Mechanical Ventilation

(VentiLife Workshop)

Pre-Test

After-Test

Feedback @ VentiLife

Lung Mechanics

Lung Volumes

Process of respiration

It includes

Pulmonary Ventilation - Gas exchange between the Atmosphere and alveoli
Alveolar Gas Exchange - Gas exchange between Alveoli & Blood
Systemic Gas Exchange - Gas exchange between Blood & Tissue
Gas Transport - CO2 transport from Tissue to Alveoli

Inspiration is an active process, driven by the contraction of the diaphragm and intercostal muscles, increasing thoracic cavity volume and reducing pressure inside the alveoli. In mechanical ventilation, air is pushed into the lungs, creating positive pressure in the alveoli during inspiration. Expiration is a passive process in both spontaneous and mechanical ventilation. It occurs due to the elastic recoil of the lungs.

Pressure-Time Scalar in Spontaneou V/S Mechanical Ventilation

Compliance = △V / △P
Resistance = Force opposing the flow of gas. R = 8µL/⌅ r4

Alveolar Gas Exchange determining factors are

Partial pressure of oxygen; Dalton's Law states that in a mixture of non-reacting gases, the total pressure exerted by the mixture is equal to the sum of the partial pressures of each individual gas.

PV = P1V1+P2V2. The partial pressure of oxygen tends to decrease as it travels from the atmosphere to the alveoli. PAO2 = PiO2 - (PACO2 / R ) ; This is the alveolar gas equation.

Solubility of the gas; Henry's Law states that solubility of gas depends on the partial pressure of the gas and solubility coefficient.

CO2 is 24 times as soluble as O2 (so diffusion CO2 is faster)

Ventilation-perfusion. Alveoli normally distend in response to the flow of the gas resulting in adequate alveolar gas exchange. Abnormalities are either highly distended but not well-perfused (e.g.; PE) or well-perfused but not ventilated alveoli (e.g.; Atelectasis). Both conditions lead to a V-Q mismatch
Surface area of alveoli
Thickness of the alveolar membrane. Fick's Law states that the rate of gas transfer (or diffusion) across a tissue plane is directly proportional to the surface area, and is inversely proportional to the thickness of the tissue." In alveolar gas exchange, oxygen (O2) moves from the alveolar air into the bloodstream, while carbon dioxide (CO2) moves from the bloodstream into the alveolar air. Fick's Law helps explain how the efficiency of this gas exchange process depends on factors such as the alveolar surface area, the thickness of the alveolar membrane, and the pressure difference of the gases on either side of the membrane.
Laplace's Law: It describes the relationship between pressure, tension, and the geometry of a structure, whether it is spherical or cylindrical. For a spherical structure, P = 2T / r

P represents the pressure inside the sphere, T represents the tension in the wall of the sphere. r represents the radius of the sphere.

smaller spheres (alveoli) require higher pressure to maintain their shape compared to larger ones (if no surfactant). The surface tension in alveoli is primarily due to the presence of surfactant, reduces surface tension and prevents alveolar collapse.

The law can help explain how the pressure inside alveoli relates to their size and the surface tension in their walls. It also explains why smaller blood vessels (capillaries) have thicker walls to withstand the higher pressure necessary for blood flow, whereas larger vessels (arteries and veins) have thinner walls for efficiency (for cylindrical structure, the laplace law is 2T/r (h/2)).

Conditions requiring mechanical ventilation

(are 1. inadequate ventilation, 2. inadequate oxygenation or 3. Airway protection)

Alveolar filling process; Pneumonitis, Pulmonary oedema etc
Pulmonary vascular disease; Embolism
Airway obstruction (central); Laryngeal edema, tumor
Airway obstruction (peripheral); COPD, Asthma
Hypoventilation (central); Drug overdose
Hypoventilation (peripheral); GBS, Myasthenia gravis
Hypoventilation (chest wall & pleura); Massive pleural effusion
Increased ventilatory demand; Sepsis, Severe metabolic disease
Miscellaneous; Airway obstruction

Respiratory failure (either Hypoxia or Hypercapnoea) nature

Oxygenation failure (hypoxia)
Ventilation failure (hypercapnia)
Diffusion defect
V-Q mismatch

Respiratory failure can result from issues in any part of the respiratory system, including the brain->neuromuscular control->muscles->airways-> alveoli.

Ventilator set up

Interface; ET tube for IPPV or Face Mask for NPPV
Circuit; Inspiratory limb and Expiratory limb
Humidifier; Heated humidifier and HME (Heat Moist Exchanger)
Temperature probe; Sense the temperature in the inspired limb
Flow sensor; A pressure transducer attached to a restrictor in the air passage
Accessories (Nebuliser, ETCO2)

Ventilator parameters

Tidal Volume (VT/Vt)
Respiratory Rate (RR/f)
Inspiratory time (Ti)
Peak inspiratory pressure (PIP)
Positive End Expiratory Pressure (PEEP)
Flow (V)
Fraction of inspired oxygen (FiO2)

Tidal Volume

Tidal Volume determines the ventilation (VT x RR).
VT is based on the Predicted Body Weight used
Normal value 6-10 ml/Kg
Low VT (4-7 ml/Kg) is used in Restrictive disease (Pneumonia, ARDS)
Hight VT (8-12) is used in Obstructive disease (Asthma, COPD)
Lung protective strategies with low VT (~6ml/Kg) decrease mortality
Inspiratory VT = Expiratory VT
VTi > VTe indicates circuit leak

Respiratory Rate

Minute Ventilation = VT x RR
Alveolar ventilation = (VT - Anatomical dead space) x RR
In adults, the normal anatomical dead space is 150 ml
Alveolar ventilation equation = CO2 / (VT-Anatomical dead space) x RR
PaCO2 ⍺ 1/Alveolar ventilation. Alveolar ventilation is a true measure of CO2 removal

Breath Cycle Time (BCT)

The Ti varies with age
Normal I:E = 2
High I:E used ARDS with refractory hypoxia
Low I:E used in Obstructive disease

Breath determinants (Phase variables)

Trigger: This determines how the ventilator recognizes the patient's effort to initiate a breath (inspiration), simply what causes the breath to begin? It can be either Time-triggered (machine-initiated) or Flow/Pressure-triggered (patient-initiated).
Limit: The limit is what the ventilator aims to control, often focusing on minute ventilation. It can be either Volume-limited or Pressure-limited. What regulates gas flow during the breath?
Cycling: Cycling refers to how the ventilator switches from inspiration to expiration.

Trigger

Ventilator triggers can be set based on different parameters:

Time: Ventilation can be initiated at predetermined time intervals.
Flow: Ventilation is triggered when a specific flow rate is detected.
Pressure: Ventilation starts when a certain pressure threshold is reached.

Pressure triggers are typically set within the range of -0.5 to -2 cmH2O, while flow triggers are set in the range of 0.5 to 2 L/min.

Cycle

Mechanical ventilation's cycling phase controls the transition from inspiration to expiration and is regulated by the ventilator. There are three main modes:

Time-Cycled Ventilation: The ventilator manages the duration of the inspiratory phase (I:E ratio), primarily employed in Pressure Control Ventilation.
Volume-Cycled Ventilation: The ventilator delivers a predetermined tidal volume, transitioning to exhalation upon delivery, commonly used in Volume Control Ventilation.
Flow-Cycled Ventilation: Typically found in Pressure Support Ventilation (PSV), this mode triggers exhalation when the preset flow rate decreases to about 25% of its peak, enabling patient-triggered ventilation.

Limit/Control

It can be Pressure control or Volume control

In pressure-limit ventilation the pressure is constant and flow is variable, in volume-limit ventilation, the flow is constant and pressure is variable, thus volume-limit is actually flow-limit ventilation.

Limit/Control

It can be Pressure control or Volume control

Flow

When the inspiratory flow equals the expiratory flow, any difference between them indicates the presence of a leak.
Low Flow: Inadequately low flow rates can lead to air hunger, patient-ventilator asynchrony, and increased work of breathing.
High Flow: Conversely, excessively high flow rates can cause turbulence, inadvertent PEEP (positive end-expiratory pressure), air trapping, and increased resistance in the respiratory system.

Pressure

PEEP (Positive End-Expiratory Pressure): This is the pressure set above the baseline, which prevents the closure of recruited alveoli during expiration. It increases Functional Residual Capacity (FRC), improves ventilation-perfusion (V-Q) mismatch, and enhances oxygenation. Typically, PEEP starts at 3-5 cmH2O and is adjusted based on lung compliance. Lower PEEP targets are used in conditions like asthma or air trapping, while high PEEP can lead to overdistension, air trapping, reduced venous return, and potential hypotension.
Peak Inspiratory Pressure (PIP): PIP is the pressure needed to overcome airway resistance and lung compliance. It impacts oxygenation by increasing Mean Airway Pressure (MAP) and affects ventilation as a component of ΔP (pressure difference). The usual range is 18-25 cmH2O, with levels above 30 potentially causing barotrauma or air leaks.
Plateau Pressure (Pplat): Pplat is the equilibrium pressure reached when the expiratory tube is occluded at the end of inspiration. It represents the end-inspiratory alveolar pressure, helping to maintain open alveoli after overcoming airway resistance. Pplat-PEEP is called the driving pressure

Compliance

Compliance measures lung distensibility, calculated as ∆V/∆P or VT divided by the difference between end-inspiratory and end-expiratory pressure. Factors affecting it include lung tissue, surfactant production, and thoracic cage mobility.
Total Compliance of the Respiratory System is the sum of lung compliance and thoracic compliance. There are two types of compliance: Static and Dynamic Compliance
Static Compliance: Measured when there is no airflow, it assesses the distensibility of the entire respiratory system (including the chest wall and lungs) and excludes resistance. Static compliance is calculated as Cstat = VT / (Plateau Pressure - PEEP).
Dynamic Compliance: Measured when airflow is ongoing, dynamic compliance reflects both chest wall stiffness and airway resistance. It can be thought of as a measure of impedance. Dynamic compliance is calculated as Cdyn = VT / (Peak Inspiratory Pressure - PEEP).
Resistance measures the opposition to airflow and is calculated as the difference between Peak Inspiratory Pressure (PIP) and Plateau Pressure (Pplat) divided by the flow rate.

Time constant

The time constant (TC) represents the duration required for the lungs to either inflate or deflate to a specific extent. It is calculated as the product of Compliance (C) and Resistance (R): TC = C x R
The time constant helps describe how quickly pressure changes occur within the lungs during breathing. Specifically:
1 TC corresponds to the time taken for alveolar pressure to reach 63% of the pressure at the proximal airway. 2 TCs reach approximately 86%. 3 TCs reach around 95%. 4 TCs reach about 98%.
5 TCs indicate complete lung inflation or deflation.
Increased resistance leads to a prolonged TC (common in conditions like asthma), as it takes more time to overcome the resistance during airflow.
Decreased compliance results in a shortened TC (common in conditions like ARDS), as the lungs are less distensible and inflate or deflate more rapidly.

Oxygenation

Oxygenation depends on two factors; FiO2 and Mean arterial pressure

FiO2; Alveolar gas equation PAO2 = PiO2 - (PACO2/R)

Ventilator Mode

Modes:

Controlled mandatory
Intermittent mandatory
Spontaneous

Breaths:

Pressure control
Volume control

Controlled Mandatory Mode

All the breaths delivered are mandatory
The trigger, limit and cycle are controlled by the ventilator.
The patient cannot trigger spontaneous breath
The patient cannot change any set variables; RR, VT, P
It may cause asynchrony in patients who have active breathing efforts

Assist control Mode

Modification of controlled mandatory ventilation
All breaths are machine controlled and mandatory, however, the patient trigger is possible
Preset volume or pressure delivered to the patient in a time-cycled manner similar to controlled mandatory ventilation
Problem; Hyperventilation

Intermittent mandatory Mode

Mandatory + Spontaneous mode
The ventilator delivers
- - - Mandatory - Time triggered, Preset VT, P, & Ti
    - Spontaneous - Patient triggered, Patient determined VT, P, &Ti
No support for patient-triggered breath
Problem; Random chance of breath stacking and asynchrony (in order to overcome this issue, Synchronous IMV is used)
SIMV has better synchrony than IMV and AC mode. It is used as a weaning mode and as a primary mode when it is used with pressure support (especially in paediatrics).

Spontaneous mode

All breaths are spontaneous (patient triggered)
The patient effort determines the VT, Ti and flow rate
Patient triggered, patient (P or V) controlled, and patient (flow) cycled
Pressure support can be provided by the machine
Weaning mode

Ventilator Waveforms

Scalars are variables measured over time, typically represented as 1. Flow-time scalars, 2. Pressure-time scalars, and 3. Volume-time scalars.
Loops, on the other hand, depict the relationship between two dependent variables. 1. Pressure-volume loops and 2. Flow-volume loops.
Conditions to diagnose from graphs are
- - - - Obstructive versus Restrictive lung diseases
        Flow starvation
        Leak
        Air tapping

Flow-Time Scalar

In pressure control ventilation, the flow is variable ('decelerating ramp' flow pattern), while in volume control ventilation, the flow remains constant ('square wave' flow pattern).

The expiratory limb is determined by compliance of the lung and resistance of the airways. Four points to be noted regarding the expiratory limb 1. peak expiratory flow, 2. slope of the expiratory flow, 3. expiratory time, and 4. does the waveform reach the baseline?

In the square wave pattern, the mean pressure is generally lower and has a limited impact on cardiac output and venous return. In contrast, the decelerating ramp flow pattern is linked to higher mean pressure, potentially affecting cardiac output and preload due to increased airflow resistance during inspiration.

A gradual rise in flow associated with discomfort and flow hunger

Pressure-Time Scalar

In pressure control ventilation, consistent pressure is administered, whereas, in volume control ventilation, the pressure varies.

High resistance is observed in obstructive lung diseases, and low compliance conditions can lead to both increased Peak Inspiratory Pressure (PIP) and Plateau Pressure (Pplateau).

Volume-Time Scalar

Volume increases during inspiration and decreases during expiration. However, in some patients, expiration may not return to baseline, which could be attributed to either air leaks or air trapping. To distinguish between these conditions, it is essential to examine the flow-time scalar.

Auto PEEP (positive end-expiratory pressure) can result in progressive dynamic hyperinflation, a phenomenon often observed in obstructive lung diseases

Loops

Pressure-Volume Loops

In a spontaneous breath, inspiration generates a negative pressure, and expiration causes a positive pressure.

In CMV, both inspiration and expiration generate a positive pressure

Flow-Volume Loops

The loop typically forms almost a symmetric shape, with inspiration and expiration following similar paths.

Asynchrony

The mismatch between patient needs and the assistance delivered by the ventilator.

Ventilator related
- - Settings
  - Humidification
  - Circuit connection

or Patient-ventilator interaction

Types of asynchrony;

Trigger related

1. 1. Delayed triggering
  2. Ineffective triggering (most common)
  3. Auto triggering (cause; Leaks or condensates)
  4. Reverse triggering

5. Flow related

Cycle related

6. Premature Cycling

7. Delayed Cycling

Modes of mechanical ventilation

CMV - Continuous/Controlled Mandatory Ventilation (machine triggered along with control of limit and cycle)
AC - Assisted Controlled (like controlled mode, but patient trigger is possible, but cannot change other variables like VT or Ti)
IMV - Intermittent Mandatory Ventilation (Controlled + Spontaneous; pt determines variables too)
PSV - Pressure Support Ventilation
CPAP - Continuous Positive Airway Pressure
APRV - Airway Pressure Release Ventilation; CPAP with intermittent release phase

Airway Pressure Release Ventilation (Inverse ratio ventilation strategy)

High level of CPAP with timed intervals of pressure release.

Brand names:

Bivent in Maquet
BiLevel in Covidien
DuoPAP in Hamilton

Indication: Restrictive lung disease (low compliance), contraindicated in obstructive disease

Variables: 1. P high (as PIP in PC mode), 2. P low (similar to PEEP), 3. T high - inhalational phase (CPAP phase), and 4. T low- duration of exhalation phase/release phase.

Applying a constant high-pressure (P high), approximately 80-90% of cycle time, results in the persistent application of elevated Mean airway pressure (MAP), which allows almost constant lung recruitment (open lung approach - open the lung but not overdistended). We have to achieve FRC but not exceed TLC.
P high is set as P mean + 3 cm H2O.
T high is usually set for 4-6 seconds.
P low is set to between 0 and 5, but optimise expiratory flow so it does not reach the baseline. More important is to target expiratory flow than pressure.
T low - do not allow the termination of expiratory flow to go < 25% of PEFR (peak expiratory flow). This intrinsic PEEP allows P low to set at 0 without causing derecruitment. T low is minimised in poor compliance state and prolonged in obstructive disorders. T low can be as low as 0.3 seconds (closer to 75% PEFR) in restrictive lung disease and as high as 1.5 seconds (closer to 25% PEFR) in obstructive disease.

Pressure Regulated Volume Control (PRVC)

It is a form of adaptive controlled ventilation; it automatically adjusts pressure over several breaths to maintain a selected volume target.
It is a hybrid mode with the advantage of both PC and VC modes.
It provides lower PIP and guaranteed VT by close-loop regulation.

Be cautious in

high flow demand system
Obstructive airway
Leak

Brand names:

Pressure-regulated volume control (PRVC) - Maquet
Adaptive pressure ventilation - Hamilton
Autoflow - Drager
Volume control plus
Variable pressure control

Adaptive Supprt Ventilation (ASV)

Variables defining a ventilator

Trigger variable
Control variable, and
Cycling variable

Modes of mechanical ventilation

Types of breath in mechanical ventilation

Mandatory; The ventilator initiates breaths, and the ventilator performs the work of inspiration
Assisted; The patient initiates breaths, but the ventilator performs at least some of the work of inspiration.
Spontaneous; The patient initiates breaths, and the patient performs the entire work of inspiration.

Breath delivery is either volume-limited or pressure limited

A mandatory breath is started, controlled, and ended by the ventilator, which does all the work. An assisted breath is initiated by the patient but controlled and ended by the ventilator.
Volume-limited breaths can be ventilator-initiated (called Volume controlled; VC or volume cycled) or patient-initiated (called Volume assisted; VA).
A new version of volume-limited ventilation is PRVC (Pressure regulated volume control); Here, the TV is set, and the applied airway pressure changes to attain the target tidal volume (resulting in variable inspiratory flow).
Assist-control ventilation (ACV) – With ACV, every breath is fully supported by the ventilator, regardless of whether the breath is initiated by the patient or the machine. The clinician determines a base ventilatory rate, but the patient is able to breathe faster than this preset rate. Potential dangers include diminished cardiac output and inappropriate hyperventilation.

Ventilator settings

Ventilator settings are adjusted to provide adequate minute ventilation (MV).

Trigger; An assisted breath can be triggered by either pressure or flow. Regardless of the trigger mechanism, the threshold must be appropriate. If it is too low, the ventilator will "auto-cycle" (ie, deliver a continual series of breaths), resulting in respiratory alkalosis; if it is too high, the patient will be "locked out" from ventilator-supported breaths.
Modes (breath types); A mandatory breath is started, controlled, and ended by the ventilator, which does all the work. An assisted breath is initiated by the patient but controlled and ended by the ventilator. A spontaneous breath is initiated, controlled, and ended by the patient

Respiratory Rate;
I:E ratio; Inspiratory time (I time) is equal to TV divided by flow rate (I time = TV/FR). Decreasing the TV or increasing the flow rate decreases the inspiratory time and decreases the I:E ratio. The normal I:E ratio is 1:2 or 1:3.
Flow rates; of 60 L/minute are standard. Higher flow rates (up to 100 L/minute) may be required in certain conditions, such as obstructive airway disease (OAD).
Cycling; Ventilator "cycling" refers to the mechanism by which a breath changes from inspiration to expiration. Common methods for cycling a ventilator are volume, flow, and time. Inspiratory hold can also be included in the cycling mechanism. This function keeps air in the lungs at end-inspiration for longer intervals to maximize gas exchange.
Positive end-expiratory pressure (PEEP); Applied or extrinsic positive end-expiratory pressure (PEEPe) can be provided by the ventilator to prevent premature airway closure and alveolar collapse at end-expiration. PEEP allows for improved oxygenation by increasing functional residual capacity (FRC)

Fraction of inspired oxygen (FiO2); FiO2 is typically set at 100 percent when mechanical ventilation is initiated. Then reduce to a nontoxic levels (generally 0.6 or less), provided that an oxygen saturation (SpO2) of 90 percent or greater can be maintained (PaO2 above 60 mmHg).

Typical initial settings (LTVV)

Assist control mode
Initial TV of 6 mL/kg PBW (can be as low as 4 mL/kg if ALI/ARDS).
Respiratory rate 16 breaths/minute (range 14 to 22 breaths/minute) to meet the patient’s MV requirements (can titrate up to 35 in order to keep pH above 7.25, as long as there is adequate expiration time to prevent intrinsic PEEP [PEEPi]
Inspiratory flow rate 60 L/minute (range 40 to 90 L/minute)
FiO2 100 percent (titrate to 60 percent or below as quickly as possible)
PEEPe of 5 cm H2O (range 5 to 10 cm H2O)
Trigger sensitivity; 2 L/mt (flow)
Keep plateau pressures at 30 cm H2O or less

General Principles

Minimize plateau pressures and tidal volumes (TVs), allowing hypercapnia, if necessary (except in brain-injured patients), to reduce the risk of lung injury.
Optimize extrinsic positive end-expiratory pressure (PEEPe) to prevent alveolar collapse and improve oxygenation.
Reduce inspired oxygen to nontoxic levels (≤60 percent) as quickly as possible.
Minimize the risk of ventilator-associated pneumonia (VAP) by maintaining the head in an elevated position whenever possible

Ventilatory Strategies

Three fundamental strategies for mechanical ventilation:

Positive-pressure noninvasive ventilation (NIV)
LTVV; also known as lung-protective IPPV (Invasive positive-pressure ventilation)
General IPPV.

General rules of mechanical ventilation

Avoid TV > 10ml/Kg
In ARDS, typically set the TV as 6 ml/PBW (range 4-8). Randomized trials have shown improved mortality with LTVV (low tidal volume ventilation)
In Non-ARDS patients, the optimal TV is 6-8 ml/PBW
Initially set ventilatory rate between 12 and 16 breaths per minute is reasonable.
For patients with ARDS, higher rates may be needed in order to facilitate LTVV
Excessively high rates may be complicated by auto-PEEP
For patients receiving SIMV, at least 80 percent of the patient's total minute ventilation is delivered by the ventilator.
A typical initial applied PEEP is 5 cm H2O. However, up to 24 cm H2O may be used for ARDS undergoing LTVV.

Plateau pressure goal: Pplat ≤30 cm H2O
Check inspiratory plateau pressure with 0.5 second inspiratory pause at least every four hours and after each change in PEEP or tidal volume.
Oxygenation goal: PaO2 55 to 80 mmHg or SpO2 88 to 95 percent.
Oxygenation goals should be individualized and hyperoxia should be avoided. Both hyperoxia (PaO2 > 120) and hypoxia (PaO2<80) are associated with increased mortality.
For patients with hypoxemic respiratory failure who are difficult to oxygenate on high FiO2, or patients with hypercapnic hypoxemic respiratory failure, the limit of acceptable oxygenation may be lower. For example, an arterial oxygen tension (PaO2) of 55 mmHg and a SpO2 of 88 percent may be acceptable in patients with hypercapnia from chronic obstructive pulmonary disease (COPD).
In Pressure-limited ventilation, The inspiratory pressure level (typically 12-25 cm H2O) is set to target an approximate tidal volume (4-8ml/PBW).

During volume-limited ventilation, the maximum rate of flow of air into the lung during inspiration is predetermined (ie, a peak inspiratory flow rate is set). An initial peak inspiratory flow rate is typically set between 40 and 60 L per minute (targeting an I:E ratio of 1:2 to 1:3). These settings are sufficient to overcome the pulmonary or ventilator impedance
An insufficient peak flow rate is characterized by dyspnea (from increased work of breathing)
A higher peak flow rate is needed in patients who have obstructive airways disease- it shortens the inspiratory time, ie, decreases I:E.
Peak flow rates >75 L/minute may be harmful.
During pressure-limited ventilation, the inspiratory flow rate is not set but is determined by the inspiratory pressure limit, the inspiratory time, as well as the compliance/resistance of the respiratory system and patient effort. Thus, unlike volume-limited ventilation, the flow is variable.

Supportive care of mechanically ventilated patients

Sedation, Analgesia and Delirium management
Hemdynamic monitoring
Glucose control
Nutritional support
Measures to prevent VAP
Venous thromboembolism prophylaxis
Gastrointestinal prophylaxis
Venous or Arterial access
Temperature management

Weaning Criteria

Patients who are mechanically ventilated for more than 24 hours should undergo daily Ventilator Liberation Protocol Assessment. Liberation from mechanical ventilation is a three-step process that involves

Readiness testing
Weaning, and
Extubation.

Ready for weaning criteria:

Evidence for some reversal of the underlying cause of respiratory failure
Adequate oxygenation (PaO2 >60 mmHg on FiO2 <0.4; extrinsic positive end-expiratory pressure [PEEPe] <5 cmH2O; PaO2/FiO2 >150 to 300)
Stable cardiovascular status (heart rate <140; stable blood pressure; no or minimal use of vasopressors)
No significant respiratory acidosis (pH ≥7.25)
Adequate hemoglobin (generally >7 g/dL in patients without ischemic cardiac disease or hemoglobin >10 g/dL in patients with ischemic cardiac disease)
Adequate mentation (arousable; can follow commands reliably; no continuous sedative infusions)
Ability to take spontaneous respirations
Stable metabolic status (acceptable electrolyte levels)
Core temperature < 38 degrees C
Rapid shallow breathing index (RSBI), a weaning predictor should be < 105.

RSBI

RSBI Calculation; Set the ventilator to PSV of 0 cm H2O and a PEEP of 0 cm H2O, without flow or pressure trigger for one minute. The tidal volume can then be determined by the ventilator. However, the respiratory rate should be manually counted since the ventilator may underestimate the respiratory rate if the patient makes inspiratory efforts that are not sensed by the ventilator. RSBI = RR/TV breaths/mt/L

Interpretation; An RSBI ≥105 breaths/minute/L (ie, a negative RSBI) indicates that a patient is likely to fail to wean while a positive test RSBI <105 breaths/minute/L is more likely to undergo successful weaning. Evidence suggests that a negative RSBI (RSBI ≥105 breaths/minute/L) is better at identifying patients who will fail weaning than a positive RSBI

Extubation failure

The pathophysiologic causes of extubation failure include an imbalance between respiratory muscle capacity and work of breathing, upper airway obstruction, excess respiratory secretions, inadequate cough, encephalopathy, and cardiac dysfunction.

Mechanical ventilation - Complications

Pulmonary complications;
- Barotrauma
- Ventilator-induced lung injury (VALI)
- Ventilator-associated pneumonia
- Respiratory muscle weakness, and Secretion retention
Nonpulmonary complications;
- Diminished cardiac output and hypotension
- Gastrointestinal stress ulceration, gut hypomotility, and decreased splanchnic perfusion
- Acute kidney injury
- Peripheral neuromuscular weakness
- Increased intracranial pressure, disordered sleep, memory and cognitive impairment, inflammation, and reduced immunity.

Physiologic effects;
- Auto-positive end-expiratory pressure (auto-PEEP; ie, intrinsic PEEP); due to incomplete exhalation.
- Heterogenous ventilation
- Altered ventilator/perfusion mismatch (increased dead space, decreased shunt)
Equipment malfunction

Physiology

Physiological dead space = Anatomical dead space + Volume of alveoli which are ventilated but not perfused.

Compliance and Resistance

Lung Compliance

Change in Volume / Change in pressure

TV/Pplateau - PEEP

Airway pressure:

Ventilation pressure = Resistive pressure (pushes airflow through airways) + Elastic pressure (inflates lungs and chest wall).

Ppeak = Maximum pressure in the proximal airway at the end of inspiration. Ppeak = Airway resistance / Compliance

Pplateau = It is the end-inspiratory alveolar pressure, or It is the equilibrium pressure reached if the expiratory tubing is occluded at the end of inspiration. Pplateau = 1/Compliance

If Ppeak increased, but Pplateau is normal => it means the airway resistance.

If both Ppeak and Pplateau increased => it means lung compliance is decreased.

Gas Exchange:

Alveolar ventilation, determined by PaCO2
Alveolar gas equation, determined by PAO2 = FiO2 [Pi-PH2O] - PaCO2/RQ
A-a gradient is a measure of how effectively oxygen moves from the alveoli into the pulmonary vasculature = PAO2-PaO2 = [Age/4] + 4

Lung Mechanics

Compliance

Compliance measures lung distensibility, calculated as ∆V/∆P or VT divided by the difference between end-inspiratory and end-expiratory pressure. Factors affecting it include lung tissue, surfactant production, and thoracic cage mobility.
Total Compliance of the Respiratory System is the sum of lung compliance and thoracic compliance. There are two types of compliance: Static and Dynamic Compliance
Static Compliance: Measured when there is no airflow, it assesses the distensibility of the entire respiratory system (including the chest wall and lungs) and excludes resistance. Static compliance is calculated as Cstat = VT / (Plateau Pressure - PEEP).
Dynamic Compliance: Measured when airflow is ongoing, dynamic compliance reflects both chest wall stiffness and airway resistance. It can be thought of as a measure of impedance. Dynamic compliance is calculated as Cdyn = VT / (Peak Inspiratory Pressure - PEEP).
Resistance measures the opposition to airflow and is calculated as the difference between Peak Inspiratory Pressure (PIP) and Plateau Pressure (Pplat) divided by the flow rate.

Page updated

Google Sites

Report abuse