Weaning
is a systematic, gradual reduction in ventilatory support using a variety of different methods that allow a patient to take over the work of breathing (WOB).
The number one criteria in considering reductions in and withdrawal of ventilatory support is
the reversal of the cause that initiated the onset of mechanical ventilation.
There are four broad major indicators for the initiation of mechanical ventilation:
Apnea
Acute ventilatory failure
Impending ventilatory failure
Severe oxygenation defects
Other factors that impact the strength of the respiratory muscles include: (besides diaphragm)
Disease
Disuse
Hypoxia
Electrolyte imbalances
Respiratory muscle fatigue is evidenced by:
Tachypnea
Abnormal respiratory movements
Increased arterial carbon dioxide (Paco2) due to inadequate ventilation
-A patient with respiratory muscle fatigue is at a high risk for respiratory failure.
Which of the following factors encountered during mechanical ventilation may result in weakened respiratory muscles and an impaired ability to breathe spontaneously?
A.Increased O2
B. Electrolyte imbalance
C. Use of assist control (AC) mode of ventilation
D. Use of synchronized intermittent mandatory ventilation (SIMV) mode
E. Inadequate nutritional intake
B, C, D, E
Respiratory load
is the WOB imposed on the respiratory muscles. Several different factors contribute to an increased respiratory load, which may prevent or otherwise make weaning from mechanical ventilation difficult.
- Minute ventilation
- Increased Resistance load
- Increased Elastic load
can be increased by pain and anxiety associated with illness or mechanical ventilation itself.
Ve
- Sepsis
- Deadspace: Increased dead space, where air in the lungs is unable to undergo gas exchange, can be caused by disease or factors relating to the ventilator circuit.
- Excessive feeding: can cause an increase in carbon dioxide production and a subsequent need for an increased V̇e.
Sepsis results in
an increased metabolic demand, with increases in oxygen consumption and carbon dioxide production leading to an increased V̇e.
Increased resistive load
Bronchospasm narrows the airways, creating resistance to inspired gas flow and increasing the WOB.
Secretions in the airway also contribute to narrowing the airway lumen and potentially blocking airflow to portions of the lungs.
The use of artificial airways in general increases resistance to flow through the narrow tubes.
Increases elastic load
Low lung compliance (CL) increases the elastic load, requiring higher levels of pressure generation to inflate the lung.
Low chest wall compliance (CCW) increases the elastic load by impairing the distensibility of the underlying lungs.
Auto-PEEP can increase the elastic load by elevating airway pressure and raising the trigger threshold, making it difficult to trigger a breath.
Respiratory Capacity
is the body’s capability to perform the effort required to overcome the resistive load to breathing.
- Decreased Respiratory drive
- Neuromuscular disturbance
- Thoracic wall abnormality
Decreased respiratory drive
A decreased drive to breathe is common during mechanical ventilation due to the sedatives and other medications given to maximize patient comfort.
Patients with injuries to the brain stem may not have a normal physiologic drive to breathe or respond appropriately to increased demands.
Neuromuscular disturbance
A host of different potential neuromuscular disturbances can disrupt the drive to breathe or response to increased need. Cervical spine or phrenic nerve injury may delay or prevent electrical impulses from signaling the diaphragm to contract, leading to potential apnea or hypoventilation
Critical illness weakness results in generalized weakness that impacts the skeletal muscles of the body, including the diaphragm.
Malnutrition and electrolyte imbalances can weaken the respiratory muscles, preventing them from handling the WOB.
Neuromuscular diseases such as amyotrophic lateral sclerosis (ALS) and muscular dystrophy (MD) have varying effects on the strength of the respiratory muscles.
Thoracic Wall abnormalities
Abnormalities of the thoracic wall, such as flail chest and the pain associated with breathing, may lead a patient to guard their breathing and fail to respond to an increased need for ventilation.
Strategies to minimize the respiratory load during and after mechanical ventilation include:
- Use of medications to treat pain, anxiety, and bronchospasm to reduce the need for high a V̇e
- Nutritional approaches that minimize the amount of excess carbon dioxide production
- Secretion clearance therapy and airway suctioning to reduce airway resistance
- Use of pressure support to provide gradually decreasing levels of ventilatory support
One method of evaluating the strength of the respiratory muscles is measuring
Maximum inspiratory pressure (MIP) or negative inspiratory force (NIF)
- Using an aneroid pressure manometer and a one-way valve, the patient maximally exhales and then forcefully inhales as hard as they can.
- Just prior to inhalation, the valve closes and completely obstructs the airway, preventing inhalation while measuring the negative pressure generated by the diaphragm and other respiratory muscles.
normal MIP
More negative than -20
Secretions increase the what instead of the what
load not the capacity
Which of the following are potential strategies for balancing respiratory muscle load and capacity? A. Airway Suctioning B. Use of Humidity C. Use of pressure support D. Use of bronchodilator E. Optimizing nutritional status
A, C, D, E
Adequate oxygenation
Pao2/Fio2 ratio greater than 150 to 200
PEEP of 5 to 8 cm H2O
Fio2 less than or equal to 40% to 50%
pH greater than 7.25
Hemodynamic stability:
Absence of acute myocardial ischemia
Absence of significant hypotension
No need for low-dose vasopressor therapy
benzodiazepines
Sedative that could dull respiratory drive,making a patient less responsive to elevations in carbon dioxide or decreases in oxygenation.
-so it is important to assess the patient’s sedation level prior to initiating any weaning attempts.
There are several different ways to measure spontaneous parameters such as
Vt, V̇e, and VC. One method of measuring spontaneous volumes is to disconnect the ventilator and connect the artificial airway to a respirometer, allowing the clinician to directly measure VT and V̇e without the application of ventilatory support.
- Another method involves switching the ventilator to a spontaneous mode and turning all support settings to zero.
- Many ventilators are equipped with packages that facilitate measurement of parameters such as MIP and the rapid shallow breathing index (RSBI), among others.
