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兵庫医科大学医学会

Clinical clerkship at Hyogo College of Medicine 2016

Ruben Kovačさん(第5学年次)

I had been in Intensive care unit (ICU) for one month. My desires had become realized. I have got opportunities to observe invasive monitoring, positive pressure ventilation, CPPR, biochemistry lab parameters, CT and x rays diagnostic, to learn more about complex acid-basis disturbances, acute kidney injury, sepsis, ARDS, cardiac surgery. I attained also an infection control meetings and also have got a view about antimicrobial treatments and approaches. Also I got opportunities to watch one-lung ventilation operations, bladder operation by DaVinci robot, and to watch intervention of TAVI and also to watch off-pump bypass operation. I will write about hemodynamic variable SVV and the ASV ventilation mode to clarify the importance of physiology that is implemented in modern intensive care unit and clinical medicine.
I have learned the hemodynamic variable stroke volume variation that is relevant to the volume responsiveness. The reason to give a patient a fluid is to increase stroke volume (volume responsiveness). If administration of a fluid does not increase stroke volume, volume loading is of no benefit and may be harmful. According to the Frank-Starling principle, the preload increases left ventricular stroke volume until the optimal preload is achieved at which point the stroke volume remains relatively constant. Once the left ventricle is functioning near the flat part of the Frank-Starling curve, fluid loading has little effect on the stroke volume. When the ventricle is operating near the flat part of the curve, there is no preload reserve and fluid infusion has little effect on the stroke volume. The stroke volume variation can show indirectly is left ventricle functioning near the flat or steep Frank- Starling curve. If the patient has been on the intermittent positive-pressure ventilation that induce cyclic changes in the loading conditions of the left and right ventricles then estimation of stroke volume variation is possible because mechanical insufflation decreases preload and increases afterload of the right ventricle that conequently leads to the reduction of the RV preload due to the decrease in the venous return pressure gradient that is related in the inspiratory increase in pleural pressure. The increase in RV afterload is related to the inspiratory increase in transpulmonary pressure. The reduction in RV preload and increase in RV afterload both lead to a decrease in RV stroke volume, which is at a minimum at the end of the inspiration. The inspiratory reduction in RV ejection leads to a decrease in LV filling after a phase lag of two or three heart beats because of the long blood pulmonary transit time. Thus, the LV preload reduction may induce a decrease in LV stroke volume, which is at its minimum during the expiratory period when conventional mechanical ventilation is used. The cyclic changes in RV and LV stroke volume are greater when the ventricles operate on the steep rather than the flat portion of the Frank-Starling curve. The magnitude of the respiratory changes in LV stroke volume is an indicator of biventricular preload dependence.
Simply speaking in heart-lung physiology, there is normal drop of blood pressure in inspiratory phase by maximum 15 mmHg, in mechanical ventilation inverse, in expiratory phase. So, if blood pressure drops more that 15mmHg that is sign that the left ventricle operate on the stepper part of Frank-Starling curve and the fluid administration will optimize the stroke output.
Limitation is that both arrhythmias and spontaneous breathing activity will lead to misinterpretations of the respiratory variations SVV. Furthermore, for any specific preload condition the PPV/SVV will vary according to the tidal volume.
I have also learned about adaptive ventilation (ASV). ASV maintains an operator set minute volume and automatically determines an optimal tidal volume / respiratory rate combination based on the minimal work of breathing principle described by Otis 1954. ASV takes into account the patient’s respiratory mechanics, which are measured breath-by-breath by the proximal flow sensor and ensures optimal ventilation for each patient during passive ventilation, spontaneous breathing, and weaning. The expiratory time constant that is product of lung compliance and bronchial resistance is calculated to determine the most optimal respiratory rate and tidal volume. The pressure-controlled SIMV (PC-SIMV) is employed, but with automatically adjusted pressure levels and SIMV rate based on measured lung mechanics.Adjustments are made with each breath. The user is asked to set just two main controls, the Ideal Body Weight (IBW) and the minimum Minute Ventilation (MinVol), plus the controls for oxygenation (FiO2), PEEP, and trigger sensitivity. Based on the user settings for IBW and MinVol, ASV calculates optimal targets for tidal volume (Vt) and respiratory frequency (f), thus automatically selecting the target ventilator pattern. This target ventilator pattern corresponds to the best combination of Vt and f, from the energetic standpoint.
Department also attained interesting education about Adaptive Support Ventilation (ASV).

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