Breathe Easier with Ventilation Knowledge During COVID-19
Sunday, April 19, 2020
Posted by: Scott Crawford, MD, FACEP, CHSOS
Historical example of 'Iron Lung,' the first type of artificial respirator. https://en.wikipedia.org/wiki/Iron_lung#/media/File:Iron_lung_CDC.jpg
Mechanical ventilation has received a lot of attention with the recent COVID-19 outbreak. Much has changed from early mechanical ventilation provided by the ‘Iron Lung’ that provided support to polio patients. This original device designed in 1927 provided negative pressure (air is pulled into the body) to allow the lungs to fill with air for patients with a paralyzed diaphragm from the effects of the polio virusi. Just as the need for respirators exceeded their supply in the 1930s, a similar problem plagues health care providers today.
Today, ventilation is provided in various critical care settings using sophisticated positive pressure (air pushed into the lungs) ventilation devices. Positive pressure ventilation was previously thought safe only under general anesthesia but was able to be provided in the ICU setting during the 1950s. Now the knowledge required to provide assisted respirations requires an allied healthcare graduate specialty degree – respiratory therapist. This article will attempt to describe the basic features of lung function and ventilation using simple terms and descriptions.
Respiratory physiology (skip this section if you don’t like chemistry):
Breathing is an active process. The diaphragm muscle contracts, which increases the volume (space) inside the thoracic cavity. This creates an area of low pressure and thus ‘pulls’ air in from the higher-pressure area outside. The average human breathes 12-20 times per minute, with higher rates achieved during exertion or illness. Breathing does more than provide oxygen to our blood for use by tissues; it serves a critical role in removing carbon dioxide (a byproduct of cellular metabolism) from the body. CO2 is also more than a waste product; it is an important regulatory component of the acid-base balance of the body. Breathing too quickly (hyperventilating) can cause increased removal of CO2 and a conversion of (and therefore decrease in) H+ (acid) and bicarbonate (HCO3), a buffer within the blood (equation is driven to the right to replace lost CO2).
A decrease in CO2 leads to alkalosis (increased pH [>7.4]), while a decrease in respiratory rate can lead to retained CO2 and acidosis (pH < 7.4) (equation driven to the left because of increased CO2). Both conditions can cause significant physiologic problems. The level of CO2 and O2 in the blood can now be quickly measured using blood-gas analysis, arterial (ABG) or venous (VBG), to allow adjustment of ventilatory support for patients.
- Respiratory rate (RR) – The number of breaths taken in one minute.
- Tidal Volume (VT) – The volume of air (ml) taken in a single breath.
In general, this ranges from 4-8 ml/kg (ideal body weight). Example for a 60kg patient would be volumes of 240-480 ml per breath.
- Airway Pressure – The amount of pressure (cm H2O) felt pushing outward on the airways.
The pressure in the airway changes during a breath. The highest pressure is called the peak pressure, the sustained pressure during inhalation is the plateau pressure. During normal exhalation the pressure drops to near zero.
- Positive End-Expiratory Pressure (PEEP) – An externally applied pressure to the airway and lungs to help push small airways and alveoli open. Often improves oxygenation in diseased lungs.ii
- Minute Ventilation (MV) – The total volume of air moved in 1 minute (MV = RR * VT)
- Management concept: If airway pressure gets too high, lowering the tidal volume and increasing the respiratory rate can keep pressure low and deliver the same amount of air.
- Compliance – How elastic the lung is. It is defined as how much the lung changes volume (stretches) as pressure is changed.iii
A car tire has a low compliance, while a balloon has high compliance.
- Resistance – How hard it is for air to flow into or out of the lungs. This can be made worse in diseases like asthma or chronic obstructive pulmonary disease (COPD) when the airway is decreased in size.iv
- Inspiratory to Expiratory (I:E) ratio – A comparison of the time spent during inspiration to the time required for exhalation. In a normal patient this is usually 1:2 but can vary from 2:1 to 1:4 depending on patient pathology and ventilation parameters sought.
Ventilation breathing modes (simplified):
A person who is getting assistance breathing from a ventilator may need different amounts of help. These are given the terms – support, assist, and control:
- Support: Ventilator gives some, but not all help once it detects an attempt to breathe. (If you don’t have enough effort you could be under ventilated)
- Assist: Ventilator waits until it senses an attempt to breathe (slight inspiratory effort) and then will deliver the full breath specified (volume or duration) and usually at a minimum set respiratory rate.
- Control: In this mode, the ventilator does all of the work, delivering the full volume or duration of breath even if no effort is made by the patient.
Breath delivery types:
The two external breathing types are pressure support and volume support.
Pressure support: A fixed pressure (cmH2O) is provided to ‘push’ air into the lungs. The actual delivered volume may vary depending on the physiology of the lungs (compliance and resistance).
Volume Support: A fixed volume (ml) of air is provided to the lung with an adjustment (increase or decrease) in pressure to achieve this.
There are several methods to monitor a patient who is receiving mechanical ventilation. The first includes monitors and alarms that will identify if a breath is not detected for a period of time that would indicate that the patient is not breathing or has become disconnected form the ventilator.
External to the ventilator, respiratory rate can be detected by movement of the patient’s chest through the EKG leads. A beam of light that shines through a fingertip or earlobe can be used to measure the wavelength of light transmitted as a marker of the percentage of oxyhemoglobin (red blood cells carrying oxygen) in the blood. This will provide a reading of percent oxygen saturation and is recorded as pulse oximetry or ‘pulse ox.’ A final, more direct method of measuring respiration is through end-tidal CO2 (ETCO2) monitoring. ETCO2 allows measurement of the amount of carbon dioxide in exhaled air and can provide information about respiratory rate as well and metabolic activity of the patient.
Example CO2 tracing showing a normal respiration, followed by a brief period of apnea (not breathing) and then a high CO2 level during exhalation from increacing amounts not exhales during apnea.
COVID-19 pulmonary disease:
Patients can develop respiratory difficulty from infection with COVID-19. This difficulty is believed to be due to lung injury from inflammatory response of the body’s immune system.v This inflammation causes fibrosis, decreased compliance, and impaired oxygen exchange within the small airways and alveoli. Patients with COVID-19 related lung injury are reported to have marked hypoxia (low oxygen levels in the blood) with potentially few symptoms. This leaves patients with a limited ability to sustain worsening without rapid respiratory failure. Although compliance is expected to be low, this is in general not a restrictive pulmonary disease process and therefor has little effect on airflow into or out of the lungs. Resistance and compliance are also expected to change throughout the disease state. COVID-19 has been described as an acute respiratory distress (ARDS) type pulmonary process; however, this may be an oversimplification.
Clinicians and researchers are describing two types of COVID-19 pulmonary disease – Type L and Type H. Type L has low-elastane (high compliance), while the Type H disease has high elastane (low compliance) and matches a more typical ARDS type presentation.vi These two types may be differentiated by radiographic imaging or monitoring of pulmonary parameters.
Radiographic images showing two descibed COVID-19 pneumonia type. Type L on right (blue border) and Type H on left (red border). vi
When creating simulated parameters for COVID-19 respiratory disease, one pulmonologist suggested near normal resistance settings of 5-10 (cm H2O/L/s), but low, less elastic compliance settings 10-20 (mL/cm H2O) to match the more severe Type H presentation. Conversion from Type L to Type H may be part of a disease spectrum noted by high airway pressures. You do not need to memorize these values or units but could reference them if using simulation equipment that allows adjustment of these settings (see end of this post).
Concepts in training for COVID-19 respiratory distress:
Important steps in treating patients with COVID-19 respiratory failure include early recognition of hypoxia using a pulse oximeter. Reports of O2 saturations in the 80s with few symptoms are common.vii Use of personal protective equipment early, and worn correctly, is important to prevent infection and exposure risks to healthcare providers. Even the American Heart Association has modified their algorithm to emphasize that PPE is required before going into a code or starting CPR.viii
Another emphasis is on the use of viral filters, masks, and shields if performing airway procedures such as intubation and mechanical ventilation. A specific concern of infection through aerosol transmission (fine particles, not just large droplets) has been brought forward and healthcare providers in a room with a patient confirmed or suspected of having COVID-19 should wear appropriate PPE. Please do not use or dispose of real PPE during training at this time to save the needed equipment for hospital personnel.
Training devices that allow providers to understand ventilatory parameter changes such as compliance and resistance will be important.
It has also been suggested that many patients have improved oxygenation when in the prone position (face down). ix While this was done in a study to avoid intubation initially, it did use masks or other oxygen delivery devices and may lead to an increased challenge for healthcare providers in setting up ventilatory equipment, securing tubes, monitoring the patient, or responding to clinical changes. Chest compressions from the back may be effective but require specific positioning and provider training.x
Tools for Simulation Training:
In addition to standard manikin and vital signs displays for simulation training, task trainers that train providers on the ability to provide ventilation and oxygenation to patients safely should be considered. IngMar Medical is a major player in the respiratory training arena and produces tools for simulating lung ventilation, such as their QuickLung. This device allows easily adjustable settings to simulate different compliance and resistance settings. This could allow providers to obtain physical feedback in what it is like to provide manual ventilations using a Bag-Valve mask device. A quick lung could also be connected to a ventilator with different settings to see how the ventilator would need to be adjusted to provide appropriate tidal volume (VT) and safe pressures.
Another more sophisticated device, the IngMar RespiTrainer Advance will pair with the QuickLung described above. The RespiTrainer Advance has pressure and flow sensors that communicate with a computer via Bluetooth to monitor parameters of respiration with real-time visual feedback of numeric and graphical data about ventilations provided. In this way a provider can see the effect of automated or manual ventilations provided and the resistance and compliance settings can be set in software to match the physical settings of the attached QuickLung to ensure safe volumes and pressures are being delivered.
https://www.youtube.com/watch?v=NRUuXE-gKSI&t=56s (video demonstrating IngMar QuickLung and RespiTrainer Advance)
IngMar makes one more very sophisticated device to support respiratory simulation training. The ASL5000 is a device that not only detects volumes, flows, and pressures, but can also simulate patient respiratory effort, both inspiration and exhalation. Many have sought to use this device to test and measure the effect of ventilators across multiple programmable simulated lung parameters and patient conditions.
https://youtu.be/UdZljrAi5-8 (Video demonstrating some of the IngMar ASL5000 measurement parameters)
Although more complicated to set up, many of the tools mentioned can also be integrated with some Laerdal manikins for an even more immersive scenario design.
Because these devices are specialized in nature, designed for respiratory therapists and critical care specialties, the use and setup may seem daunting because of all the terms and settings that can be programmed. This article hopefully helped to introduce some of the terms associated with respiratory therapy and management and will allow an informed discussion with educators and providers in your region about how to better prepare health care providers to treat patients with COVID-19 respiratory failure.
i Drinker, P. A., & McKhann, C. F. (1986). The iron lung: first practical means of respiratory support. JAMA, 255(11), 1476-1480.
iiCavalcanti, A. B., Suzumura, É. A., Laranjeira, L. N., de Moraes Paisani, D., Damiani, L. P., Guimarães, H. P., ... & de Oliveira, R. P. (2017). Effect of lung recruitment and titrated positive end-expiratory pressure (PEEP) vs low PEEP on mortality in patients with acute respiratory distress syndrome: a randomized clinical trial. Jama, 318(14), 1335-1345.
iii Desai JP, Moustarah F. Pulmonary Compliance. [Updated 2019 Feb 18]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK538324/
iv Hurley JJ, Hensley JL. Physiology, Airway Resistance. [Updated 2019 Aug 4]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK542183/
v Huang, C., Wang, Y., Li, X., Ren, L., Zhao, J., Hu, Y., ... & Cheng, Z. (2020). Clinical features of patients infected with 2019 novel coronavirus in Wuhan, China. The Lancet, 395(10223), 497-506.
vi Gattinoni, L., Chiumello, D., Caironi, P., Busana, M., Romitti, F., Brazzi, L., ... & Gattinoni, L. COVID-19 pneumonia: different respiratory treatment for different phenotypes?.
viii Edelson, D. P., Sasson, C., Chan, P. S., Atkins, D. L., Aziz, K., Becker, L. B., ... & Escobedo, M. (2020). Interim Guidance for Basic and Advanced Life Support in Adults, Children, and Neonates With Suspected or Confirmed COVID-19: From the Emergency Cardiovascular Care Committee and Get With the Guidelines®-Resuscitation Adult and Pediatric Task Forces of the American Heart Association in Collaboration with the American Academy of Pediatrics, American Association for Respiratory Care, American College of Emergency Physicians, The Society of Critical Care Anesthesiologists, and American Society of .... Circulation.
ix Ding, L., Wang, L., Ma, W., & He, H. (2020). Efficacy and safety of early prone positioning combined with HFNC or NIV in moderate to severe ARDS: a multi-center prospective cohort study. Critical care, 24(1), 28.
x Bhatnagar, V., Jinjil, K., Dwivedi, D., Verma, R., & Tandon, U. (2018). Cardiopulmonary resuscitation: Unusual techniques for unusual situations. Journal of emergencies, trauma, and shock, 11(1), 31.