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A comprehensive overview of the key concepts and parameters related to mechanical ventilation for covid-19 patients. It covers important topics such as airway resistance, lung compliance, derecruitment, recruitment, ventilation phases, pressure measurements (peak inspiratory pressure, plateau pressure, peep, driving pressure), flow rates, tidal volume, respiratory rate, minute ventilation, and various ventilation modes (assist control, simv, prvc, pressure support, nippv, cpap, bpap). The detailed explanations and definitions of these ventilation-related terms and concepts can be valuable for healthcare professionals, medical students, and researchers working in the field of critical care and respiratory management of covid-19 patients.
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Airway resistance - correct answer the resistive forces encountered during the mechanical respiratory cycle and is ≤5 cm H2O. Lung compliance - correct answer describes the ease with which lungs stretch and expand to accommodate a change in volume or pressure. People with high lung compliance have more difficulty with the exhalation process while those with low compliance have difficulty with inhalation. Derecruitment - correct answer the loss of gas exchange surface area due to atelectasis. It can be minimized by increasing PEEP. Recruitment - correct answer the restoration of gas exchange surface area by applying pressure Initiation phase - correct answer the start of the mechanical breath, whether triggered by the patient or the machine. Inspiratory phase - correct answer the portion of mechanical breathing during which there is a flow of air into the patient's lungs to achieve a maximal pressure, the peak airway pressure (PIP or Ppeak), and a tidal volume (TV or VT). Plateau phase - correct answer does not routinely occur in mechanically ventilated breaths but may be checked as an important diagnostic maneuver to assess the plateau pressure (Pplat). With cessation of air flow, the plateau pressure and the tidal volume (TV or VT) are briefly held constant. Exhalation - correct answer a passive process in mechanical breathing. The start of the exhalation process can be either volume cycled (when a maximum tidal volume is achieved), time cycled (after a set number of seconds), or flow cycled (after achieving a certain flow rate). Peak inspiratory pressure - correct answer the maximum pressure in the airway at the end of the inspiratory phase. Since this value is generated during a time of airflow, the PIP is determined by both airway resistance and compliance. By convention, all pressures in mechanical ventilation are reported in "cm H2O." It is best to target a PIP <35 cm H2O. Plateau pressure - correct answer the pressure that remains in the alveoli during the plateau phase, during which there is a cessation of air flow, or with a breath-hold. To calculate this value, the clinician can push the "inspiratory hold" button on the ventilator. The plateau pressure is effectively the pressure at the alveoli with each mechanical breath and reflects the compliance in the airways. To prevent lung injury, the Pplat should be maintained at <30 cm H2O. Positive end-expiratory pressure (PEEP) - correct answer the positive pressure that remains at the end of exhalation. This additional applied positive pressure helps prevent atelectasis by preventing the end-expiratory alveolar collapse. PEEP is usually set at 5 cm H2O or greater, as part of the
initial ventilator settings. PEEP set by the clinician is also known as extrinsic PEEP, or ePEEP , to distinguish it from the pressure than can arise with air trapping Intrinsic PEEP (iPEEP or auto-PEEP) - correct answer the pressure that remains in the lungs due to incomplete exhalation, as can occur in patients with obstructive lung diseases. This value can be measured by holding the "expiratory pause" or "expiratory hold" button on the mechanical ventilator. Driving pressure (ΔP) - correct answer the term that describes the pressure changes that occur during inspiration, and is equal to the difference between the plateau pressure and PEEP (Pplat - PEEP). It is also known as the pressure required to expand the lungs. Inspiratory time (iTime) - correct answer the time allotted to deliver the set tidal volume (in volume control settings) or set pressure (in pressure control settings). Expiratory time (eTime) - correct answer the time allotted to fully exhale the delivered mechanical breath I:E ratio - correct answer or the inspiratory to expiratory ratio, is usually expressed as 1:2, 1:3, etc. The I:E ratio can be set directly or indirectly on the ventilator by changing the inspiratory time, the inspiratory flow rate, or the respiratory rate. By convention, decreasing the ratio means increasing the expiratory time. Peak inspiratory flow - correct answer the rate at which the breath is delivered, expressed in L/min. A common rate is 60 L/min. Increasing and decreasing the inspiratory flow is a means of indirectly affecting the I:E ratio. A patient with a respiratory rate set at 20, who is not over breathing, has 3 s for each complete cycle of breath. If you increase the inspiratory flow, the breath is given faster, and that leaves more time for exhalation. Thus, inspiratory flow indirectly changes the I:E ratio. Tidal volume (TV) - correct answer the volume of gas delivered to the patient with each breath. The tidal volume is best expressed in both milliliters (ex: 450 mL) and milliliters/kilogram of predicted body weight, much as one might describe a drug dosage in pediatrics. Clinicians can choose to set the ventilator in a volume control mode, where the tidal volume will be constant for each breath. In pressure control modes, the pressure is constant, but the tidal volume is an independent variable, and will vary slightly with each breath. Respiratory rate (RR or f) - correct answer the mandatory number of breaths delivered by the ventilator per minute. However, it is important to be mindful that the patient can breathe over this set rate, and therefore one must report both your set RR and the patient's actual RR; both of these values can be found on the ventilator screen. In addition, it is important to remember that the RR is a key factor in determining time for exhalation. For example, if a patient has a RR of 10 breaths per minute (bpm), he will have 6 s per breath: ((60 s/min) / 10 bpm = 6 s/breath). A RR of 20 bpm only allows 3 s for the entire respiratory cycle. Minute ventilation (MV) - correct answer the ventilation the patient receives in 1 min, calculated as the tidal volume multiplied by the respiratory rate (TV x RR), and expressed in liters per minute (L/min). Most healthy adults have a baseline minute ventilation of 4-6 L/min, but critically ill
maintain alveolar distention. The patient must be awake, minimally sedated, and able to take spontaneous breaths during this mode of ventilation.