Principles of Mechanical Ventilation PDF

Chapter 1 Principles of Mechanical Ventilation  These are 5 components 5 of mechanical ventilation (MV): Components of V = Volume Mechanical F = Flow Ventilation C = Compliance (VF-CPR) P = Pressure...

Chapter 1 Principles of Mechanical Ventilation  These are 5 components 5 of mechanical ventilation (MV): Components of V = Volume Mechanical F = Flow Ventilation C = Compliance (VF-CPR) P = Pressure R = Resistance  We will study these in more detail later, but in a nutshell... 5 Volume is flow multiplied by time Components Flow is volume divided by time of Mechanical Pressure is flow multiplied by resistance Ventilation Resistance is the change in pressure (VF-CPR) divided by flow Compliance is volume divided by change in pressure  5 Components of Or as mathematical expressions... Mechanical Ventilation (VF-CPR)  Here's a brief article that helps reinforce these concepts... 5 https://derangedphysiology.com/main/ Components cicm-primary-exam/required-reading/re spiratory-system/Chapter%20531/flow- of volume-pressure-resistance-and-compli Mechanical ance Ventilation (VF-CPR) (This file has been uploaded into BB).  Airway Airway resistance is Resistance calculated by Pressure Change / Flow i g h t t m Wha se an cau e in e a s. incr irway Raw = ∆P / V a ? a n c e s i s t ∆P = pressure. re Raw = airway resistance change (Peak Inspiratory Pressure − Plateau Pressure) V = Flow  Airway Resistance. Given: Raw = ∆P / V Raw and ∆P have a direct relationship – ∆P reflects the work of breathing – An increase in Raw would increase the work of breathing – A decrease in Raw would decrease the work of breathing. Given: Raw = ∆P / V Raw and V have an Airway inverse relationship An increase in Raw would Resistance decrease the flow A decrease in Raw would increase the flow  Airway Resistance. Given: Raw = ∆P / V ∆P and V have a direct relationship – An increase in ∆P (work of breathing) would be required to increase the flow – A decrease in ∆P (work of breathing) would lead to a decrease in flow  Clinical conditions that increase Airway airway resistance. Resistance  Severe and uncorrected airflow obstruction would lead to a persistent increase in the work of breathing. Airway Resistance When the patient is unable to maintain the increase in the work of breathing, ventilatory and oxygenation failure will occur.  Compliance Compliance is calculated by: Volume Change / Pressure Change C = compliance* C = ∆V / ∆P ∆V = volume change ∆P = pressure change *Use PIP for dynamic compliance and PPLAT for static compliance. Sometimes, dynamic compliance is noted as Cdyn.  Given: C = ∆V / ∆P C and ∆V have a direct relationship Compliance An increase in C would increase the volume without the need to increase the work of breathing (∆P) A decrease in C would decrease the volume unless the work of breathing (∆P) is increased  Given: C = ∆V / ∆P Compliance C and ∆P have an inverse relationship An increase in C would decrease the work of breathing (∆P) A decrease in C would increase the work of breathing (∆P)  Compliance can be defined as either static or dynamic. Static compliance represents the compliance when there is NO gas Compliance flow, such as during an inspiratory hold. Dynamic compliance represents compliance during the movement of air.  Dynamic Compliance Compliance Static Compliance Compliance  An article for review on compliance... Compliance https://www.hamilton-medical.com/en_U S/E-Learning-and-Education/Knowledge-B ase/Knowledge-Base-Detail~2019-02-04~ Static-compliance-%28Cstat%29-vs--dyna mic-compliance-%28Cdyn%29~23980f36- edb7-41ea-b04f-fb9fe4d990b5~.html#Dat aTables_Table_0=od3  Low compliance indicates a stiff lung (one with high elastic recoil) and can be thought of as a thick balloon. – Exa: Fibrosis. Compliance High compliance indicates a pliable lung (one with low elastic recoil) and can be thought of as a grocery bag. – Exa: Emphysema.  Compliance Compliance is highest at moderate lung volumes, and much lower at volumes which are very low or very high. The compliance of the lungs demonstrate lung hysteresis; that is, the compliance is different on inspiration than on expiration for identical volumes.  Clinical conditions that decrease Compliance the compliance:  Compliance Severely low compliance (“stiff lungs”) would lead to a persistent increase in the work of breathing. When the patient is unable to maintain the increase in the work of breathing, ventilatory and oxygenation failure will occur.  In ventilator graphics, a decrease in lung compliance would increase the plateau Compliance pressure (PPLAT) and peak inspiratory pressure (PIP) by the same proportion.  An increase in peak inspiratory pressure Compliance (PIP) with an unchanged plateau pressure (PPLAT) suggests an increase in airflow vs. Resistance resistance.  In a pressure/volume loop, shifting of the P/V loop toward the pressure axis (arrow A in next slide) correlates with a decrease in compliance. Compliance Shifting of the P/V loop toward the volume axis (arrow B in next slide) correlates with an increase in compliance. Compliance  Because of the presence of elastic and collagen fibers in its parenchyma, the lung has principles of elasticity. Elastance / Elasticity is the tendency of a material to try to maintain its Elasticity shape and offer resistance to stretching forces. Mathematically, E = ∆P / ∆V  Here's one more article that may Elastance / help you out... Elasticity http://www.ubccriticalcaremedici ne.ca/rotating/material/Lecture_ 1%20for%20Residents.pdf  Normal Values Normal Compliance = 50-100 mL/cmH20 Normal Resistance = 1-8 cmH20/L/s  Dead Space Ventilation Dead space is the volume of a breath that does not participate in gas exchange. In other words, it is ventilation without perfusion. Physiologic or total dead space is the sum of anatomic dead space and alveolar dead space.  Dead Space Ventilation Anatomic dead space is the volume of gas within the conducting zone, which includes the trachea, bronchus, bronchioles, and terminal bronchioles. It is approximately 2 mL/kg in the upright position. Alveolar dead space is the volume of gas within unperfused alveoli. – It is usually negligible in the healthy, awake patient. The ratio of physiologic dead space to tidal volume is usually about 1/3; i.e. 20 – 40% is normal.  Dead Space Ventilation Anatomic Deadspace – Normally 1 mL per lb. body weight – VD/VT ratio is increased when the VT is reduced – For example, the VD/VT ratio increases from 30% to 50% when the VT is reduced from 500 mL to 300 mL: – 150 mL/500 mL = 30% – 150 mL/300 mL = 50%  Dead Space Ventilation Alveolar Dead space – Lung volume that is unable to take part in gas exchange because of reduction or lack of pulmonary perfusion (e.g., pulmonary embolism) – Affected by clinical conditions – Anatomic VD + Alveolar VD = Physiologic VD Dead Space Clinical conditions that increase the physiologic deadspace Ventilation  Dead Space Ventilation Factors that increase dead space: – General anesthesia – multifactorial, including loss of skeletal muscle tone and bronchoconstrictor tone – Artificial airway – Positive pressure ventilation (i.e. increased airway pressure) – Upright posture as opposed to supine (because of decreased perfusion to the uppermost alveoli) – Pulmonary embolus, PA thrombosis, hemorrhage, hypotension, surgical manipulation of pulmonary artery tree – anything that decreases perfusion to well-ventilated alveoli – Emphysema (blebs, loss of alveolar septa and vasculature) – Age – Anticholinergic drugs  Dead Space Ventilation A high physiologic VD/VT ratio would lead to a persistent increase in the work of breathing. When the patient is unable to maintain the increase in the work of breathing, ventilatory and oxygenation failure will occur.  Ventilatory Failure Four major causes of ventilatory failure – Hypoventilation – V/Q mismatch – Intrapulmonary shunting – Diffusion defect  Ventilatory Failure Hypoventilation – Causes: CNS depression, neuromuscular disorders, airway obstruction, drugs, etc. – Characterized by an increase in PaCO2 – May lead to acute ventilatory failure (e.g., drug overdose) or chronic ventilatory failure (e.g., COPD)  Ventilatory Failure V/Q mismatch – Abnormal distribution of ventilation and/or pulmonary blood flow – High V/Q mismatch may be due to reduction in pulmonary perfusion (e.g., pulmonary embolism, hypoxic vasoconstriction)  Ventilatory Failure V/Q mismatch – Low V/Q mismatch may be due to reduction in ventilation (e.g., airflow obstruction) – Mild or uncomplicated V/Q mismatch usually responds well to oxygen therapy  Ventilatory Failure Intrapulmonary Shunting – “Wasted” pulmonary perfusion – Causes refractory hypoxemia  Ventilatory Failure Intrapulmonary Shunting: Severity of shunting may be measured by: Estimated shunt equation for non- critically ill patients Estimated shunt equation for critically ill patients Classic physiologic shunt equation  Ventilatory Failure Diffusion Defect – Decrease in P(A-a)O2 gradient (e.g., high altitude) – Thickening of A-C membrane (e.g., edema) – Decreased surface area of A-C membrane (e.g., emphysema) – Insufficient time for gas diffusion (e.g., tachycardia)  Oxygenation Failure Oxygenation failure is defined as severe hypoxemia (PaO2 < 40 mm Hg) in spite of high FIO2 (50% to 100%) Hypoxia can occur with a normal PaO2 (e.g., CO poisoning) Oxygenation Failure To be clear, divide SaO2 by 100. Oxygenation status should be monitored with arterial oxygen content (CaO2) because it includes the amount of oxygen combined with hemoglobin and dissolved in plasma Clinical Conditions Leading to Mechanical Ventilation: Depressed Respiratory Drive  Clinical Conditions Leading to Mechanical Depressed Respiratory Drive Ventilation Clinical Conditions Leading to Mechanical Ventilation Excessive Ventilatory Workload Clinical Conditions Leading to Mechanical Ventilation Excessive Ventilatory Workload Clinical Conditions Leading to Mechanical Ventilation Failure of Ventilatory Pump