting face mask, nasal cannula or helmet.
NON-INVASIVE VENTILATION with CONTINUOUS POSITIVE AIRWAY PRESSURE (CPAP):
- PEEP – positive pressure is maintained throughout inspiration and expiration
- reduces breathing effort requirement
- improves oxygenation
- improves compliance
NON-INVASIVE VENTILATION with BI-LEVEL POSITIVE AIRWAY PRESSURE (BiPAP):
- 2 positive pressure settings include IPAP – inspiratory positive airway pressure, and EPAP – expiratory positive airway pressure
- increases tidal volume
- reduces PaCO2
- improves oxygenation
- reduces breathing effort requirement
Mechanical Ventilation Ventilator Variables
CONTROL: ventilator controls the pressure and the volume
TRIGGER: what starts off inspiration, where the flow, pressure or volume are generated by the patient, whist the time is triggered by the ventilator itself if the inspiration is not initiated by the patient (in other words, either the patient triggers inspiration, or the ventilator)
CYCLING: time, flow and volume trigger expiration
Volume Controlled Ventilation
In Volume Controlled Ventilation, a preset gas volume is forced into the lungs, whilst pressure is dependable on lung compliance.
Volume Controlled Ventilation ensures delivery of a constant tidal and minute volume, and is set based on the patient’s ideal body weight, height and sex.
However…
When a patient’s lung compliance decreases, an increased amount of pressure is required. Volume Controlled Ventilation will deliver the preset tidal volume with no regards to a patient’s airway condition change, which may lead to VILI (Ventilator-Induced Lung Injury)
Pressure Controlled Ventilation
In Pressure Controlled Ventilation, the lungs are inflated up to a preset pressure. The tidal volume is variable, depending on both lung compliance and resistance in breath delivery.
Pressure Controlled Ventilation helps prevent excessive airway pressure whilst reducing the risk of VILI (Ventilator-Induced Lung Injury).
However…
It does not guarantee minute volume, and this may lead to the patient experiencing hypoventilation leading to hypoxia.
To prevent this from happening, Volume Guaranteed Pressure Control Ventilation ensures that with each mandatory breath, a set tidal volume (VT) is applied with minimum pressure. Additionally, pressure adapts gradually to resistance and/or compliance changes so the set tidal volume is administered.
Mechanical Ventilation Ventilator Settings
FiO2 – FRACTION OF INSPIRED OXYGEN
FiO2 is the fraction of oxygen in each delivered breath. FiO2 should be set somewhere between 21% (0.21) and 100% (1.0). FiO2 is used to maintain oxygenation along with PEEP.
Oxygen toxicity risk increases when the patient’s dependency and duration on FiO2 are high. Additionally, oxygen metabolites may lead to tracheobronchitis (inflammation of the trachea and bronchi), absorptive atelectasis (loss of lung volume caused by the resorption of air within the alveoli), hypercarbia (increase in carbon dioxide in the bloodstream), lung fibrosis (lung tissue damage and scarring), and diffuse alveolar pulmonary membrane damage (changes which occur to the structure of the lungs).
PEEP – POSITIVE END-EXPIRATORY PRESSURE
Positive pressure is maintained throughout inspiration and expiration, thus, pressure is not allowed to drop to zero at the end of expiration. This prevents the alveoli from collapsing, improves oxygenation without increasing the FiO2, helps prevent oxygen toxicity, whilst increasing the availability of alveolar surface area for gaseous exchange.
However, PEEP should be used with caution in patients with either a head injury and/or poor cardiac output. This is because PEEP increases the intra-thoracic pressure and hence reduces venous return, therefore cardiac output is reduced, and intracranial pressure is increased.
Similarly, patients with COPD and/or asthmatic patients who are not able to completely exhale tidal volume should also receive PEEP with caution.
Mechanical Ventilation Modes
CMV – COntinuous Mandatory Ventilation
CMV delivers a preset number of breaths with a preset tidal volume or pressure. In CMV the patient’s inspiratory efforts make no difference since all settings are preset. Settings involved include TV, Inspiration Pressure, FiO2, PEEP and RR.
a/c – Assist / control Ventilation
In A/C ventilation, the patient can trigger the ventilator to deliver the breath. This type of setting has the ability to sense the natural negative pressure generated by the patient, delivering a breath with a set volume or pressure. However, if the patient does not trigger any breaths, a set number of breaths is still delivered.
simv – Synchronised Intermittent Mandatory Ventilation
In SIMV, a preset number of ventilator breaths per minute are delivered. Spontaneous breaths may be initiated by the patient at any point between ventilator breaths.
To augment the tidal volume of spontaneous breaths, pressure support is often used. Settings involved include a set rate, tidal volume, FiO2, with optional pressure support and PEEP.
NOTE: monitor total breathing rate, minute volume, and airway pressure.
PS/ spn-cpap: Pressure support ventilation
SPN refers to spontaneous mode of ventilation in which respirations are started and ended by the patient. SPN requires no preset rate and TV since both are determined by the patient, thus, need to be monitored well.
SPN may be combined with pressure support, where spontaneous breaths are aided by an extra push from the ventilator. Thus, if apnoea is detected, the ventilator starts providing backup mandatory ventilation.
BiPAP – Bi-Phasic Positive Airway Pressure
In BiPAP, the ventilator alternates between IPAP (Inspiratory Pressure) and PEEP. BiPAP provides mandatory breaths synchronised with the patient’s breathing attempts for both inspiration and expiration. With this setting in place, the patient can breathe spontaneously at any time, supported by pressure support. This helps reduce the need for patient sedation whilst improving oxygenation.
Complications of Mechanical Ventilation
Weaning Patient from Mechanical Ventilation
A patient on mechanical ventilation can be weaned off of the ventilator if he/she:
- is conscious and cooperative
- has an FiO2 of <50%
- has adequate minute ventilation
- is able to cough
- has minimal or clear secretions
- has no evidence of septic shock
- has an adequate fluid status
- has no significant acid-base or electrolyte imbalance
- has minimal vasopressor requirement
- is showing evidence of resolution of the primary reason which required mechanical ventilation
If the patient meets the criteria mentioned above, ventilatory support is decreased (mandatory breaths, FiO2 and Pressure Support), and replaced with spontaneous ventilation. Spontaneous breathing trials can be performed with the use of T-piece humidifier and flow inflated ventilation bags.
Whilst attempting to wean patient off of mechanical ventilation, it is very important to monitor for signs of respiratory distress!
Signs of respiratory distress include:
- tachypnoea
- tachycardia
- hypoventilation
- bradycardia
- hypertension
- hypotension
- hypoxaemia – SPO2 <90%
- agitation
- altered level of consciousness
- labored breathing
- use of accessory muscles of breathing
References
Ashurst, S. (1997). Nursing care of the mechanically ventilated patients in ITU: Part 1 and 2. British Journal of Nursing 6(8, 9): 447-454, 475-485.
Byrd, R.P., Eggleston, K.L., Hnatyuk, O.W. (2006). Mechanical Ventilation.
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