Silverstein and Hopper Chapter 32: Mechanical Ventilation - Core Concepts

Mechanical Ventilators vs Spontaneous Breathing

Mechanical ventilators use an increase in airway pressure to move gas into the lungs in contrast to spontaneous breathing when airway pressure decreases below atmospheric pressure in order to generate the inspiratory phase of a breath

Oxygenation

• Movement of oxygen from the alveoli into the pulmonary capillaries

  • Primarily dependent on the surface area available for gas exchange and preservation of the gas-exchange barrier

Ventilation

• Removal of carbon dioxide

  • Primarily dependent on fresh gas movement into the alveoli

Intrathoracic Pressure

• During spontaneous breathing, the intrathoracic or pleural pressure falls (becomes subatmospheric) during inspiration as a result of the expansion of the chest wall and movement of the diaphragm

  • This pressure change results in the inspiratory flow of air into the lungs

• In contrast, PPV utilizes positive airway pressures to generate inspiratory gas flow

Equation of Motion

Pvent + P muscles = Elastance x Volume + Resistance x Flow

• Pvent - pressure generated by the ventilator

• Pmuscles - pressure generated by inspiratory muscles

• Elastance is the inverse of compliance

Total pressure needed to generate a ventilator breath

Resistance

Reflects the pressure required to generate a given flow

Ohm’s Law Equation for Resistance

Resistance = driving pressure/flow

Airway Resistance During Mechanical Ventilation

• Any narrowing of the airways will increase resistance including airway collapse or narrowing or diffuse bronchoconstriction

• The endotracheal tube is often the source of greatest resistance present during mechanical ventilation

Compliance

• A measure of the distensibility of the lung, defined as the change in lung volume for a given change in pressure

  • In ventilated patients, this translates to delivered V_T/pressure required to generate the V_T

  • Elastance - inverse of compliance

  • A lung with high compliance will accept a large increase in volume for a small pressure change

  • Low compliance would require a large pressure change to create a small increase in volume

Compliance Equation

Compliance = delta volume/delta pressure

Dead Space

• Portion of the tidal volume that does not participate in gas exchange

• Can be categorized as apparatus, anatomic, and alveolar in origin

Apparatus Dead Space

The volume of the circuit from the Y-piece to the nose of the patient

Anatomic Dead Space

The volume of the conducting airways from the nose to the level of the alveoli

Alveolar Dead Space

Alveoli that are ventilated but not perfused

What are the two major types of ventilator breath?

Mandatory

Spontaneous

Mandatory Breath

When the machine controls initiation and/or termination of inspiration

Assisted Breath

When a mandatory breath is initiated by the patient

Spontaneous

Patient is responsible for both initiation and termination of inspiration

Supported Breath

A spontaneous breath in which the inspiratory flow is augmented by the machine

Mandatory Breath Initiation (Trigger)

Ventilator

Mandatory Breath Inspiratory Flow

Ventilator

Mandatory Breath Termination (Cycle)

Ventilator

Assisted Breath Initiation (Trigger)

Patient

Assisted Breath Inspiratory Flow

Ventilator

Assisted Breath Termination (Cycle)

Ventilator

Spontaneous Breath Initiation (Trigger)

Patient

Spontaneous Breath Inspiratory Flow

Patient

Spontaneous Breath Termination (Cycle)

Patient

Supported Breath Initiation (Trigger)

Patient

Supported Breath Inspiratory Flow

Ventilator

Supported Breath Termination (Cycle)

Patient

What are the three possible breath patterns?

Continuous mandatory ventilation (CMV)

Intermittent mandatory ventilation (IMV)

Continuous spontaneous ventilation (CSV)

Continuous Mandatory Ventilation (CMV)

All mandatory breaths are delivered

Intermittent Mandatory Ventilation (IMV)

Both mandatory and spontaneous breaths

Continuous Spontaneous Ventilation (CSV)

All spontaneous breaths

Assist/Control

When all breaths delivered are mandatory while allowing the patient to trigger a mandatory breath to allow ventilation to be synchronized with the patient’s efforst

What are the four phases of breath that can be controlled by the ventilator?

The start of inspiration

Inspiration

The end of inspiration

Exhalation

Pressure-Time Scalar of the Respiratory Cycle

What variables are not under ventilator control?

Compliance and resistance of the system are inherent to the patient and not under ventilator control

What are the three interdependent variables that can be manipulated by the ventilator machine?

In any one breath, the ventilator can only directly control one of these variables at a time, and the other two variables become dependent variables

What are the three types of control that ventilator breath can be considered?

Pressure-controlled

Volume-controlled

Flow-Controlled

Control Variable

• The magnitude of the remaining two dependent variables will be determined by the set value of the control variable and the compliance and resistance of the system

Pressure-Controlled Breath

• In a pressure-controlled breath, the machine will maintain airway pressure as determined by the operator, and inspiration ends when a preset inspiratory time is reached

  • The tidal volume and gas flow rate generated during the breath are dependent on the magnitude of the preset airway pressure as well as the resistance and compliance inherent to that animal

Volume-Controlled and Flow-Controlled Breaths

• Volume-controlled and flow-controlled breaths are essentially the same, the machine will deliver the preset tidal volume over the preset inspiratory time

  • Airway pressure reached during these breaths is dependent on the magnitude of the preset tidal volume and subsequent flow rate, as well as the resistance and compliance of the patient's respiratory system

Cycle Variable

• Parameter by which inspiration is terminated

• Time is the most common cycle variable

  • Determined by the preset respiratory rate and the I:E

  • Inspiratory time of ~1 second commonly recommended

Trigger Variable

• Parameter that initiates inspiration

• In animals that are not making respiratory efforts of their own, the trigger variable will be time and is determined from the set respiratory rate

• If the animal is making respiratory efforts, the trigger variable may be a change in airway pressure or gas flow in the circuit resulting from the patient's attempt to initiate inspiration

• On most machines, the trigger sensitivity can be set by the operator

• An airway pressure decrease of 2 cm H2O or gas flow change of 2 L/min are usually effective trigger sensitivities, but lower settings may be necessary in smaller patients

• Appropriate trigger sensitivity ensures that ventilator breaths are synchronized with genuine respiratory efforts made by the patient

• If trigger variable is too sensitive, it can lead to initiation of breaths after nonrespiratory movements such as patient handling - avoid

Limit Variable

• Parameter that the breath cannot exceed during inspiration, but is different from the cycle variable because it does not terminate the breath

• May be found on modern intensive care ventilators

Baseline Variable

• Controlled during exhalation

• Airway pressure is the most common baseline variable manipulated

• If airway pressure during exhalation is maintained above atmospheric pressure, it is referred to as positive end expiratory pressure (PEEP)

Continuous Mandatory Ventilation

• A minimum respiratory rate is set by the operator

• If the trigger sensitivity is set appropriately, the patient can increase the respiratory rate, but all breaths delivered will be of a mandatory breath type

Controlled Ventilation

Patient is unable to trigger breaths

A/C (Assist/Control)

Patients are allowed to trigger their own respiratory rate

Intermittent Mandatory Ventilation

• A set number of mandatory breaths are delivered

  • Between these breaths, patients can breathe spontaneously

Synchronized Intermittent Mandatory Ventilation (SIMV)

• The machine tries to synchronize the mandatory breaths with the patient's inspiratory efforts (assisted breaths)

  • The ventilator has a window of time in which it will deliver a mandatory breath

    • If the patient triggers a breath during this period, it will be assisted appropriately

    • If no breath is triggered by the end of this time period, a mandatory breath will be given

    • Between these mandatory breaths, the patient can breathe spontaneously as often or as few times as desired

  • Operator can control only the minimum respiratory rate and minimum minute ventilation

    • No control over the maximum rate or maximum minute ventilation

Continuous Spontaneous Ventilation

• Every breath is triggered and cycled by the patient

• Respiratory rate, inspiratory time and tidal volume are also determined by the patient

What are the two most common forms of continuous spontaneous ventilation?

Continuous positive airway pressure (CPAP)

Pressure support ventilation (PSV)

Continuous Positive Airway Pressure (CPAP)

• Provides a constant level of positive pressure throughout the respiratory cycle

• Increases the functional residual capacity and compliance, enhancing gas exchange and oxygenation

Pressure Support Ventilation (PSV)

• Inspiratory flow is augmented to a preset level of inspiratory pressure

• Reduces the effort required to maintain spontaneous breathing in patients with adequate respiratory drive but inadequate ventilatory (inspiratory) strength

• Can help overcome the resistance of breathing through the endotracheal tube and ventilator breathing circuit

• Can be used alone or to augment the spontaneous breaths during SIMV or CPAP

• Cycle variable is a set reduction in inspiratory flow rate, a patient dependent variable

I:E Ratio and Respiratory Rate

• Normal respiratory rate of 15-20 breaths is usually selected when assisted ventilation is initiated

  • Then can be changed as appropriate for the patient

• I:E ratio may be preset by the operator or in some older ventilators is a default setting within the machine

• An I:E ratio of 1:2 is commonly used to ensure that the patient has exhaled fully before the onset of the next breath

  • As respiratory rates are increased, the expiratory time will be sacrificed to "squeeze" in the necessary number of inspirations

Reverse or Inverse I:E Ratio Ventilation

• When inspiratory time exceeds the expiratory time

  • Can occur as a result of the patient's respiratory pattern, such as fast respiratory rates

  • Can result in breath stacking or intrinsic PEEP because the animal is not able to exhale fully before the start of the next inspiration

  • Have been used as a ventilation strategy to improve oxygenation

Positive End-Expiratory Pressure (PEEP)

• PEEP maintains positive pressure in the airway during exhalation that prevents the lung from emptying completely so the lung is "held" at a higher volume and pressure during exhalation

• PEEP can increase the oxygenating efficiency of diseased lungs by recruiting previously collapsed alveoli, preventing further alveolar collapse, and reducing ventilator-induced lung injury

• Appropriate magnitude of PEEP depends on the severity of the lung disease, the clinical response of the patient, and the presence of comorbidities in the patient (e.g. hypotension and intracranial hypertension)

• Low levels of PEEP are commonly provided in animals with normal lung function to help prevent atelectasis

• PEEP can have detrimental effects

  • PEEP maintains elevated intrathoracic pressures during exhalation and as a result may compromise venous return

Parameters that the Operator Can Preset on the Ventilator

• With volume-controlled ventilation the tidal volume (or minute ventilation) is preset by the operator and peak airway pressure is a dependent variable

  • A peak airway pressure alarm limit is set to alert the operator of excessive airway pressures

• With pressure-controlled ventilation, the airway pressure generated during inspiration is preset and tidal volume is a dependent variable

• I:E ratio can be preset directly on some ventilators, but with some machines it is the consequence of the inspiratory time and respiratory rate that is chosen by the operator

Rise Time

Time in which the airway pressure increases from baseline to peak pressure

Faster rise times are indicated in patients with rapid respiratory rates

• Caution in animals with small endotracheal tubes because of increased resistance to flow

What is normal tidal volume for a healthy dog and cat?

~10-15 ml/kg

Peak Airway Pressure Setting

• Peak airway pressure is generally kept below 20 cm H2O, often closer to 10 cm H2O in patients with normal lungs

  • Animals with pulmonary disease have reduced pulmonary compliance and therefore require higher pressures in order to deliver an adequate tidal volume

    • Airway pressures up to 30 cm H2O may be necessary in animals with severe, diffuse lung disease

What are common causes for the low pressure alarm?

Patient disconnect

Leak

What are common causes for the high pressure alarm?

Patient-ventilator dyssynchrony

ET tube kink or obstruction

Small size ET tube with high inspiratory flow rate

VC mode: pneumothorax

Common Causes for Low PEEP/CPAP Alarm

Leak

Common Causes for Low Tidal Volume Alarm

Leak

PC mode: patient-ventilator dyssynchrony, decreased compliance, pneumothorax, ET tube obstruction

VC mode: patient-ventilator dyssynchrony, ET tube obstruction

Common Causes for the Apnea Alarm

In spontaneous breathing modes, will need appropriate settings for mandatory rescue breaths

Common Causes for Low Minute Ventilation Alarm

Causes of low tidal volume

Spontaneous breathing modes: low respiratory rate

What are the three main indications for mechanical ventilation?

Severe hypoxemia despite oxygen supplementation

Severe hypoventilation despite therapy

Excessive respiratory effort with impending respiratory fatigue or failure

What indicates severe hypoxemia?

• Indicated by cyanosis, PaO2 of less than 60 mmHg or a SpO2 of less than 90%

• Cannot use venous values to diagnose hypoxemia

Indications for Mechanical Ventilation - Severe Hypoxemia Despite Oxygen Supplementation

• Most of these animals have primary lung disease

• Inspired oxygen concentrations of greater than 60% for a prolonged period (24-48 hours) can lead to oxygen toxicity and subsequent pulmonary damage

  • Animals that require high concentrations of inspired oxygen for longer than 24 hours in order to achieve adequate oxygenation may also benefit from mechanical ventilation

Hypoventilation

• An elevation in the partial pressure of carbon dioxide (PCO2)

  • In patients that are hemodynamically stable, venous CO2 is an accurate reflection of arterial PCO2 and can be used to evaluate hypoventilation

  • Severe hypoventilation is defined as a PaCO2 higher than 60 mmHg and may be an indication for mechanical ventilation if the patient is unresponsive to therapy for the primary disease

Indications for Mechanical Ventilation - Severe Hypoventilation Despite Therapy

• Hypercapnia is a consequence of reduced effective alveolar ventilation

  • May be due to increased dead space in a breathing circuit, upper airway obstruction, sedative overdose, or neurologic or neuromuscular diseases that impair respiratory rate or chest wall movement

    • Most patients with increased apparatus dead space, upper airway obstruction, or sedative overdoses respond to appropriate therapy and do not require mechanical ventilation

    • Patients requiring ventilation in this category have neurologic, muscular, or neuromuscular disease processes such as brain disease, high cervical spinal cord disease, peripheral neuropathies, neuromuscular junction abnormalities, or primary myopathies (collectively neuromuscular diseases)

      • Animals with brain disease may not tolerate small elevations in PCO2 and mechanical ventilation may be beneficial in these patients if the PaCO2 is higher than 45 mmHg

• Another group of patients that may require PPV to prevent hypoventilation are those animals that require general anesthesia for reasons such as maintenance of an endotracheal tube or provision of effective analgesia

  • The anesthetic drugs invariably cause hypoventilation and PPV during the anesthetized period is ideal

• Many post-cardiopulmoanary arrest patients will require PPV for a time after return of spontaneous circulation

  • For short durations, manual ventilation may be sufficient, but animals with apnea, inadequate or unreliable ventilatory efforts, hypercapnia, or concerns for intracranial hypertension will benefit from mechanical ventilation

Indications for Mechanical Ventilation - Excessive Respiratory Effort with Impending Respiratory Fatigue or Failure

• Animals with pulmonary disease may be able to maintain adequate oxygenation and ventilation by increasing their respiratory effort

  • If respiratory effort is marked, patients can become exhausted and respiratory arrest may occur despite acceptable blood gas values

  • Intervention before the arrest and initiation of mechanical ventilation may successfully stabilize these patients

What is a fourth indication for PPV?

Severe hemodynamic compromise that is refractory to therapy

• Anesthesia is often feasible in these patients with opioid and benzodiazepine drugs alone

• Mechanical ventilation can decrease oxygen consumption and support left heart function and may allow ongoing support of the patient while definitive measures to improve the hemodynamic state can be made

Initiation of Mechanical Ventilation

• If the patient is in a life-threatening state, it maybe necessary to anesthetize, intubate, and provide manual PPV while ventilator setup is performed

• The operator should anticipate that animals with primary lung disease may require more PEEP and higher inspired oxygen concentrations that patients with neuromuscular disease

• Turn on machine and test to ensure its functioning properly with an artificial lung or rebreathing bag

• Always start mechanical ventilation with 100% oxygen until appropriate machine function and patient stability can be confirmed

  • After initial stabilization, the FiO2 can be tailored appropriately

• A separate source of 100% oxygen with a means to provide manual ventilation should be available at all times in case of machine failure

• Immediately after the patient is connected to the ventilator, the chest should be observed for appropriate movements

  • Ventilator settings should be adjusted if the chest wall movements appear excessive or inadequate

• Auscultation performed to confirm the presence or absence of breath sounds bilaterally

  • If breath sounds are not audible bilaterally, endobronchial intubation may have occurred and the endotracheal tube should be repositioned appropriately

• Patients require general anesthesia in order to start mechanical ventilation unless they have severe neurologic deficits

  • Anesthesia is required to enable intubation, keep the patient immobile, and allow the patient to tolerate positive pressure ventilation

• Animals that are immobile and unable to fight the ventilator, such as patients with respiratory paralysis, may benefit from placement of a temporary tracheostomy tube

  • Allows for reduction (or even removal) of anesthetic agents and makes neurologic evaluation and patient treatment simpler to interpret

    • Patients with normal neurologic function cannot be ventilated without general anesthesia, even with a temporary tracheostomy tube

  • Brachycephalic animals may benefit from the placement of a temporary tracheostomy tube for the weaning process

• Monitoring tools should be evaluated and significant abnormalities addressed immediately

• Once the patient is considered stable, an arterial blood gas is evaluated and the ventilator settings modified accordingly

  • In the absence of arterial blood gases, ventilator management is based on physical examination findings, venous PCO2 levels, and pulse oximetry

What is the goal of mechanical ventilation?

To maintain adequate arterial blood gas levels (PaCO2 35-50 mmHg, PaO2 80-120 mmHg) with the least aggressive ventilator settings possible

Carbon Dioxide in Mechanical Ventilation

• Arterial PCO2 is directly proportional to CO2 production and is inversely proportional to alveolar minute ventilation

• In most disease state, CO2 production is relatively stable, and minute-to-minute changes of PaCO2 are the result of change in alveolar minute ventilation

• Endotracheal tube obstruction caused by kinks or the accumulation of airway secretions may also reduce the volume of effective alveolar ventilation

• In the absence of these equipment issues, hypercapnia is considered to be a result of inadequate V_A

• Because minute ventilation is equal to the product of the respiratory rate and tidal volume, one or both of these ventilator settings can be increased and the PCO2 concentration reevaluated to determine if the new ventilator protocol is adequate

• If PCO2 is low, V_A should be decreased

• If end-tidal CO2 correlates reliably with PCO2, it can be an excellent real time monitor

Total Minute Ventilation (V_T)

Equal to the product of the respiratory rate and the tidal volume, but a portion of this inspired gas volume does not participate in gas exchange (dead space) and so does not contribute to the elimination of CO2

Alveolar Minute Ventilation

Respiratory rate x (VT - dead space volume)

What can cause significant increases in dead space in small animals?

• In small patients, excess tubing length between the breathing circuit Y-piece and the animal's mouth can cause significant increases in dead space

  • Can be a consequence of excessive endotracheal tube length, extension pieces, or monitoring devices connected to the endotracheal tube

What is the first priority with oxygen in mechanical ventilation?

• First priority is to reduce the FiO2 to less than or equal to 60% as soon as possible to reduce the risk of oxygen toxicity

  • Magnitude of reduction in FiO2 dictated by the measured PaO2

  • PaO2 should be reevaluated after any reduction in oxygen concentration

  • If SpO2 correlates well with the PaO2 (or arterial blood samples are unavailable), pulse oximetry can be used to help guide changes in ventilator settings as well

What is the oxygen goal with mechanical ventilation once FiO2 is reduced to less than 60%?

• Once FiO2 is reduced to less than 60%, the focus becomes reducing the magnitude of the ventilator settings, namely PEEP and the peak inspired airway pressure, can be considered if the PaO2 is significantly higher than the targeted range

Hypoxemia Despite Ventilation with 100% Oxygen

• In severe cases, hypoxemia will persist despite ventilation with 100% oxygen

  • Changes in the ventilator settings are required

  • Increases in PEEP, peak inspired airway pressure/tidal volume, and/or respiratory rate may help improve the oxygenating efficiency of the lung

• Prone positioning will maximize lung function in most patients

• Animals with hypoxemia should be maintained in sternal recumbency until stabilized

Mechanical Ventilation Complications - Cardiovascular Compromise

• Cardiovascular compromise as a result of impairment of intrathoracic blood flow is often an issue for patients with cardiovascular instability or when aggressive ventilator settings are necessary

Mechanical Ventilation Complications - Ventilator-Induced Lung Injury

• Volutrauma and repetitive alveolar opening and collapse are believed to be the major causes of ventilator-induced lung injury and may be reduced with protective ventilation strategies

Mechanical Ventilation Complications - Ventilator-Associated Pneumonia

• Aseptic airway procedures, intensive oral care, and reducing the incidence of gastric regurgitation are all important in preventing the incidence of ventilator-associated pneumonia

• Patients should be monitored continuously for evidence of a nosocomial infection and changes in pulmonary function

• Routine sampling of airway fluid for cytology and culture and sensitivity testing may help to detect early ventilator-associated pneumonia

Mechanical Ventilation Complications - Pneumothorax

• Has been shown to occur more often when very high airway pressures are used in human patients (plateau airway pressure >35 cm H2O)

• When more conventional ventilator settings are used, there is no correlation among airway pressure, PEEP, or other settings, and the occurrence of pneumothorax

• Development of pneumothorax is more likely the result of underlying lung disease rather than the ventilator setting used

• Pneumothorax should be a primary consideration when a patient has an acute decline in oxygenating ability, elevation in PCO2, decreased chest wall movement and compliance, and patient-ventilator asynchrony

• Tension pneumothorax can be rapidly fatal if not diagnosed and treated in animals receiving PPV

• Unilateral or bilateral thoracostomy tubes with continuous drainage are indicated when managing these ventilated animals

• If an acute, life-threatening pneumothorax develops in the ventilator patient, an emergency thoracotomy to create an open pneumothorax may be required to prevent cardiopulmonary arrest

  • Thoracostomy tubes placed after stabilization of cardiovascular parameters

Mechanical Ventilation Troubleshooting - Patient-Ventilator Asynchrony (Bucking the Ventilator)

• Occurs when the patient is breathing against the machine

• Can prevent effective ventilation and may lead to hypoxemia, hypercapnia, and hyperthermia

• Increases the work of breathing and can increase patient morbidity

Mechanical Ventilation Troubleshooting - Sudden Decrease in Oxygenation

• Oxygen supply to the machine should be checked and confirm the breathing circuit is intact and the ventilator is delivering breaths as prescribed

• If the patient has become hypoxemic, the FiO2 should be increased immediately to 100% and the animal placed in sternal recumbency until the condition is improved

Mechanical Ventilation Troubleshooting - Sudden Elevations in PCO2

• Can occur as a consequence of equipment faults, patient problems (e.g. pneumothorax, airway obstruction), or inappropriate ventilator settings

• If no mechanical abnormalities are evident and patient disease such as pneumothorax is ruled out, the ventilator settings should be changed to increase minute ventilation

• Hypercapnia may be an acceptable consequence of some protective ventilation strategies (permissive hypercapnia)

Causes of Hypoxemia in the Ventilator Patient

Loss of oxygen supply

Machine or circuit malfunction

Deterioration of the underlying pulmonary disease

Development of new pulmonary disease

• Pneumothorax

• Pneumonia

• VILI

  • ARDS

Potential Responses to Hypoxemia in the Ventilator Patient

Increase FiO2

Sternal positioning

Thoracic auscultation

Verify machine function

Evaluate for leaks

Increases in PEEP

Increases in PIP/tidal volume

Causes of Hypercapnia in the Ventilator Patient

Pneumothorax

Bronchoconstriction

Endotracheal tube or airway obstruction

Circuit leak

Increased apparatus dead space

Increased alveolar dead space

• Alveolar overdistension

• Pulmonary embolism

Inadequate ventilator settings

Low VT and/or RR

Potential Responses to Hypercapnia in the Ventilator Patient

Evaluate patient data

Thoracic auscultation

Assess endotracheal tube

Verify machine function

Ensure adequate expiratory time

Increase VT and/or RR

Note: hypercapnia may be an acceptable consequence of some ventilation strategies

Causes of Hyperthermia in the Ventilator Patient

Impaired natural cooling mechanisms

Potential Responses to Hyperthermia in the Ventilator Patient

Rule out fever

Active cooling

Causes of Inappropriate Ventilator Settings in the Ventilator Patient

Patient efforts/requirements are not being met with ventilation strategy