Principles of Vent Support 1: Unit Exam #1

Spontaneous vs. Mechanical Ventilation

• The body's mechanism for conducting air in and out of the lungs is known as spontaneous ventilation.
• Ventilation is the conduction of air in and out of the body.
• Mechanical ventilation is the use of a machine to assist or replace spontaneous ventilation.

External vs. Internal Respiration

• External respiration involves the exchange of oxygen (O2) and carbon dioxide (CO2) between the alveoli and the pulmonary capillaries.
• Internal respiration occurs at the cellular level and involves the movement of oxygen from the systemic blood into the cells.

Muscles Involved in Respiration

• Scalene and trapezius muscles are accessory muscles of inspiration.
• External oblique and transverse abdominal muscles are accessory muscles of expiration.

Pressures Required for Alveolar Inflation

• Transpulmonary pressure (P_L) is the pressure required to maintain alveolar inflation.
• Transairway pressure (P_{TA}) is the pressure gradient required to produce airflow in the conducting tubes.
• Transrespiratory pressure (P_{TR}) is the pressure to inflate the lungs and airways during positive-pressure ventilation.
• Transthoracic pressure (P_{TT}) represents the pressure required to expand or contract the lungs and the chest wall at the same time.

Elastance vs. Compliance

• Elastance is the tendency of a structure to return to its original shape after being stretched. The more elastance a structure has, the more difficult it is to stretch.
• Compliance is the ease with which a structure distends or stretches. Compliance is the opposite of elastance.
• Viscous resistance is the opposition to movement offered by adjacent structures such as the lungs and their adjacent organs.
• Distending pressure is the pressure required to maintain inflation (e.g., alveolar distending pressure).

Calculation of Pressure

• The relationship between change in compliance (ΔC), change in volume (ΔV), and change in pressure (ΔP) is given by: \Delta C = \frac{\Delta V}{\Delta P}
• Therefore, the change in pressure can be calculated as: \Delta P = \frac{\Delta V}{\Delta C}
• Example: To achieve a tidal volume of 400 mL for an intubated patient with a respiratory system compliance of 15 mL/cm \text{H}2\text{O}, the required pressure is: \Delta P = \frac{400 \text{ mL}}{15 \text{ mL/cm H}2\text{O}} = 26.67 \text{ cm H}2\text{O} or 26.7 \text{ cm H}2\text{O}.

Pulmonary Compliance

• Emphysema causes an increase in pulmonary compliance.
• ARDS and kyphoscoliosis cause decreases in pulmonary compliance.
• Asthma attacks cause an increase in airway resistance.

Static Compliance

• The formula for calculating static compliance (CS) is: CS = \frac{VT}{P{\text{plat}} - EEP}, where: \newline (VT) is Tidal Volume; \newline (P{\text{plat}}) is Plateau Pressure; and \newline (EEP) is End-Expiratory Pressure.

User Interface vs. Control Logic

• The user interface (or control panel) contains dials or a touch panel where the ventilator operator sets or enters information to establish how the pressure and pattern of gas flow are delivered.
• The control logic (or control system) interprets operator settings and produces/regulates the desired output.
• The input power is the ventilator’s power source that provides the energy to enable the ventilator to perform the work of ventilating the patient.
• The drive mechanism is a mechanical device that produces gas flow to the patient.

Single-Circuit vs. Double-Circuit Ventilators

• Single-circuit ventilator: Gas flows directly from its power source to the patient.
• Double-circuit ventilator: Primary power source generates a gas flow that compresses another mechanism (e.g., bellows or bag-in-a-chamber), and the gas in that mechanism flows to the patient.

Exhalation Valve Function

• The exhalation valve allows the release of exhaled gas from the expiratory line into the room air.
• Major components of a patient circuit include:
  • Main inspiratory line: Connects the ventilator output to the patient’s airway adapter.
  • Adapter: Connects the main inspiratory line to the patient’s airway.
  • Expiratory line: Delivers expired gas from the patient to the exhalation valve.
  • Expiratory valve: Releases exhaled gas from the expiratory line into the room air.

Power Transmission and Conversion System

• The power transmission and conversion system is the internal hardware that converts electrical or pneumatic energy into the mechanical energy required to deliver a breath to the patient.

Gas Flow Regulation

• Modern ICU ventilators use flow-control valves (e.g., proportional solenoid valves and digital valves with on/off configurations) to regulate gas flow to the patient.

Ventilator Control Variables

• The ventilator can control four variables: pressure, volume, flow, and time.
• The ventilator can control only one variable at a time.

Volume-Controlled Ventilation with Increased Airway Resistance

• In volume-controlled ventilation, the ventilator maintains the volume and flow, but the pressure varies with changes in lung characteristics.
• An increase in airway resistance will require more pressure to deliver the set volume.
• The set rate is independent of the changes in pressure.

Pressure-Targeted Ventilation with Increased Airway Resistance

• In pressure-targeted ventilation, pressure is unaffected by changes in lung characteristics.
• An increase in airway resistance will cause less volume to be delivered and will change the flow waveform.

Volume-Limited Ventilation with Decreased Lung Compliance

• In volume-limited mode, the ventilator maintains the volume and flow, but the pressure varies with changes in lung characteristics.
• A decrease in lung compliance will cause the amount of pressure needed to overcome elastance to increase, thereby increasing the peak pressure needed to deliver the set volume.

High-Frequency Oscillators

• High-frequency oscillators control both inspiratory and expiratory time.

Calculating Cycle Time

• If the set rate is 15 breaths/min, then the time for inspiration and expiration to occur is \frac{60 \text{ sec/min}}{15 \text{ breaths/min}} = 4 \text{ seconds}.

Hypercapnic Respiratory Failure

• Inadequate ventilation decreases the amount of carbon dioxide excreted by the lungs, causing a buildup of carbon dioxide in the blood (hypercapnia).

Causes of Respiratory Failure

• Alveolar hypoventilation leads to pure hypercapnic respiratory failure.
• Ventilation/perfusion (V/Q) mismatch, intrapulmonary shunting, and diffusion impairment lead to hypoxemic respiratory failure.

Acute Hypercapnic Respiratory Failure

• Drug overdose will affect the patient’s central nervous system, knocking out the patient’s respiratory center, which will cause an increase in the partial pressure of carbon dioxide in the arteries (PaCO2). The alveolar hypoventilation is causing the low partial pressure of oxygen (PaO2).
• If this was a chronic hypercapnic respiratory failure, the patient’s bicarbonate level would be elevated above the normal level.
• Respiratory muscle fatigue would decrease a patient’s ability to “move air” and would cause acute hypercapnic respiratory failure.
• A decreased fractional inspired oxygen (FIO_2), pulmonary shunt, and perfusion/diffusion impairment will lead to acute hypoxemic respiratory failure.

Increased Work of Breathing

• An asthma exacerbation is characterized by bronchoconstriction due to bronchospasm, edema, and inflammation of the airways. This increases the amount of work a patient must do to overcome the increase in airway resistance, thereby increasing the patient’s work of breathing.
• Myasthenia gravis is a neuromuscular disease that may paralyze the ventilatory muscles, causing a patient to not be able to move air.

Impending Ventilatory Failure

• A patient with worsening airway obstruction, deterioration in acid-base and oxygenation status, decreased peak expiratory flow rate, tachycardia, tachypnea, and sweating is in impending ventilatory failure and meets the standard criteria for instituting mechanical ventilation.
• Changing oxygen delivery devices to a nonrebreather mask will not increase the fractional inspired oxygen (FIO_2) delivered.
• Continuous positive airway pressure may address the patient’s oxygenation problem; however, it will not help to improve the patient’s increased work of breathing.

Goals of Mechanical Ventilation

• To provide support to the pulmonary system to maintain an adequate level of alveolar ventilation
• To reduce the WOB until the cause of respiratory failure can be eliminated
• To restore ABG levels to normal
• To prevent or treat atelectasis with adequate end-inspiratory lung inflation
• Support or manipulate pulmonary gas exchange:
  • Alveolar ventilation—Achieve eucapnic ventilation or allow permissive hypercapnia (Note: Permissive hypercapnia sometimes is required in the ventilation of patients with asthma, acute lung injury [ALI], or acute respiratory distress syndrome [ARDS] to protect the lung by avoiding high ventilating volumes and pressures.)
  • Alveolar oxygenation—Maintain adequate oxygen delivery (CaO_2 \times \text{Cardiac output}).
• Increase lung volume:
  • Prevent or treat atelectasis with adequate end-inspiratory lung inflation.
  • Restore and maintain an adequate functional residual capacity (FRC).
• Reduce the work of breathing.

Clinical Objectives of Mechanical Ventilation

• Reverse acute respiratory failure.
• Reverse respiratory distress.
• Reverse hypoxemia.
• Prevent or reverse atelectasis and maintain FRC.
• Reverse respiratory muscle fatigue.
• Permit sedation or paralysis (or both).
• Reduce systemic or myocardial oxygen consumption.
• Minimize associated complications and reduce mortality.

Parameters to Assess Airway Resistance

• For patients with acute asthma, the PEFR is the most frequently used parameter to assess airway resistance.
• Peak expiratory flow rate (PEFR) values:
  • Normal: 350-600 L/min
  • Critical value: 75-100 L/min

Obstructive Sleep Apnea Treatment

• Continuous positive airway pressure (CPAP) is an accepted method used to treat obstructive sleep apnea.
• Noninvasive positive pressure ventilation (NIV) would be appropriate if the patient had central sleep apnea, since there would be no respiratory efforts during the apnea periods.
• Pressure support ventilation (PSV) and pressure- controlled continuous mandatory ventilation (PC-CMV) would require the patient to be intubated.

Full Ventilatory Support

• Full ventilatory support is provided when the ventilator-initiated rates are 8 breaths/min or more.

Partial Ventilatory Support

• Continuous mandatory ventilation (CMV) is a full ventilatory support mode.
• A ventilator rate setting of 8 breaths/min or more is also considered full support.
• Volume- controlled intermittent mandatory ventilation (VC-IMV) with a set rate of 4 breaths/min is partial ventilatory support and MMV can be partial ventilatory support when the patient is participating in the work of breathing (WOB) to maintain effective alveolar ventilation.

Volume Control Ventilation and Airway Resistance

• During volume control ventilation, changes in lung characteristics cause changes in pressure.
• Increasing airway resistance causes an increase in the amount of pressure required to deliver the volume, thereby increasing peak airway pressure.

CPAP and Oxygenation

• The arterial blood gas shows that the patient is ventilating, as evidenced by the partial pressure of carbon dioxide (PaCO_2) of 25 mm Hg. Therefore, this patient does not need to be intubated and ventilated at this time. This also means that the patient does not require noninvasive positive pressure ventilation (NIV).
• The patient does have an oxygenation problem, as evidenced by the partial pressure of oxygen (PaO_2) of 59 mm Hg while on a nonrebreather mask. This is an indication for continuous positive airway pressure (CPAP).

Assisted Breath in PC-CMV Mode

• An assisted breath is always patient triggered.
• In the pressure control or targeted mode the pressure set is the pressure limit and the inspiratory time setting ends inspiration (cycle).
• Therefore, the correct answer is patient triggered, pressure limited, time cycled.

Intermittent Mandatory Ventilation (IMV) Mode

• The intermittent mandatory ventilation (IMV) mode allows the patient to breathe spontaneously between operator mandatory ventilator breaths.
• During these spontaneous breaths the baseline pressure may be set at ambient pressure or above ambient pressure. In addition, pressure support may be used during the spontaneous breathing period.
• Pressure support ventilation (PSV) has no time-triggered breaths, nor does it have volume-targeted breaths.
• Continuous mandatory ventilation (CMV) does not allow for spontaneous breathing; it only allows the patient to trigger the mandatory ventilator breath.
• Airway pressure release ventilation (APRV) does not have volume-targeted breaths. It is designed to be two levels of continuous positive airway pressure (CPAP) where the patients breathe spontaneously at both levels.

Ventilator Asynchrony

• High or low flow rate settings can cause the patient to be out of synchrony with the ventilator.
• The higher the flow rate the shorter the inspiratory time.
• Incorrect sensitivity settings can lead to auto-triggering or “locking out” the patient.

NIPPV

• In many cases, clinicians prefer using noninvasive positive pressure ventilator support in the form of bilevel positive airway pressure (bilevel PAP) rather than CPAP for patients with Raw-induced auto-PEEP. Bilevel PAP also is the method most often used to treat acute-on-chronic respiratory failure.

Mandatory Breath

• Breaths that are triggered by the mechanical ventilator are considered mandatory breaths because the ventilator is controlling the timing of the breath and delivering either a set volume or set pressure.

Tidal Volume Calculation

• Minute ventilation equals respiratory rate multiplied by tidal volume (V_T).
• Therefore, tidal volume equals minute ventilation divided by respiratory rate.
• Example: V_T = \frac{6 \text{ L/min}}{12 \text{ breaths/min}} = 0.5 \text{ L} = 500 \text{ mL}

Inspiratory Time Calculation

• TI = \frac{VT}{V_E} (convert L/min to L/sec first)

I:E Ratio Calculation

• Total Cycle Time (TCT) = \frac{60 \text{ sec}}{f}
• Expiratory Time (TE) = TCT - TI
• TI : TE = 1:X

High Flow Rates

• The flow setting on a mechanical ventilator determines how fast the inspired gas will be delivered to the patient. During continuous mandatory ventilation (CMV), high flows shorten inspiratory time (T_I) and may result in higher peak pressures and poor gas distribution.

Slow Flow Rates

• Slower flows may reduce peak pressures, improve gas distribution, and increase at the expense of increasing inspiratory time (T_I).
• Unfortunately, shorter expiratory time (TE) can lead to air trapping, while using a longer (TI) may cause cardiovascular side effects.

Constant Flow Pattern

• Generally, a constant flow pattern provides the shortest inspiratory time (T_I) of all the available flow patterns with an equivalent peak flow rate setting.

Descending Waveform

• The descending waveform occurs naturally in pressure ventilation.

Acceptable Arterial Oxygen Tension

• The goal of selecting a specific fractional inspired oxygen (FIO_2) for a patient is to achieve a clinically acceptable arterial oxygen tension (e.g., 60-100 mm Hg).

High Oxygen Concentration

• Using a high oxygen concentration following a cardiac arrest can provide a way of restoring normal oxygenation and replacing tissue oxygen storage when oxygen debt and lactic acid accumulations occur.

Auto-PEEP

• Patients may have trouble triggering a breath when unintended positive end-expiratory pressure (auto-PEEP) is present. When this occurs, adjusting the sensitivity may not alleviate the patient’s inability to trigger the ventilator.
• When auto-PEEP occurs in mechanically ventilated, spontaneously breathing patients with airflow obstruction, setting extrinsic PEEP to a level equal to about 80% of the patient’s auto-PEEP level may allow the ventilator to sense the patient’s inspiratory efforts.

Humidity Deficit

• Humidity deficit equals 44 mg/L - absolute humidity

Replace a Heat Moisture Exchanger (HME)

• If more than four HMEs are used during a 24-hour period because of secretion buildup, it is probably advisable to change to a heated humidifier that provides 100% relative humidity at 31° to 35° C.

Low Exhaled Tidal Volume

• The low exhaled tidal volume (VT) alarm should be set at 10-15% below the set (VT).

Ventilator Alarm

• When a ventilator alarm rings, the respiratory therapist (RT) must first ensure that the patient is being ventilated. If the RT doubts this, the patient should be disconnected from the ventilator and ventilated using a manual resuscitation bag. The RT should silence the alarms at that time and call for help.

Minimizing Air Trapping

• To minimize air trapping, or intrinsic positive end-expiratory pressure (PEEPi), the following steps may be taken: switch to pressure-controlled continuous mandatory ventilation (PC-CMV) with a short inspiratory time (TI), use a lower tidal volume (VT), maintain a clear airway, administer bronchodilators for bronchospasm, and increase inspiratory flow to lengthen expiratory time (T_E).

Acute Severe Asthma

• Increased airway resistance from bronchospasm, increased secretions, and mucosal edema cause air trapping, which can cause uneven hyperexpansion of various lung units. This can cause rupture or compress other areas of the lungs leading to pneumothorax, pneumomediastinum, subcutaneous emphysema, and other forms of barotrauma.