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RSPT 2372 Intro to MV: Chapter 3 Principles of MV

Objectives

• Summarize the historical milestones leading to modern mechanical ventilation
• Contrast positive vs negative pressure ventilation
• Recognize differences in patient interfaces for invasive vs noninvasive mechanical ventilation
• Define the timing components of a breath and calculate respiratory rate using TI and TE
• Describe alveolar and dead space ventilation and calculate E and A (not specified)
• Interpret the changes in volume, airflow, alveolar pressure and intrapleural pressure over the course of a single breath
• Describe the differences between an iron lung and a chest cuirass
• Identify the components of a ventilator circuit and events during lung inflation and deflation using positive pressure
• Describe the effects of lung mechanics alterations (CST and Raw) on volume and pressure in VC and PC modes
• Predict the changes in peak inspiratory pressure and plateau pressure when CST or Raw are altered
• Define PEEP and it's influence on gas exchange and hemodynamics
• Describe the variables of interest in an optimal PEEP study
• Describe patient scenarios leading to increased mean airway and peak inspiratory pressures
• Define pressure support ventilation (PSV) and its influence on the work of breathing (WOB)
• Describe the variables that trigger inspiration during mechanical ventilation
• Describe the variables that cycle a breath from inspiration to expiration.
• Contrast the difference between PC-AC and VC-AC
• Contrast the difference between PC-IMV and VC-IMV
• Describe the use of automatic tube compensation (ATC)
• Identify dual modes of ventilation
• Describe inspiratory flow waveforms used in mechanical ventilation
• Determine the ventilator variables that affect Pao2, pH and Paco2
• Identify alarms that require adjustment and their level of priority
• Describe the rationale for a sigh breath
• Explain the effects of positive pressure ventilation on the lung
• Explain the effect of positive pressure ventilation on the cardiac/cardiovascular system
• Describe central nervous system (CNS), renal and gastrointestinal effects of positive pressure ventilation
• Explain the importance of sedation protocols during weaning
• Describe the influence of Paco2 on intracranial pressure (ICP)
• Identify the effects of sleep disruption on the ICU patient
• List the complications of mechanical ventilation and explain each

Introduction to Mechanical Ventilation

• Positive pressure ventilation is now predominant
• Earlier ventilators included Bird and Bennett valves
• Advantages to positive pressure ventilation include less space required, better patient access and ability to set precise tidal volume (VT) and backup RR
• In the 1960s and 70s volume ventilators became available, triggered by time, later by the patient
• Ventilator-induced lung injury (VILI) can result from large volumes and pressures, with understanding of mediators
• Earlier VT settings were 10-15 mL/kg, now typical settings are 4-8 mL/kg

History of Mechanical Ventilation

• 1928 brought negative pressure ventilation which didn't require artificial airway and was simple to use
• 1932 saw the "Iron Lung" by John Emerson
• 1940s and 50s was a Polio epidemic across Europe and the US, resulting in 500,000 ppl/year dead or paralyzed, and negative pressure vents were in large halls
• 1952 witnessed outbreaks of Polio in Copenhagen
• 50 new admits per day with a mortality rate of 87%
• 1,500 medical students provided bag mask ventilation resulting in 165,000 hours of ventilation
• Mortality decreased by 25%
• Use of positive pressure ventilation due to polio

Methods to Reduce VILI

• Appropriate positive end-expiratory pressure (PEEP)
• Lung recruitment strategies
• Permissive hypercapnia
• Newer pressure limiting modes
• Non-invasive ventilation (NIV)

Goals and Support of Mechanical Ventilation

• Goal: balance oxygenation and ventilation
• Support depends on underlying disease process, length of time for resolution and expected outcomes

Ventilation

• Can be defined as the bulk movement of gas into and out of the lungs
• Gases of interest for ventilation include:
  • Nitrogen
  • Oxygen
  • Carbon dioxide
• Volume of Oxygen Uptake (VO2) refers to the amount of oxygen consumed by the body in a given period of time; normal is 250 mL O2/min
  • Volume of Oxygen Uptake (VO2) is measured in L/min or mL/kg
• Volume of Carbon Dioxide Output (VCO2) represents the amount of CO2 produced and exhaled by the body in a given period of time; normal is 200 mL CO2/min
  • Volume of Carbon Dioxide Output (VCO2) is measured in L/min or mL/kg
• Nitrogen is an inert gas; does not cross the AC membrane except at high altitudes

Ventilatory Cycle

• One inspired volume of air plus one expired volume of air constitute total cycle time (TCT)
• Ttot = Ti + Te
• Ti: inspiratory flow moves from 0 to peak and back to 0 at end inspiration
• Te: begins at end inspiration or 0 until next inspiratory cycle; is generally longer than Ti
• Te may include a pause at 0

Minute Ventilation and Rate

• Normal adult tidal volume (VT) = 400-700 mL or 7 mL/kg of ideal body weight (IBW) or predicted body weight (PBW)
• Normal adult respiratory rate (RR) = 12 bpm (range 12-20 breaths/min)
• Normal minute ventilation (MV) = 6 L/min (range 5-10 L/min)
• VE (MV) = VT x f (frequency or breaths per minute
• Example: 500 mL/breath x 12 breaths/min = 6,000 mL/min or 6 L/min

Gas Exchange and Alveolar Ventilation

• Only 70% of VT reaches alveoli and participates in gas exchange
• Alveolar ventilation per breath is represented by VA
• Remaining 30% fills conducting airways and equals 150mL/breath (about 1mL/lb IBW), from nares to terminal bronchioles (anatomic dead-space) or VD ant
• VD alv alveolar deadspace consists of ventilated but not perfused alveoli Physiologic deadspace, or VD phys = VD ant + VD alv, inspired gas that doesn't participate in gas exchange
• Alveolar ventilation (A) equals (VT - VDphys) x f
  • Ex. (500 mL - 150 mL) x 12 breaths/min = 4200 mL/min or 4.2 L/min
• There is a direct relationship between alveolar ventilation, CO2 production and arterial Paco2
  • A = (0.863 × CO₂) ÷ PaCO2
  • Ex. (0.863 x 200) A increases are equal to CO2 decreases, vice versa
• As CO2 increases, VA must also increase to keep PaCO2 constant

Spontaneous Breathing

• Happens without conscious awareness
• Timing and flowrate varies based, sleep/wake state and activity
• The CNS contains:
  • Phrenic nerves
  • Innervates the diaphragm
• Tidal volume (VT) is approximately 500 mL/breath
• Cough, sneeze, or exercise can result in larger volumes
• The duration of inspiratory cycle is about one second
• Average flow rate is about 0.5 L/sec or 30 L/min
• Diaphragm contracts and descends (intrapleural and intrathoracic pressure decreases)
• Quiet respiration intrapleural pressure at passive end expiration is around -5 cm H2O
• Quiet respiration intrapleural pressure during inspiration around -10 cm H2O
• Normal lung-thorax system compliance is about 100 mL/cm H2O, meaning a -5 cm H2O pressure change equals 500 mL/breath
• Normal spontaneous breathing happens with inspiration and alveolar pressure is below atmospheric
• Pressure changes allow for inspiratory and expiratory gas flow

Negative Pressure Breathing

• Mechanical ventilation can be invasive or noninvasive
• Involves the use of:
  • The Iron Lung
  • United Hayek's Biphasic Cuirass Ventilator, which consists of:
    • Video of BCV (Bronchial hygiene assistance)
    • High-frequency chest wall oscillation (HFCWO) and cough assist

Positive Pressure Ventilation (PPV)

• Requires a sealed airway from either ETT, Trach, or mask
• Inflatable cuffs used, prevents dislodgement
• Supports resuscitation bag ventilation
• Patient "Y" connects the patient to the ventilator
  • The inspiratory limb carries gas from the ventilator to a "Y" connector and endotracheal tube
  • The expiratory limb has a one way valve which closes upon inspiration, allowing expired gases to flow through
• Heated humidification required for intubated patients
• During inspiration airway pressure increases
  • Depends on machine settings, and lung compliance and resistance, such as low lung compliance increases peak pressure and plateau pressures
• Larger VT = greater peak pressure
• During expiration, the expiratory valve opens from the inspiratory phase, allowing air to escape.
• Respiratory Therapists adjust flow, volume, time and pressure
• To provide optimal gas exchange while minimizing the risk of barotrauma
• Untrained or unexperienced personnel should not make changes without supervision
• Troubleshooting must occur during ventilator malfunction
• May disconnect for ambu bag ventilation

Peak Inspiratory Pressure (PIP)

• The highest proximal airway pressure attained during the inspiratory phase
• Affected differently by ventilation type
  • Pressure control (PC) ventilation
  • PIP is set, volume varies
• In Volume control (VC) ventilation PIP PIP varies based on set tidal volume (VT), and is Influenced by: -Inspiratory flow, inspiratory flow waveform, ventilator circuit/endotracheal tube resistance, and lung mechanics (compliance and resistance).
• Aim to maintain PIP < 35 cm H₂O and decrease risk of barotrauma

Plateau Pressure

• Measured with an inspiratory hold maneuever in V/C, for 1 second or less
• Under static conditions, Pplateau= alveolar pressure, including from smallest airways
• Plateau pressure can be used to determine static compliance and overall airway resistance (airways resistance or RAW)
• Used for the calculation of static compliance and airway resistance • PIP - Pplateau = airways resistance (RAW) • RAW = PIP - P plateau / Inspiratory flow (L/sec)
• Determined by elastic lung tissue recoil in absence of airflow = static lung compliance (CST) during VC • CST = VT/Pplateau - baseline pressure

Baseline Pressure and PEEP

• Resting airway pressure
• Baseline pressure equals ambient pressure equals Zero
• Baseline pressure greater than ambient pressure equals Positive End Expiratory Pressure (PEEP)
• Use positive end-expiratory pressure (PEEP) • To maintain alveolar volumes during expiration and prevent collapse Can also:
• Improve overall oxygenation
• Extrinsic PEEP is set intentionally to improve lung volumes

Auto PEEP or Intrinsic PEEP

• Air trapping, otherwise known as dynamic hyperinflation is indicated with incomplete emptying of lungs during expiration
• Can be measured with expiratory pause maneuver
• Can lead to increased mean airway pressure (MAI)
• More prevalent with patients experiencing obstructive disease on MV like COPD

Optimal PEEP

• Appropriate level of PEEP needed to improve and maintain lung volumes and improve oxygenation restrictively to patients with restrictive pulmonary disease, is in between 3-5cm H2O
• Indicated for most MV patients
• Achieve through incrementing until adequate oxygenation
• Be cautious with high PEEP patients prone to:
  • Hypotension
  • Hypovolemia
  • Decreased Intracranial pressure

Mean Airway Pressure (Paw)

• Average pressure in the airways throughout the respiratory cycle
• Area under the expiratory curve
• Increased by:
  • Increased T
  • Increase I: E ratio
  • Decreasing time expired
  • Increasing V
  • Increased intrinsic PEEP a
  • Auto PEEP
  • Decreasing spontaneous breathing Down ramp- decreasing flow pattern
  • Low lung compliance increasing HAW (High Airway Resistance)

Invasive vs. Noninvasive ventilation

• Shared similarities, positive pressure, measuring volumes, airway pressures and sensing capabilities
• Differed based on:
  • Costs effectiveness with spontaneous efforts
  • Apneic patients and interfaces.

Ventilator Principles

• Pneumatically powered from- high-pressurized gas source or Electrically powered ventilators with internal components that rely on electricity
• Controlled via Pneumatic valves, Electrical circuits, microprocessors

Pneumatically and Electrically Powered Ventilators

• Both systems utilize the same micoprocessor to control ventilation and airflow
• Electrical has ability to be more precise than Pneumatic-based ventilators

Control Systems

• Backup batteries that last for as long as 2 hours, used for issues relating to charge failure and battery
• Vents have On-board gas compressors and liquid oxygen systems
• Utilizes combination of pressure and electrical/micro-controlled systems
• Two type of loops:
  • Open loop, without a feedback signal
  • Closed- loops; adjust gas and measure flow
• The Control Panel: can be mechanically or virtually knobs for setting adjustments

Trigger, Volume, Pressure, and Flow

• Trigger Mechanism/ Variable; to begin a breath with change is pressure or flow These actions can be triggered by time and/or the patients, as well as pressure and flow

Pressure, Flow, Time, and Volume Breath Delivery Options

• Pressure Trigger; Typical pressure releases 0.5-1.5 c/H20, but requires effort and is patient sensitive based on PEEP settings
• Flow Trigger; Bias flow is sensitive, with airflow during expiring the patient's inspiratory effects
• Time trigger; mandatory set depending on respiration rate

Cycling, Mandatory, and Patient Breathing Patterns -

• Cycle is a method where the breath is cut off, where triggering is to begin the same pattern
• Patients cycles are related spontaneous breathing patterns and comfort based on rise times.
• Mandatory Breaths are the equal same breath ventilator delivers over period

Volume and Time Cycling

• Switch from an inspiration with a set inspiration pressure can lead to cycle pressure
• Volume can shift through variable pressure that are depended on the patient
• Time to Obstructive and Restricitve lung diseases for cycling.

Volume Cycling and ARDS (acute respiratory distress syndrome)

• The term indicates a condition associated with leakages of blood.

1. During the 70's , 80's, and 90's they were treated with VT 10 and/or 15ml
2. Now, the current practice is to keep antibiotic treatment and volume constant (4 to 8ml) with 30cm of H20

Flow and Optimal Levels

• Used for ventilator flow, and decreasing the % levels of pressure and inflation

Ventilator Classification , Mode and Taxonomy

• Mode: to be connected wit(VCV) , Volume, with a constant Mandalay setting based on patients inflation,
• Taxonomically with flow and pressure controls

Ventilatory and Targeting Schemes

• A set Point depending on patients, their servo’ mechanism

Modes and Maximal Inflation

• 10 Maxims helps inflation This include flow, cycle, the timing of inspiration, triggers, with spontaneous of Mandala and flow.
• There are 3 steps involved when in the basic breathing sequence

Basic Ventilation Modes

• Control variables that depends pressure or volume, that are to be Mandala to patient

CMV (continuous mandatory ventilation)

• Also known as assist control, that is set either via pressure or every time
• Patient have time and trigger functions but is volume controlled
• Spontaneous breaths cant help initiate the ventilator from control breaths

VC(volume control)CMV

• Constant Air Flow dependent on mechanics that shift from higher peak to lower points

PC (pressure control)CMV

• Constant Pressure depending on the lungs condition that decreases Tidal volume (VT)

IMV (intermitted mandatory ventilation)

• IMV has either pressure and/or volumes

SIMV (synchronized intermittent mandatory ventilation)

• Improved patterns and vent sync- for those that do trigger

Intermitten Synchronized Volume Control

• VT controls that improve with the patients

Synschronized pressure control

• Lung mechanics and triggers are assisted by the pressure, leading to an overall enhanced breathing rate.

Recruitment Inflation

• The process can improve patients who are in ARDS, in about2-3 minutes, until decreased.
• Plateau has a lower levels, and must be measured.

Positive Pressure Ventilation

• Pressure must be sustained for the breathing functions

Automatic Tube Ventilation

• Provides airway pressure to improve oxygen delivery.

Ventilation Parameters and Waveforms

• Assists with observing levels of pressure, flow and volume.

Insipiratory and Airway pressure

• Must make sure that total Static compliance is between 60-100ml/m on H2O, to minimize Lung compilations

FIO2 and excessive Levels

• High flow and low pressures based on barometric pressure to minimize Oxygen toxicity .

Factory and Safety

• There must be multiple checks, because it is crucial

Airways and Humidity

• Hyperthermal and re-warming airways can affect and lead to airways issues and humidity complications

What is Sigh

• The term that assists alveolar units to breath and prevent atelectasis via machine.
• However, this can be controversial is setting "recommended ventilation systems settings and modes" so use this with caution.

Effect of ventilators on organ

• Primary Function: to improve or augment ventilation
• However: a patients baseline levels must be documented to understand if any changes

Oxygen and Low flow levels

• All ventilators requires low levels when possible to reduce pressure and decrease oxygen toxicity

Muscle Function for Ventilator

• Can increase pressure and reduce cardiac or pulmonary arrest. However, also comes at risk of atelectasis due to inflammation from injury. These factors must be accounted for.

Airways with high flow

• High airways require PEEP to deliver better oxygen

Respiratory and Diaghphramatic Muscle functions

• Disease.use cases the decline, but can improved via support

Mucociliary Motility Levels and WOB.

• This combination requires ventilation to assist at optimum air levels to improve motility, and proper cleaning with suction.