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24 Cards in this Set

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Mechanical ventilation pathophys
-The relationship between intrapulmonary pressures during inspiration and expiration is reversed during mechanical ventilation
-The ventilator delivers air by pumping it into the pt, therefore, pressures during inspiration are positive
-The positive pressure pumped into the lungs results in inc intrathoracic pressures and dec venous return during inspiration
-With the institution of PEEP, even greater pressures are generated during the inspiration
-During expiration the pressure in the lungs dec to the baseline PEEP level and continues to be positive throughout expiration
-Barotrauma occurs when air leaks from the alveoli into the pleural space, called a pneumothorax
Plateau Pressure
-The plateau pressure is the pressure applied (in positive pressure ventilation) to the small airways and alveoli.
-It is believed that control of the plateau pressure is important, as excessive stretch of alveoli has been implicated as the cause of ventilator induced lung injury
-The peak pressure is the pressure measured by the ventilator in the major airways, and it strongly reflects airways resistance.
-For example, in acute severe asthma, there is a large gradient between the peak pressure (high) and the plateau pressure (normal).
-In pressure controlled ventilation, the pressure limit is (usually) the plateau pressure due to the dispersion of gas in inspiration.
-In volume control, the pressure measured (the PAW) by the ventilator is the peak airway pressure, which is really the pressure at the level of the major airways
-High Pplat pressures are associated with barotrauma
-High levels of PEEP can also contribute to barotrauma
-Peak inspiratory pressurs are measurements of airway resistance and lung compliance and should reflect plateau pressures closely
-Barotrauma is primarily a result of high alveolar pressures and are best assessed by Pplat
-Keep Pplat <30-35cm/H20
Static Compliance/Plateau Pressure
-Static compliance is a measurement used to obtain compliance
-Plateau pressure is obtained by pushing the end-inspiratory hold button on a ventilator at the end of a maximum inspiration
-this holds the volume of delivered air in the patients chest by preventing exhalation
-the PIP drops to a plateau pressure with this maneuver, which reflects the pressure necessary to hold the lungs open
-static compliance is determined by dividing the effective tidal volume Vt by the plateau pressure minus the total PEEP
-Static compliance= Vt/(plateau pressure-PEEP)
-Without lung disease, peak inspiratory pressure (PIP) is only slightly above the plateau pressure.
-A high compliance means that the lung is more easily distended, whereas a lower compliance means that the lungs is stiffer and difficult to distend, higher compliance is better
-When the compliance is low, more pressure will be need to deliver a given volume of gas to a patient. Disease states resulting in low compliance include the Adult Respiratory Distress Syndrome (ARDS), pulmonary edema, pneumonectomy, pleural effusion, pulmonary fibrosis, and pneumonia among others. Emphysema is a typical cause of increased lung compliance.
MECHANICAL VENTILATORS
Ventilators are classified into two categories:
-negative pressure
-positive pressure ventilators
Volume ventilators
-A designated volume of air is delivered with each breath
-The amount of pressure required to deliver the set volume depends on the pt lung compliance and pt ventilator resistance factors
-PIP must be monitored in volume modes because it varies from breath to breath
-A respiratory rate, inspiratory time, and tidal volume are selected for the mechanical breaths
-Uses the volume to trigger the termination point when ventilating
volume modes include:
-assist control and synchronized intermittent mandatory ventilation
Assist control mode
-In AC mode, a mandatory (or control) rate is selected
-If the pt wishes to breath faster, they can trigger the ventilator and receive a full volume breath
-Designed so that the patient receives the fixed control rate, and is assisted with another full breath when the vent detects a spontaneous breath
-The advantage of this mode is that patients can breathe spontaneously without working
-For the ventilator to interact with the patient, it must sense that the patient is making a spontaneous effort – a trigger – and address this by delivering a breath
-Slight hiccups or small inhalations can trigger and additional full breath
-Effective for patients hyperventilating in a compensatory manner (DKA)
-Increased risk for barotrauma in pt’s with erratic, non-compensatory breathing
-Designed so that the pt receives a fixed control rate and will be assisted with a full breath whenever the vent detects a spontaneous breath
SIMV
-The Vt and rate are preset
-If the pt wants to breath above this rate, they can
-Unlike the AC mode, any breath taken above the set rate are spontaneous breaths taken through the ventilator circuit
-The Vt of these breaths can vary drastically from the Vt set on the ventilator, because the Vt is determined solely by the pt spontaneous effort
-Adding pressure support during spontaneous breaths can minimize the risk of increased work of breathing
Pressure ventilators
Pressure modes include:
-pressure support ventilation PSV, pressure controlled ventilation PCV, continuous positive airway pressure CPAP/PEEP, and noninvasive bilevel positive airway pressure ventilation BiPAP
-typical pressure mode delivers a selected gas pressure to the pt early in inspiration, and sustains the pressure throughout the inspiratory phase
-Pt effort is reduced and comfort increased
-Pressure is consistent with these modes, volumes is not
-With changes in resistance or compliance, volume will not change
-Therefore, exhaled tidal volume is the variable to monitor closely
-preferred for infants
-Pressure Control refers to the type of breath delivered, not the mode of ventilations
-The tidal volume is determined by the preset pressure limit, Aka pressure targeted
-Terminates the ventilation based on airway pressures
-poorly sedated or bucking pt’s can cause spikes in PIP, and will terminate the vent
-Designed to limit or prevent barotrauma
Pressure controlled ventilation PCV
-PCV mode is used to control plateau pressures in conditions such as ARDS where compliance is decreased and the risk of barotrauma is high
-it is used when the pt has persistent oxygenation problems despite a high Fi02 and high levels of PEEP
-the inspiratory pressure level, respiratory rate, and inspiratory-expiratory (I:E) ratio must be selected
-tV varies with compliance and airway resistance and must be closely monitored
-sedation and the use of neuromuscular blocking agents are frequently indicated, because any patient-ventilator asynchrony usually results in profound drops in the Sa02
-most ventilators operate with a short inspiratory time and a long expiratory tim (1:2 or 1:3 ratio)
-this promotes venous return and allows time for air to exit the lungs passively
-inverse I:E rations are used in conjunction with pressure control to improve oxygenation in pts with ARDS by expanding stiff alveoli by using longer distending times, thereby providing more opportunity for gas exchange and preventing alveolar collapse
-as expiratory time is decreased, one must monitor for the development of hyperinflation or auto-PEEP
-when the PCV mode is used, the mean airway and intrathoracic pressure rise, potentially resulting in a decrease in CO and oxygen delivery
Pressure controlled ventilation cont
-In pressure control, a pressure limited breath is delivered at a set rate.
-The tidal volume is determined by the preset pressure limit.
-This is a peak pressure rather than a plateau pressure limit (easier to measure).
-The inspiratory time is also set by the operator.
-longer inspiratory times lead to higher mean airway pressures (the “i” time (Ti) is a pressure holding time after flow has stopped
-Patients can breath spontaneously on pressure control as long a the inspiratory time has not be unduly prolonged. The trigger mechanism is the same as in volume control.
-Pressure control does not guarantee minute ventilation, and therefore requires more monitoring by the operator
-The inspiratory pressure is determined by looking for a tidal volume of 6-8ml/kg.
-The respiratory rate is determined by the minute volume requirement.
-The inspiratory time is usually set at 1sec, but can be increased if: 1. target tidal volume is not achieved with, or 2. The patient remains hypoxic in spite of a plateau pressure >30cmH2O.
-Unfortunately, there is a limit to this process, auto-PEEP.
-Longer inspiratory times and faster respiratory rates predispose to alveolar gas trapping, auto-PEEP
-This intrinsic PEEP is present in addition to applied PEEP at the beginning of inspiration, placing the patient on a less compliant (overdistended) part of the volume-pressure curve.
Thus tidal volumes fall and airway pressures rise.
Auto-PEEP
-Auto PEEP is the popular name used to describe increased alveolar pressure caused by gas trapping during mechanical ventilation
-Gas trapping occurs when there is inadequate time to exhale the mechanical tidal volume
-Recall that the time constant determines the length of time needed for a passive exhalation and that the time constant is the product of airway resistance and lung compliance
-The lower the compliance, the higher the driving pressure pushing gas out of the lungs during exhalation; the lower the resistance, the higher the expiratory flow rate can be when driven by the alveolar pressure.
-When gas trapping occurs, the functional residual capacity (FRC) is increased.
-As the FRC increases, the alveolar pressure increases by an amount of pressure determined by the patient's lung compliance.
-As the FRC rises in relation to the total lung capacity (TLC), the lung compliance will decrease.
-This decrease in lung compliance shortens the time constant for the next breath and thus shortens the time required to exhale the next breath and lessens the amount of trapping that will occur with each subsequent breath until the time constant shortens enough that gas trapping no longer occurs.
-Longer inspiratory times and faster respiratory rates predispose to alveolar gas trapping, auto-PEEP
-This intrinsic PEEP is present in addition to applied PEEP at the beginning of inspiration, placing the patient on a less compliant (overdistended) part of the volume-pressure curve.
Thus tidal volumes fall and airway pressures rise.
Acute lung injury, PCV
-acute lung injury is a foreign origin disease process
-alveoli are affected differently by disease
-Some are effectively normal, some have low compliance, some have normal compliance but long time constants, Others are not involved in gas exchange.
-The Time Constant of the lung (TC), the phenomenon whereby a given percentage of a passively exhaled breath of air will require a constant amount of time to be exhaled regardless of the starting volume given constant lung mechanics.
-In volume ventilation, gas is preferentially delivered to more compliant alveoli, causing overdistension and poor mixing. In pressure control, there is better distribution of gas to these differing lung units.
See pic:
-this contains normal alveoli (A1), non compliant alveoli (A2 e.g. consolidation) and alveoli with long time constants due, in this case, to proximal obstruction - such as a mucus plug or bronchial constriction.
-When a volume breath is delivered (with constant flow pattern) the gas passes down the path of least resistance into the most compliant alveoli
-so there is relative overdistension of A1, A2 is inflated as expected, and A3 does not have time to inflate before the ventilator cycles off.
Acute lung injury, PCV cont
See picture:
-a pressure controlled breath is delivered. In this case there is better distribution of gas - because A1 will not overdistend to the same extent as before, and there is sufficient time for A3 to inflate.
-Gas Distribution is the main reason for using pressure control ventilation
-The major drawbacks of pressure control is changing tidal volume in relation to: 1. changes in lung compliance, and 2. auto-PEEP.
Pressure support ventilation mode
-PSV mode augments or assists spontaneous breathing efforts by delivering a high flow of gas to a selected pressure level early in inspiration, and maintaining that level throughout the inspiratory phase
-The pt effort determines the rate, inspiratory flow, and Vt
-When PSV mode is used as a stand alone mode of ventilation, the pressure support level is adjusted to achieve the approximate targeted Vt and respiratory rate
-in pressure support ventilation, the patient determines the respiratory rate & the duration of inspiration.
-You set the PEEP and inspiratory pressure (pressure support level).
-Remember that the peak/plateau pressure is the PS plus the PEEP.
-The term “pressure support ventilation” describes the combination of pressure support and PEEP.
-Pressure support on mechanical ventilators is “above PEEP”, which is an incorrect term – it is really the pressure above “CPAP”.
-Thus if a patient is on PEEP 5cmH2O and pressure support of 10cmH2O the peak/plateau pressure is 15cmH20
Non-invasive positive pressure ventilation NPPV
-intervention strategy for spontaneously breathing patient who are having trouble keeping smaller bronchiole airways open
-would benefit from the displacement of excessive fluid accumulation n the alveoli back into interstitial spaces and peri-alveolar capillary network
-this intervention works by providing a constant back pressure of air against which the pt must exhale
-the back pressure helps to keep the alveoli open, diminishes alveolar fluid accumulation, assists in keeping smaller bronchioles open that in turn allow a more effective exhalation and removal of C02 from the lungs
-decreases the work of breathing necessary during inhalation due to the positive pressure of air entering into the mask
-two form of NPPV: CPAP and BiPAP
Continuous Positive Airway Pressure
-The main indications for positive airway pressure are congestive heart failure and COPD, and resp failure
-CPAP is the term used when PEEP is supplied during spontaneous breathing
-PEEP is the term used to describe positive end expiratory pressure with positive pressure mechanical ventilation
-CPAP assists spontaneously breathing pt to improve their oxygenation by elevating the end expiratory pressure in the lungs throughout the respiratory cycle
-CPAP delivers constant pressure through a mask that is positioned securely on the face during inhalation and exhalation
-PEEP is positive pressure exerted at the end of exhalation
-During inspiration, the inspiratory positive airway pressure, or IPAP, forces air into the lungs—thus less work is required from the respiratory muscles.
-The bronchioles and alveoli are prevented from collapsing at the end of expiration. If these small airways and alveoli are allowed to collapse, significant pressures are required to re-expand them.
-This is because of the Young–Laplace equation (which explains why the hardest part of blowing up a balloon is the first breath).
-Entire regions of the lung that would otherwise be collapsed are forced and held open. This process is called recruitment.
-Usually these collapsed regions of lung will have some blood flow (although reduced).
-Because these areas of lung are not being ventilated, the blood passing through these areas is not able to efficiently exchange oxygen and carbon dioxide.
-This is called ventilation–perfusion (or V/Q) mismatch. The recruitment reduces ventilation–perfusion mismatch.
-The amount of air remaining in the lungs at the end of a breath is greater (this is called the functional residual capacity).
-The chest and lungs are therefore more expanded. From this more expanded resting position, less work is required to inspire.
-This is due to the non-linear compliance–volume curve of the lung.
Positive End Expiratory Pressure
-PEEP is the term used to describe positive end expiratory pressure with positive pressure mechanical ventilation
-PEEP is most often necessary in pt with refractory hypoxemia, where the Pa02 deteriorates rapidly despite greater concentrations of oxygen administration
-PEEP is used to keep the alveoli stented open and it may recruit alveolar units that are totally or partially collapsed
-This end expiratory pressure increases the functional residual capacity by re-inflating collapsed alveoli, maintains the alveoli in an open position, and improves lung compliance
-this decreases shunts and improves oxygenation
-high levels of PEEP should rarely be interrupted because it may take several hours to recruit alveoli again and restore the FRC
-In the pt who does not have adequate circulating blood volume, institution of PEEP decreases venous return to the heart, decreases CO, and decreases oxygen delivery to the tissues
-Barotrauma is most common when high levels of PEEP are used, usually requires placement of a chest tube
Bilevel positive airway pressure ventilation BiPAP
-Alternates the positive pressure from a higher level during inhalation (to decrease work of breathing) to a lower level during exhalation ( to allow effective exhalation while promoting bronchiole patency and decreasing atelectasis
-Is noninvasive form of mechanical ventilation provided by means of a nasal mask, nasal prongs, or a full face mask
-Is used in the treatment of pt with chronic respiratory insufficiency to manage acute or chronic respiratory failure without intubation and conventional mechanical ventilation
-Is beneficial in pt with worsening hypoventilation, obstructive apneic episodes or both
-Is also used to avoid intubation in pt with respiratory failure and hypercarbia, and to avoid reintubation after extubation in borderline
-Inspiratory Positive Airway Pressure (IPAP) = pressure support
-Expiratory Positive Airway Pressure (EPAP) = PEEP
Respiratory rate
-In the pressure ventilator, the inspiratory time determines the duration of inspiration by regulating the gas flow rate
-The higher the flow rate, the faster peak airway pressure is reached and the shorter the inspiration
-Conversely the lower the flow rate the longer the inspiration
-Respiratory rate times Vt equals minute ventilation/volume
-RR x Vt = MV
-Minute volume- volume of air inhaled or exhaled in one minute
-Minute volume determines alveolar ventilation- volume of gas reaching the alveoli
-These two parameters are adjusted according to the PaC02
-Increasing the minute volume decreases the PaC02, Conversely decreasing the minute volume increases the PaC02
Vt
-Lower Vt targets 6-8ml/kg are now recommended
Pressure limit
-On volume cycled ventilators, the pressure limit dial limits the highest pressure allowed in the ventilator circuit
-Once the high pressure limit is reached, inspiration is terminated
-If the pressure limit is being constantly reached, the designated Vt is not being delivered to the pt
-The cause of this can be coughing, accumulation of secretions, kinked ventilator tubing, decreasing compliance, or a pressure limit set to low
Peak Inspiratory Pressure
-The maximum pressure delivered by a ventilator during each inspiratory cycle.
-PIP can give an indication of both airway resistance and lung compliance
-Climbing PIP signifies high airway pressures and a lower compliance of the lungs
-A steady Pplat is suggesting that lung tissue is normal
-High PIP – check for tight airways
-High Pplat – causes barotrauma, check volumes
-Sudden elevation in PIP and Pplat indicates the lungs are collapsing
ETC02 MONITORING
-End tidal carbon dioxide ETC02 measures the level of C02 at the end of exhalation
-when the percentage of C02 dissolved in the arterial blood (PaC02) approximates the percentage of alveolar C02 (PAC02)
-therefore samples of exhaled C02 measured at the end of exhalation (ETC02) can be used to approximate levels of alveolar C02
-levels of alveolar C02 and arterial C02 are similar, therefore ETC02 can be used to estimate PaC02
-ETC02 is usually lower than PaC02 by 2-5mmHg
-although the difference between PaC02 and ETC02 (PaC02-ETC02 gradient) may be attributed to several factors, pulmonary blood flow is the primary determinant
-ETC02 provides continuous estimates of alveolar ventilation
Capnograph waveform
-on a capnograph the waveform is composed of four phases
First phase:
-is the baseline phase, represents both the inspiratory phase and the very beginning of the expiratory phase, when C02 free air in the anatomical dead space is exhaled
-this value should be zero in a healthy adult
Seconde phase:
-is the expiratory upstroke, which represents the exhalation of C02 from the lungs
-any process that delays the delivery of C02 from the pts lungs to the detector prolongs the expiratory upstroke
-conditions such as COPD and bronchospasm are known physiological causes of prolonged expiratory upstroke
-mechanical obstructions such as kinked ventilator tubing may also cause prolonged expiratory upstroke
Third phase:
-begins as C02 elimination rapidly continues
-a plateau on the capnogram indicates the exhalation of alveolar gases
-the ETC02 is the value generated at the very end of exhalation, indicating the amount of C02 exhaled from the leased ventilated alveoli
Fourth phase:
-known as the inspiratory downstroke
-the downward deflection of the waveform is caused by the washout of C02 that occurs in the presence of the oxygen influx during inspiration