Artificial ventilation of the lungs, Some practical aspects
M.R.Rajagopal, Dept of Anaesthesiology
The importance of artificial ventilation of lungs as a lifesaving supportive treatment in life-threatening illness is more and more being recognized. Here are a few practical aspects about it that every doctor should know.
First, let us clarify that artificial ventilation should be instituted only in salvageable situations. It should not be considered a last rite in incurable illness. Everyone has the right to death with dignity.
Artificial ventilation may be initiated:
* To replace normal ventilation, when spontaneous ventilation has stopped,
* To aid normal ventilation when the patient is hypoventilating, or
* Prophylactically, when respiratory failure is a possibility, as following extensive surgery.
When to initiate?
Cyanosis by itself may not indicate artificial ventilation. Oxygen therapy may be all that is needed if the volume of ventilation is normal. Generally artificial ventilation is considered necessary if
Tidal volume is too low or
The vital capacity is less than twice the tidal volume, because then effective cough will not be possible.
In this context, it must be emphasized that clinical signs of respiratory distress unrelieved by oxygen therapy must be considered an indication that one of the above conditions exist. And in measured variables, an arterial carbon dioxide tension of 50 mmHg or more (particularly if rising) is generally considered to warrant artificial ventilation, of course with exceptions.
Major disadvantages of artificial ventilation:
Use of an artificial airway like the endotracheal or tracheostomy tube bypasses the nose, deprives the inspired gases of humidification, and results in poor ciliary activity leading on to progressive atelectasis.
Unlike in spontaneous ventilation, inspiration is achieved with a substantial positive pressure. This has several disadvantages:
Ventilation becomes uneven, the smaller and distal alveoli tending to get underfilled, contributing to progressive atelectasis
The high intrathoracic pressures may cause barotrauma, resulting in surgical emphysema and pneumothorax.
The high intrathoracic pressures decrease venous return to the heart and can contribute to poor cardiac output, especially in the presence of hypovolaemia or cardiac tamponade.
Some common modes of ventilation:
Total ventilatoy support is one where the ventilator does all the work.
Partial ventilatoy support: Here the patient breathes, and the ventilator supports it. This partially obviates many of the disadvantages mentioned above. But naturally, this would be feasible only when the patient retains reasonable ability to breathe. Some commonly used modalities of partial ventilatory support are:
i. Continuous Positive Airway Pressure (CPAP): Here the patient breathes spontaneously, but the airway pressure is never allowed to come down to 0.
ii. Assist mode ventilation (Triggered ventilation): Here the patient initiates the breath, but the negative pressure generated by the spontaneous breath triggers the ventilator to deliver a positive pressure.
iii. Pressure support ventilation (PSV): In this, the initiation of ventilation is as in assist mode; but the machine generated positive pressure stops as soon as the patientís spontaneous effort stops. This is a good weaning mode, but may also be the sole supportive mode of ventilation in some patients with borderline ventilatory failure.
iv. Synchronised Intermittent Mandatory Ventilation (SIMV): In this, the spontaneous ventilatory efforts may be alternated with varying rates of mechanically delivered breaths. The frequency of mechanically ventilation can be gradually decreased during the weaning phase.
There are many more sophisticated modes of ventilation in common use, but the above must be considered essential.
Care of the patient on ventilator:
n Artificial ventilation and intensive care can be a harrowing experience for the patient. The importance of adequate explanations to the patient and family and sedation cannot be overemphasized.
n Relieve pain.
n Daily clinical examination from head to toe to rule out new abnormalities.
n Care of skin; eyes.
n Nutrition: vitally important. In its absence muscle wasting can make weaning very difficult.