APPENDIX A: DEFINITIONS AND VENTILATOR TERMS

MECHANICS/PHYSIOLOGY OF BREATHING

1. Respiratory drive: The normal respiratory rate is 12-20 bpm. Respiratory drive is controlled by central and peripheral receptors based on arterial concentrations of carbon dioxide (CO2); oxygen (O2), and hydrogen ions. Increased intracranial pressure, administration of opioids and other medications may cause a central decrease in respiratory drive. This could result in insufficient ventilation to maintain an adequate level of oxygenation and clearance of CO2.1-3
2. Work of breathing: Work of breathing is the mechanical work needed to maintain oxygenation and ventilation. Pain, acidosis, and hypermetabolic states will cause an increased work of breathing. This is not necessarily pathologic but does indicate additional respiratory support is needed while working up the underlying etiology. Prolonged tachypnea, with etiology undiagnosed and untreated, can lead to inspiratory muscle fatigue which could require mechanical ventilator support (e.g., hemorrhagic shock, severe rib fractures, untreated asthma, etc.) 1-3
3. Lung compliance: Lung compliance is the ability of the lung to expand when given a certain amount of air pressure. The more compliant the lung, the less pressure needed for it to expand to a certain volume. Lungs with low compliance will require higher pressures to expand to the same volume as a normal lung. Internal causes such as pneumothorax and fluid/blood in the alveoli can decrease lung compliance. There are also external causes that decrease lung’s ability to expand due to the stiffness of the chest wall, such as obesity, pregnancy, burns, chest wall injury.1-5 A decrease in lung compliance from any cause can lead to hypoxemia and hypercapnia.
4. Tidal volume (TV or VT): Is the volume of air that is exchanged in one breath. Decreases in tidal volume can result from external pressure (i.e. Pneumothorax, hemothorax, tension pneumothorax) by effectively reducing lung volume. Dynamic hyperinflation also known as “breath stacking” is caused by the inability to completely exhale and can lead to “auto-positive end expiratory pressure (auto-PEEP).” This may be due to inadequate exhalation time, airflow obstruction, or both. This condition leads to decreasing tidal volumes and can cause hemodynamic compromise.1-3
5. Oxygenation: The successful binding of oxygen to hemoglobin at the cellular level in the alveoli, drives SaO2 (arterial oxygen saturation) and SpO2 (oxygen saturation) values. Successful alveolar gas exchange enables efficient aerobic respiration at the cellular level in all perfused body tissues. 1-3
6. Diffusion/exchange: The process where oxygen (O2) is exchanged with CO2 on red blood cells in the alveoli/pulmonary capillaries for transport to body tissues. Pathologic conditions such as pulmonary edema, pneumonia, and acute respiratory distress syndrome (ARDS) can impair diffusion of oxygen across the alveolar membrane leading to reduced oxygen saturation of hemoglobin. 1-3
7. Fraction of Inspired Oxygen (FiO2): Normal atmospheric air contains 21% oxygen or an FiO2 of 0.21. By increasing the percentage of oxygen delivered to the patient (supplemental oxygen), you can potentially increase the arterial oxygen saturation and oxygen content of the bloodstream. 1-3
8. Dead Space: Any part of the airway where gas exchange does not occur, pharynx, larynx, trachea, bronchi, and ventilator tubing are examples.
9. Hypoxia: A state of O2 deficiency in the tissue significant enough to cause impairment of function. Causes can be multifactorial to include low PaO2 due to elevation/altitude(>10,000 feet), decreased available RBCs such as hemorrhage or decreased RBC functionality such as CO poisoning, inadequate circulation or perfusion such as G force pooling or hypotension causes without RBC deficits, and decreased tissue level oxygen transport such as in cyanide poisoning. 1-3

VENTILATION DEFINITIONS

1. Minute ventilation (VE): Tidal volume multiplied by the respiratory rate (normal is 60cc/kg/min), usually expressed in liters (8-10L.min). The body regulates carbon dioxide through changes in minute ventilation. Increases in carbon dioxide leads to increased respiratory rate and/or tidal volume and increased minute ventilation (amount of air exchanged during one minute of ventilation). 1-3
2. Peak Inspiratory Pressure (PIP): The greatest pressure within the lungs during inspiration. Pressures above 35mmHg have been shown to cause pressure-related lung injury (barotrauma). Ideally, pressures should remain below 30 mmHg. Increased peak pressures are usually due to increases in resistance or decreased lung compliance within the respiratory system (e.g., kink in the circuit, mucous plugging, laryngospasm/bronchospasm, tension pneumothorax, inability for adequate exhalation, edema). 1-3
3. Plateau pressure: It is the static pressure achieved at the end of a full inspiration. To measure plateau pressure, we need to perform an inspiratory hold on the ventilator to permit the pressure to equalize through the system. Plateau pressure is a measure of alveolar pressure and lung compliance. Normal plateau pressure is below 30 cm H20, and higher pressure can generate barotrauma. Checking a plateau pressure is helpful to delineate between a resistance or compliance problem. If peak pressures are high and plateau pressures are normal, this is indicative of a resistance problem (kinked circuit, ventilator asynchrony, laryngospasm, mucous plugging, etc.). If peak and plateau pressure are high, this is more likely from compliance issues such as pneumonia, pulmonary edema, atelectasis, pneumothorax, abdominal compartment syndrome, etc.).
4. End Tidal CO2 (EtCO2 ): Measurement of carbon dioxide on end tidal expiration. Normal values are 35mmHg-45mmHg. Exhaled gasses are analyzed by either vital signs monitor or portable EtCO2 devices (e.g., EMMA) a quantitative capnograph or capnometer is the clinical standard of care with invasively ventilated patients. EtCO2 is one of the most useful measures to determine overall adequacy of ventilation. Anyone who is intubating a patient and putting them on mechanical ventilation must be able to monitor EtCO2 .
5. Arterial Blood Gas (ABG): Although continuous pulse oximetry and EtCO2 can reliably confirm adequate oxygenation, ventilation, and guide most ventilator changes – ABGs are the gold standard for evaluating acid-base status, oxygenation, ventilation and adjusting ventilation settings. If a point of care blood gas analyzer is available, this can enable targeted ventilator setting changes. Knowing these values will greatly improve critical care guidance via telemedicine resources. At facility-based care (Role 2 care and beyond) an arterial line for continuous blood pressure monitoring and ABG sampling should be placed.
6. Normal ABG values:

- pH (7.35-7.45)
- PaO2 (75-100 mmHg)
- PaCO2 (35-45 mmHg)
- HCO3 (22-26 meq/L)
- Base excess/deficit (-2 to +2)
- SaO2 (95-100%)

ABG DEFINITIONS

While in the PH and rotary wing transport environments, it is rare to obtain an ABG – knowing the normal values is important to check a patient prior to Role 2 to Role 3 RW transport. The ABG is extremely useful in trauma resuscitations because the pH, lactate and base deficit will give an overall picture of perfusion.

1. pH: Measure of hydrogen ion concentration (i.e. acid-base status). Acidosis (low pH) leads to coagulopathic states in trauma patients as well as development of potentially fatal cardiac arrhythmias.
2. PaO2: Measurement of dissolved oxygen in blood, also a measurement of adequacy of gas exchange at the cellular level.
3. PaCO2: Measurement of dissolved carbon dioxide in blood, also a measurement of adequacy of gas exchange at the cellular level.
4. HCO3: Measurement of bicarbonate in the blood, serves as a buffer against acid.
5. Base excess: Gives indication of metabolic component of blood gas results, most likely will not change field ventilator management but can provide information for telemedicine consultation regarding adequacy of resuscitation.
6. SaO2: Percentage of oxygen bound to hemoglobin in arterial blood, correlates closely with SpO2 values.

VENTILATOR TERMS

1. Volume-targeted modes: Volume constant, inspiration terminates when preset VT delivered. Peak airway pressure is variable and increases as needed to deliver prescribed VT. This is generally represented by a constant flow waveform.
2. Pressure-targeted modes: Volume variable and dependent on pulmonary compliance. The vent terminates flow of air when the pressure is met. Peak airway pressure is fixed, determined by set pressure level. This is generally represented by a decelerating flow waveform.
3. Tidal volume (VT): Is the volume of gas, exchanged during a breath and commonly expressed in milliliters. VT is generally set between 4-8 ml/kg ideal body weight (IBW), to prevent lung over distension and barotrauma.
4. Frequency (f): Is the rate, per minute, of breathing (patient or ventilator). Known as respiratory rate (RR).
5. Minute Ventilation (Ve): Is the average volume of gas entering, or leaving, the lungs per minute, commonly expressed in liters per minute. The product of VT and RR (respiratory rate). Normal Ve is 5 – 10 L/min.
6. Inspiratory (I) and Expiratory (E) time and I:E ratio: Is the period of time over which the VT is delivered. Setting a shorter inspiratory time (I) results in a faster inspiratory flow rate in volume cycled ventilation. Average adult inspiratory time is 0.7 to 1 second. I:E ratio is usually 1:2.
7. Positive end-expiratory pressure (PEEP): Is the amount of positive pressure that is maintained at end-expiration. It is expressed in centimeters of water (cmH2O). The purpose of PEEP is to increase end-expiratory lung volume and reduce air-space closure at end-expiration. Normal physiologic PEEP is 5 cmH2O.
8. Pressure Support (PS): Delivers flow at a set pressure, generally to overcome resistance of the airway and ventilator circuit. PS can also be used to support a spontaneously breathing patient, such as with Bi-PAP.
9. Flow: Is the velocity at which gas is delivered to the patient, expressed in liters per minute. When the flow rate is set higher, the speed of gas delivery is faster and inspiratory time is shorter.
10. Peak Inspiratory Pressure (PIP): Represents the total pressure that is required to deliver the VT and depends upon various airway resistance, lung compliance, and chest wall factors. It is expressed in centimeters of water (cmH2O).
11. Sensitivity or trigger sensitivity: Effort, or negative pressure, required by the patient to trigger a machine breath, commonly set so that minimal effort (-1 to -2 cmH2O) is required to trigger a breath.1,3

REFERENCES

1. Grossbach I, Chlan L., Tracy MF. Overview of mechanical ventilatory support and management of patient-and ventilator-related responses. Critical care nurse, 2011. 31(3), 30-44.
2. The National Heart, Lung, and Blood Institute. How the Lungs Work Video. Nov 10, 2020 https://www.youtube.com/watch?v=C0mYCssvYpE
3. Wilcox SR, Richards JB, Fisher DF, et al. Initial mechanical ventilator settings and lung protective ventilation in the ED. The American journal of emergency medicine, 2016. 34(8), 1446-1451.