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FATIGUE (POWER FAILURE)

Pathophysiology:

Diagnosis:

Shortness of breath Increasing respiratory rate Decreasing tidal volume Use of accessory muscles Signs of patient patient fatigue - weak cough Hypercarbia Diffuse atelectasis on X-ray

Alveolar ventilation - total ventilation - dead space ventilation

Is the Cause an Increased Carbon Dioxide Production or Increased Dead Space?

If mechanical factors are not present, the differential diagnosis includes: an increase in carbon dioxide production, and an increase in dead space ventilation, V_D/V_T.

Increased dead space can be due to a decrease in regional lung perfusion relative to ventilation, often due to lung over expansion. Vascular occlusion from pulmonary emboli must also be considered. The distinction can be made by directly measuring carbon dioxide production and then calculating V_D/V_T. Thus, diagnosis includes:

Assessing tidal volume, vital capacity, inspiratory force Measuring carbon dioxide production: VDVT, respiratory quotient

Serial measurements of tidal volume, vital capacity, and inspiratory force will allow one to detect early deterioration. In addition, the measurements of carbon dioxide production will allow one to determine whether the production is in excess of that predicted for the burn size alone. Oxygen consumption can also be measured directly using available spirometric techniques or the Fick method and the respiratory quotient directly calculated.

CAUSES OF HYPERCAPNIA

Extrapulmonary Impaired Ventilatory Response to CO2     - Central nervous system-depressant drugs, head trauma, starvation,      hypothyroidism, hypophosphatemia, metabolic alkalosis     - Increased CO2 Production     - Increased temperature (6% - 10% per C)     - Increased muscle activity: shivering, seizures     - Increased respiratory quotient from excess carbohydrate calories     - Sepsis Pulmonary Airway obstruction Impaired Chest Wall Motion     - Chest wall trauma, instability     - Chest wall pain, splinting     - Neuromuscular disorders     - Increased Dead space Ventilation     - Shallow breathing     - Vascular obstruction, pulmonary emboli     - Low cardiac output, hypovolemia, impaired perfusion     - Positive-pressure ventilation     - Increased airway pressure, impaired perfusion

 

Hypercarbia in patients receiving mechanical ventilation

Correct Mechanical Problems     Endotracheal tube leak, plugging     Air leak from lung, chest tubes     Leak in ventilator     Airways plugging Correct Increased Dead space     Treat hypovolemia     Decrease mean airway pressure     Control pulmonary emboli Correct Increased CO2 production     Control fever     Decrease activity     Control sepsis     Avoid excess CHO leading to increasing RQ

CHO, carbohydrate, RQ, Respiratory Quotient

 

Mechanism of Power Failure

Increased oxygen demands Increased energy demands Increased CO2 production Catabolism induced weakness

Increased Work of Breathing

Fatigue and Anxiety

Decreased cough Decreased tidal volumes Atelectasis, pneumonia Added catechol release

Respiratory Distress     - hypoxemia     - lung sepsis

Treatment

Optimize nutrition (avoid excess CO2 production) Maintain adequate rest periods Consider partial ventilatory assist to avoid fatigue                (tracheostomy useful)

Protection of the lung against processes that will impair function is the best form of support. Controlling edema and infection while maintaining nutrition and adequate rest as well as chest wall exercise are key components. Excess carbon dioxide production should be controlled by avoiding excess carbohydrate calories and controlling excessive hyperthermia. The nutrient mix should be well controlled in order to avoid too few or too many calories. Fatigue and early evidence of respiratory compromise should be treated with assisted ventilatory support. An increase in V_DV_T due to low blood volume or excessive positive end-expiratory pressure (PEEP) can be in part corrected by volume loading.

Partial ventilatory support via a tracheostomy may be useful, especially if the anticipated problem will last several weeks, as is the case with a large body burn. Adequate rest must be assured as well as control of pain and anxiety, which can lead to a further increase in catechols and resulting hypermetabolism. The patient receiving an anesthetic must be accurately evaluated preoperatively to determine intraoperative ventilatory needs. In addition, added ventilatory support should be provided in the early postoperative period until the patient can resume sufficient spontaneous ventilation.