Start your understanding about the setting of ventilators. The respiratory support to a patient by mechanical means, i.e., through machines ( Ventilators) is known as Mechanical Ventilation. There are basically 3 modes of ventilation, i.e., CMV, SIMV and CPAP. Once youunderstand this, then it is very easy to know all about mechanical ventilation in ICU. CMV is abbreviation for continuous mechanical ventilation. This mode of ventilation is used when patient has no spontaneous breathing. It means that patient is totally dependent on ventilator. It is continuous,i.e., patient cannot survive without ventilator. In continuous mechanical ventilation ( CMV) mode the respiratory rate fixed on machine is same which patient's reading shows. It means if respiratory rate fixed by ( set on ) machine is 18/minute then patient will breath only 18 per minute. This CMV mode is used in patient who has no sponteous breathing, as during the cranial surgery where no disturbance of the brain is required in majority of neurosurgical procedures. This mode is used during elective hyperventilation in higher respiratory rate is fixed and patient is paralysed for 24 or 48 hour to reduce intracranial pressure. Hyperventilation causes CO2 ( carbon Dioxide) wash out which causes vasoconstriction causing reduced ICP. Patient is paralysed by using muscle relaxants like Norcuronium, which does not allow patient to have own breathing and there is no fight between the machine and patient.
CMV mode can be 2 types: Pressure regulated or Volume regulated.In patient where is risk of Barotrauma to the airway or lungs like in children with Asthma, or in emphysema where there is risk of rupture of air pockets in lungs, pressure is controlled. The airway pressure does not rise beyond certain limit. Volume controlled is used when a fixed tidal volume like 500 mL is used during each breathing.
Second mode of creating on ventilatior is used is known as SIMV ( Synchronized Intermittent Mechanical Ventilation),i.e., there is synchrony between machine and patient. Patient has some breathing of its own but that is not adequate. In adequate breathing is like less tidal volume. So, if patient is breathing 18 mies per minute but Tidal Volume is only 200 mL , so the minute volume is 200mL X 18= 3.6 Litre only. If you fix SIMV mode on Tidal volume 500mL and respiratory rate 12, the 12 times machine will deliver 500mL and patient will have minute volume at least 6 Litre. Machine will augment the respiratory effort of patient at least 12 times because patient is conscious having own breathing and machine is not fighting with patient. This is safe mode to use without it ensures adequate minute volume and even a paralysed patient in elective hyperventilation if regains consciousness even then there will no fight between machine and patient.
In CPAP mode, patient is conscious and having own breathing. But patient is on ventilator. This may be used during weaning off the patient from ventilator. Here poise airway pressure is given by the ventilator. Continuos positive airway ventilation is ensured.
