Introduction
Most critically ill patients with mechanical ventilation develop diaphragmatic weakness. However, the exact cause of the disease is not known. It is a complication of ventilator-induced inactivity of the diaphragm. Diaphragmatic weakness may lead to respiratory failure and increase the time to wean from mechanical ventilation if left untreated. Diaphragmatic weakness or dysfunction is often underdiagnosed as it is asymptomatic. Bedside assessment of respiratory muscles improves the diagnosis of diaphragm weakness and helps the healthcare team prepare preventive strategies and pharmacological interventions.
What Is Diaphragmatic Weakness?
The respiratory system muscles are organized to facilitate inspiratory and expiratory airflow for respiration, speech, and airway clearance. The diaphragm is a dome-shaped, thin muscular structure below the heart and lungs, separating the thoracic and abdominal cavities. The diaphragm is the primary inspiratory muscle allowing the lungs to expand and contract during respiration. The pressure between the thoracic and abdominal cavities is called trans-diaphragmatic pressure (Pdi).
Diaphragmatic weakness is the inability of the diaphragm to generate the maximal force of normal limits. The action of the diaphragm is usually measured in terms of force generation or shortening. Approximately 80 % of patients with prolonged mechanical ventilation exhibit diaphragmatic weakness. Patients with paralyzed or weakened diaphragm have breathing difficulties and an inability to control voluntary breathing.
Types of Diaphragmatic Weakness:
The following are the two types of diaphragmatic weakness.
- Unilateral diaphragmatic weakness is caused when one side of the diaphragm is weak or paralyzed.
- Bilateral diaphragmatic weakness is caused when both sides of the diaphragm are weak.
What Are the Causes of Diaphragmatic Weakness?
Diaphragmatic weakness is caused by pressure or damage on the phrenic nerve, which controls the diaphragm. The diseases or conditions which damage the phrenic nerve include the following.
- Nervous system diseases like multiple sclerosis or ALS (amyotrophic lateral sclerosis).
- Cancers spread and compress the phrenic nerve.
- Cervical spine arthritis.
- Surgical damage to the phrenic nerve.
- Congenital central hypoventilation syndrome.
- Upper spinal cord injury that spreads to the phrenic nerve.
The causes of diaphragmatic weakness in critically ill patients include the following:
- Hyperinflation.
- Neuropathies.
- Myopathies.
- Metabolic abnormalities.
- Decreased oxygen delivery.
- Medications.
- Sepsis before ICU admission.
What Are the Mechanisms of Diaphragmatic Weakness During ICU Admission?
The mechanisms of diaphragmatic weakness during the time of ICU admission include the following.
Hypercapnic Respiratory Failure:
- Acute hypercapnia (excess carbon dioxide in the blood) increases the neural drive and affects the contractility of the diaphragm.
- Diaphragm force is reduced when there are acute changes in carbon dioxide.
Cardiogenic Shock:
- Cardiogenic shock is associated with reduced diaphragmatic oxygen delivery and increased diaphragmatic work.
- The imbalance between oxygen supply and demand results in diaphragmatic weakness or fatigue.
Sepsis:
- Sepsis is associated with diaphragmatic weakness in most mechanically ventilated patients.
- Diaphragmatic force is impaired due to sepsis.
Surgery:
- Surgeries in the upper abdomen can cause post-operative complications such as diaphragm dysfunction.
- Injuries to the phrenic nerve during surgeries such as coronary bypass surgery lead to unilateral or bilateral diaphragmatic weakness.
What Are the Risk Factors Associated With Diaphragmatic Weakness in ICU?
The risk factors associated with diaphragmatic weakness in the ICU include the following.
Sedatives, Steroids, and Neuromuscular Blocking Agents:
- Sedatives such as Propofol decrease the diaphragms' capacity to generate pressure.
- High doses and longer duration of steroids and neuromuscular blocking agents can cause diaphragm weakness in ICU patients.
Mechanical Ventilation: Diaphragmatic weakness due to mechanical ventilation is due to the following three mechanisms.
- Disuse atrophy due to diaphragm inactivity from excessive ventilator support.
- Load-induced injury due to insufficient ventilatory support.
- Hypercapnia under controlled mechanical ventilation.
What Are the Symptoms of Diaphragmatic Weakness?
The diaphragm plays an essential role in breathing. Therefore, the symptoms of diaphragmatic weakness are more apparent when it is bilateral. The symptoms include the following.
- Breathing difficulties (during rest and active state).
- Difficulty breathing.
- Fatigue.
- Recurrent pneumonia.
- Difficulty in breathing while lying down in cases of unilateral diaphragmatic weakness.
How to Diagnose Diaphragmatic Weakness?
The diagnostic tests used to detect diaphragmatic weakness include the following.
- Diaphragm electromyography (EMG) assesses the diaphragm activity and is also used for bedside monitoring of diaphragm activity.
- Diaphragm ultrasound helps to visualize the diaphragm directly and detect changes.
- Maximal inspiratory pressure. Measurement of inspiratory pressure at the mouth is used to test respiratory muscles' functions.
- Lung function testing is the first line of the assessment tool to detect diaphragmatic weakness.
- Fluoroscopy provides a dynamic evaluation of diaphragmatic weakness. It is more commonly used for the detection of unilateral diaphragmatic weakness.
- Ultrasonography is a simple, non-invasive bedside procedure for evaluating the diaphragm.
- Sleep studies detect sleep-disordered breathing and unilateral diaphragmatic weakness.
- Cardiopulmonary exercise training quantifies the level of respiratory impairment.
What Are the Preventive and Treatment Strategies for Diaphragmatic Weakness?
The preventive and treatment strategies for diaphragmatic weakness include the following:
- Inspiratory muscle training (IMT) with resistive or threshold loading increases the maximal inspiratory pressure.
- Continuous positive airway pressure (CPAP) for sleep-disordered breathing improves respiration.
- Non-invasive bi-level airway pressure is used for treating more severe diaphragmatic weakness.
Treatment for Unilateral Diaphragmatic Weakness:
- Asymptomatic patients do not require treatment. However, diaphragmatic surgical plication is indicated for patients with dyspnea. In this procedure, the weak diaphragm is immobilized by surgically created folds. This reduces paradoxical movements during breathing, further decreasing the workload and fatigue of the diaphragm.
Treatment for Bilateral Diaphragmatic Weakness:
- Bilateral diaphragmatic weakness improves with intrinsic muscle training.
- Non-invasive positive pressure ventilation (NPPV) is the treatment for patients with chronic respiratory failure. It improves the rate of lung function, quality of life, and survival.
- Diaphragm pacing is indicated for patients with bilateral diaphragmatic weakness who are highly ventilator-dependent.
Preventive Strategies:
Prevention of disuse atrophy:
- Spontaneous breathing is preferred for maintaining inspiratory efforts.
- Phrenic nerve pacing (not recommended).
The pharmacological approach involves the use of antioxidants (N-acetylcysteine).
Conclusion
Diaphragmatic weakness is frequent in the intensive care unit as many factors influence the diaphragm force in the ICU. It is challenging for physicians to balance mechanical ventilation strategies and diaphragm activities. Therefore, regular monitoring of diaphragm activity and associated care is vital for preventing diaphragmatic weakness. More studies are necessary to provide guidelines for future therapies.
