How to Select a Medical Ventilator to Save Critical COVID-19 Patients

by

Dr. Sharad N. Pachpute

 

 Introduction to Ventilator

• A ventilator is a machine which helps for artificial breathing of air in and out of the lungs if a patient is not able to breathe naturally
• Some critical patients need help to breathe or functioning of the lungs. To achieve this, ventilator assist breathing process of such patients. Ventilator is also called breathing machine.
• The ventilator supply air at positive pressure into the lungs and helps in maintaining oxygen level in the blood
• In ventilators, some pure oxygen mixes with surrounding air and that is supplied with positive pressure through small flexible hose whose one end is fitted mouth or nose mask
• Two flexible hoses are provided on ventilators:
  • Inspiratory pipe: to provide fresh oxygen rich air to lungs
  • Expiratory pipe: to expel the used air from lungs
• The following figures shows a schematic of ventilator machine
• The amount of oxygen is controlled with help sensors attached the patient’s body

 

• Medical staffs regularly monitor the conditions of patients:
  • breathing rate or oxygen level,
  • functioning of ventilation,
  • comfort level of patient,
  • Consequences of ventilation on lungs and other body parts

 Scope of the ventilator for Saving Critical Patients

Ventilator is needed during the failure of the respiratory system

• Lung issues: Asthma, chronic obstructive pulmonary disease (COPD), severe pneumonia
• Body wide problems: Drug overdose, poisoning, severe trauma, Acute respiratory distress syndrome (ARDS), Sepsis (infection in your bloodstream)
• Brain Issues: Amyotrophic lateral sclerosis (ALS), head injury or stroke,
• Heart Failure: Sudden cardiac arrest, heart failure
• Nerve or muscle problems: Spinal cord injury, polio myasthenia gravity
• Neonatal respiratory distress syndrome: breathing problems in newborns, or premature babies.

 Ventilation for COVID-19 Patients

• During the pandemic period of coronavirus, many COVID- 19 patients face shortness of breathing. These viruses cause highly viscous fluids and mucus in the smaller airways of the lung and block the oxygen supp to lung tissue.
• Medical ventilation will help COVID-19 patients to overcome breathing problems until infection lungs is completed out and recovery of immune system
• Many hospitals have shortage of ventilation. Medical staffs need to be aware selection of ventilation machine, monitoring and precautions during mechanical ventilation.

Flow Physics of Ventilation

• When air flows during inspiration and expiration, the amount of resistance to airflow through respiratory duct is called as airway resistance
• Air flows through ventilators, trachea and airways of the lungs. Its pressure decreases along its flow direction. That means the resistance to flow increases.
• To minimize the airflow resistance, the pressure drop should be minimized in order to maintain a smooth flow of oxygen or air to patients
• Major factors that breathing rate and airway resistance should be considered while selecting a ventilation system for patients

 

Determinants of Airway Resistance and Pressure Drop

• The Ohm’s law finds the rate of electricity flowing through the electrical circuit, current = voltage/resistance
• Similarly, we can find the rate of airflow through the flow circuit as: Flow = Pressure gradient / Resistance

Where R_AW is the airway resistance. P_atm and P_A denote atmospheric and airways pressure. V is the volume flow rate flowing through airways.

• The Hagen-Poiseuille equation of pipe flow can be used for air flow calculation using hydraulic diameters of airways, viscosity and length of duct. However, the size of airways changes consciously due to contraction and relaxation of muscles. That is controlled by nervous control system.
• The flow characteristics of the respiratory system, depend on pressure drop across airways and other physical parameters:
  • Lung volume (∆V_L)
  • Pressure at the airway opening (P_mou)
  • Pressure within the intrapleural space (P_pl)
  • Atmospheric pressure (P_atm)
  • Alveolar pressure (P_alv)
  • Muscle pressure difference (∆P_mus);
  • volumetric flow rate(Q): positive during inspiration, negative during expiration
  • Resistance of the respiratory system (R_r)
  • Lung compliance (C_L): change in pressure (∆P) /change in volume (∆V_L)
  • Chest wall compliance (C_ch)

Inspiration and Expiration

• Air flow resistance is different for inspiration and expiration due changes in the size (diameter) of airways
• During inspiration, the positive pressure is within the alveoli causes. The flow resistance decreases due to increase in diameters of smaller airways. But this is opposite during expiration, narrow airways increase the flow resistance because of reduced pressure.
• The air ventilation helps patients to maintain the breathing cycle of inspiration and expiration by delivering the oxygen and expelling the used air

• The following figure shows the tidal volume and pressure of artery during mechanical ventilation

 Type of Medical Ventilator

• A detailed classification  of medical ventilatiors is given below. There are two types of medical ventilators.
• Mechanical ventilators are used for long term and continuous supply of oxygen to patients. Bag-valve mask ventilators are small which are pretty common in ambulance crews and critical units of hospitals
• Mechanical ventilators are further classified as
  • Positive pressure ventilation system pushes the air or oxygen into the lungs with a certain pressure
  • Negative pressure ventilation system sucks the air oxygen into the lungs by expansion and contraction of chest.
• Negative pressure ventilations were popular in old day (before 1950). Nowadays most devices are based on positive pressure ventilation due to efficient functioning and easy controlling of pressure and flow rate of oxygen

4.1 Types of Positive Mechanical Ventilator

• In invasive ventilation, an endotracheal tube is inserted into the mouth or nose to reach the lungs. The procedure of inserting tube is called intubation.
• The ventilation tube can be directly inserted into the bigger airways of respiratory system (trachea). This procedure is called a tracheostomy.

• Mechanical ventilators are adjusted to meet the needs of oxygen supply as per the patient.
• Non-invasive ventilation system, ventilator support supplies air or oxygen without any tracheal intubation. This is easy to use for patients who need some medical ventilation and who are not hypoxemic.
• In non-invasive ventilator, oxygen-rich air is supplied to the lungs with the help face mask.

Positive Mechanical Ventilator

• A mechanical ventilator blows air with increased oxygen percentage compared to normal air. This mixture passes through a humidifier which maintains the pressure, humidity and pressure as per the need of the patients
• Two flexible hoses are provided: 1) to supply oxygen-rich air, 2) to expel used air which contains carbon dioxide
• The breathing rate, pulse rate, and blood pressure are monitored continuously during the ventilation

• The various sensors used for the ventilation system to monitor the volume flow rate of air, pulse rate, and blood pressure of the patient

Bag Valve Mask (BVM) Ventilator

• Bag-Valve-Mask (BVM) ventilator is also called as manual resuscitators. Because, this device used a self-inflating resuscitation system.
• This device is operated manually by squeezing the self-inflating bag
• Due to portability and low cost, BVM Ventilator is used in many ambulance crew and critical care units
• The BVM ventilator uses a self-inflating bag which in order maintain continuous flow of oxygen
• Ventilation (self-inflating) bags are available
  • Preterm (for new-born): 140ml
  • Paediatric (for infant or children): 250ml
  • Adult: 1600ml
• The pure oxygen passes through Positive end expiratory pressure (PEEP) valves and face-mask
• Oxygen source (100% oxygen) is attached to the behind the resuscitator bag as a reservoir bag

 

 

• To achieve successful ventilation by using this method depends, the following things should be checked:
  • A patent airway
  • Proper mask seal
  • Proper ventilation setting
  • PEEP valve to improve oxygenation

Limitations of BVM Ventilator

• Requires manual fitting by medical staff
• Cannot use if a patient needs long term steady air supply
• Loose-fitting of face mask seal reduces the percentage of oxygen that a patient inspires (FiO2)
• limited ability to gauge tidal volumes and hyperventilation of patients
• Other issues like aspiration, claustrophobia and gastric distension by forced ventilation
• Exhaled secretions or moisture in airways can cause the dysfunction in exhalation valve and increases the air flow resistance for exhalation
• Possibility of Risk  due to  barotrauma to patients

Installation and Monitoring the Ventilation 

Correct Installation of Ventilation unit

• The ventilation unit is installed such a way that the area  is divided between high and low level
• Ventilators unit should  be designed  so that they cannot close air supply
• Check hygiene of patients
• For a mechanical ventilator with positive ventilation,  the head side of bed is tilted in angle between 30-45° for improved breathing
• Cleanness of air in rooms
• Monitor the oxygen level and pulse rate

 Recording and Monitoring

• Ventilators Setting
• Airways Pressures
• Consciousness
• Vitals (Pulse, BP, Temperature, RR)
• Intake or output charting
• Compliance
• Suctioning
• Humidification
• Dates of intubation and catheterisation
• Central Venous Pressure (CVP), Blood sugar monitoring
• Treatment (Drug chart, RT Feeding chart)

How to Check Patients for Ventilation

• Pulse oximeter is used to check the saturation oxygen level and pulse rate. If it falls below 90%, then the patients need to be put on ventilation units
• Follows the major process for different patients as per age and level of illness

 

• Process for Bag-valve Mask (BVM) ventilator 

Scope of CFD Modeling  for Ventilator design

• The velocity and pressure drop across the ventilator system can be determined based on CFD simulations
• The predicted velocity can be used to optimize the performance of the ventilator
• click on some articles on flow prediction inside the lungs

   

• Felicity predicted in the upper part of the lung

     

Reference

L. Hao, Y. Shi, M. Cai, S. Ren, Dynamic Characteristics of a Mechanical Ventilation System With Spontaneous Breathing, IEE (2019)