Detailed explanation of the workingprinciple of the ventilator and its four major functions

The ventilator is a lung ventilation device(Lungventilator), because it can only play the role of sending air into and outof the lungs, and does not participate in the entire process of breathing, itcan not replace the full function of the lungs (refer to replacement Gasfunction). Therefore, some people think that it is more accurate to call it aventilator. The function of the ventilator we are talking about actually refersto its ventilating function.

The functions of ventilator can be dividedinto several categories: primary function, secondary function, specialfunction, auxiliary function.

（一） Main functions

(1) Adjusting the ventilation pressure orvolume: The fixed-pressure ventilator prioritizes the pressure setting, and theventilation volume depends on the ventilation pressure, while the fixed-volumeventilator sets the ventilation priority, and the ventilation pressure dependson the ventilation volume. size. Higher-end multifunctional ventilators nowhave both functions, but because the constant pressure ventilator maintainsconstant airway pressure during mechanical ventilation, its ventilation volumeis directly proportional to lung compliance. When there is secretion in therespiratory tract Or airway spasm caused by increased resistance, and pulmonaryconsolidation or fibrous hyperplasia and poor compliance, its ventilation isnot constant enough, difficult to grasp, has been gradually phased out.

(2) Adjust the breathing frequency orbreathing cycle: Most ventilators can directly set the ventilation frequency,but some ventilators can change the ventilation frequency by setting theventilation cycle. The ventilation cycle refers to the total time required tocomplete one inhalation, exhalation, and quiescence. If the ventilation cycleis set to 3s, the breathing rate is 20 times per minute. At present, the breathingfrequency of some high-end ventilators can be adjusted quickly, up to 100-3000times / min, which is much higher than the normal breathing rate of people.This function can cope with some special situations, such as difficulty intracheal intubation and bronchoscopy. Examination, severe air leak in theartificial airway, post-pulmonary resection, and pneumothorax patients.

(3) Adjust the inspiratory / expiratoryratio: The inspiratory / expiratory ratio during mechanical ventilation dependson three factors, namely the ventilation frequency, ventilation volume (orpressure), and inspiratory flow rate. On the premise of setting the ventilationfrequency and ventilation volume, The inspiratory / exhalation ratio can bechanged by adjusting the inspiratory flow rate. More specific examples areinverse ratio ventilation (IRV), which means that the inspiration time islonger than the expiration time, and it is suitable for patients with pulmonarysclerosis or fibrosis.

(4) Adjust the sensitivity of auxiliary ventilation:The sensitivity usually depends on the negative pressure in the circuit duringinhalation, so the greater the negative pressure is set, the lower thesensitivity, and vice versa. The sensitivity of adult assisted ventilationshould be adjusted between 0 ~ -3cm water column.

(Two) secondary functions

(1) Adjust the oxygen concentration in theinhaled gas: Mix 100% pure oxygen with compressed air with an oxygen mixer inthe air to adjust the inhaled oxygen concentration to 21-100%. The oxygen concentrationadjusted by the device is constant. It is mostly used to indirectly drive theventilator; while the direct drive ventilator uses the Venturi device, that is,the speed of the pure oxygen jet is used to suck the surrounding air to reducethe oxygen concentration, but the adjusted oxygen concentration is notconstant, and it must have an oxygen concentration. Direct monitoring measuresto prevent oxygen poisoning.

(2) Humidification and warming of inhaledgas: Most respirators use thermal humidifiers to heat water to generate steam,which is mixed into the inhaled gas and into the inhaled gas, and at the sametime plays the role of heating and heating. The general adjustment temperatureis 32 ~ 35 ℃. But some ventilator do not have the function of heating.

(Three) special functions

(1) Positive end-expiratory pressure(PEEP): This function can support the small airways and alveoli, keeping theinternal pressure at a level higher than atmospheric pressure at the end ofexhalation, and preventing the collapse of the small airways and alveoli. Andcan increase the amount of functional residual gas, increase lung compliance,thereby improving the diffuse function of the lung. Mostly used in patientswith ARDS (acute respiratory distress syndrome) and patients with atelectasis.

(2) Continuous positive airway pressure(CPAP): Its effect is similar to PEEP, which can prevent and reverse theclosure of small airways and alveolar collapse, increase intrathoracicpressure, save effort and feel comfortable.

(3) Pressure support (PSV): This is anauxiliary ventilation pressure function, that is, the patient first triggersventilation, and the ventilator gives the patient a certain level of positivepressure support during expiration to reduce the work of the patient duringinspiration. Conducive to the recovery of respiratory muscle function, patientsare easy to accept. Slowing the breathing rate is a way to evacuate theventilator.

(4) Sigh function (SIGH): This function isonly used for long-term intermittent positive pressure ventilation (IPPV),which can fully expand the alveoli, but it is easy to cause barotrauma. Itshould be used with caution in patients with bullae.

(5) Intermittent forced ventilation (IMV)and synchronous intermittent forced ventilation (SIMV): It can organicallycombine spontaneous breathing and IPPV to ensure the effective ventilation ofpatients. Patients in early respiratory failure are easy to accept SIMV, anddrone countermeasures. Used with CPAP to treat ARDS. These two functions aregenerally used in patients with good spontaneous breathing, and more oftenbefore going offline.

(6) Minute Mandatory Ventilation (MMV):This function guarantees the minute ventilation. If the autonomous ventilationis lower than the setting index, the insufficient part is automaticallyreplenished by the ventilator. If the autonomous ventilation is greater thanthe set value or equal to the set value Value, the ventilator automaticallystops gas supply. Best for patients with spontaneous breathing instability.

(7) Ventilator instead of ventilation(BUV): When the ventilator is running, if its self-test system finds asystematic error or the power supply voltage of the ventilator is lower than90% of the rated voltage, it will automatically switch to BUV. The ventilatorreplacement ventilation conditions are preset by the ventilator manufacturer.When the ventilator is replaced with a ventilator replacement ventilation, theventilator automatically performs ventilation according to the set conditions.

(8) Divided lung ventilation (DLV, ILV):The two lungs are separated by double-lumen cannula, and different forms ofventilation are given, which is called separated lung ventilation. It is mainlyused in patients with severe pulmonary bullae or lung abscesses on one side ofthe lung, and in patients with normal lungs on the other side.

9 (9) Bi-level positive airway pressure(BiPAP): Adjusts two pressure levels and time respectively. Both pressures arepressure-controlled and the air velocity is variable. This is a newer form ofventilation and has a greater prospect for development.

10 (10) Safety valve opening (SVO): Whenthe power supply is interrupted or the ventilator finds a serious error, thesafety valve automatically opens, and the patient can still breathe air.

(Four) auxiliary functions

(1) Monitoring function: Modern ventilatorshave comprehensive monitoring functions. In addition to monitoring the basicventilation functions of the ventilator, such as the frequency, tidal volume,and airway pressure, it can also monitor blood oxygen saturation, airwayresistance, and lungs. Compliance and vital capacity monitoring. The medicalstaff can grasp the working condition of the ventilator and the change of thepatient's condition in a timely manner.

(2) Alarm function: The multifunctionalventilator uses a combination of optics and acoustics to perform alarms. Thecontents of the alarm generally include power supply, air source conditions,breathing frequency, tidal volume; airway pressure, temperature, and exhalation/ absorption ratio.

(3) Recording function: The high-endmultifunctional ventilator also has a recording function, which can be directlyconnected to a printer. It can review and print important parameters,waveforms, trend charts and charts of mechanical ventilation in the past 12hours, and can be linked with the monitoring system. To store and displayclinical data.