Ventilators and breathing parameters

A piece of equipment that should ASSIST in breathing. Assist is the operative word. A good ventilator determines the requirement of the patient and only provides assistance. This is the most advanced mode of ventilation and is termed “patient-synchronized ventilation.”  

The important AVERAGE parameters for ventilation are:

  • Number of breaths per minute: Typically 15-20 per minute at rest.
  • Inhalation to Exhalation time ratio: Typically 1 second to 1.5-2 seconds.
  • Pressure (Critical parameter): 10 cm to 30 cm of H2O column above atmospheric pressure.
  • Volume per breath: 500 ml at atmospheric pressure for an average adult
  • Volumetric flow rate (important for sizing flow controllers): 60 SLPM.

Ventilator Modes

  • Pressure Control Mode: In this mode, air is forced into the patient at a certain pressure during inhalation and then withdrawn during exhalation. A pulse of pressure is applied with a certain frequency (the breathing rate). For the electrical engineer, this is a voltage pulse. Given the resistance of thebreathing circuit, it forces a volume of air (current) through the patient.
  • Volume Control Mode: In this mode, a fixed volume of air is forced into the patient in a fixed period. The pressure (potential) is raised to permit this flow rate (current).
  • “Patient synchronized” modes require sensing of various parameters at the patient end and then using PID control to deliver the required volumes of air at a controlled volumetric rate, pressure and frequency. Modern ventilators have many such modes.
  • Pressure Regulated Volume Control: Pressure-regulated volume control (PRVC) ventilation is a mode of mechanical ventilation that combines volume and pressure control ventilation. PRVC is an appropriate mode of ventilation for patients who require a specific tidal volume (VT) with the lowest effective pressure, such as those with acute respiratory distress syndrome (ARDS) which is a condition found among COVID patients. In the PRVC mode, the ventilator delivers a volume-controlled breath at the lowest delivery pressure,according to the target VT.
  • Pressure support ventilation: Pressure support ventilation (PSV) is a spontaneous mode of ventilation in which each breath is initiated by the patient but is supported by constant pressure inflation. It allows the patient to determine inflation volume and respiratory frequency (but not pressure, as this is pressure-controlled), thus can only be used to augment spontaneous breathing.
  • Synchronized Intermittent-Mandatory Ventilation: SIMV assists the spontaneous breathing of the patient with a preset number of ventilator-delivered breaths each minute. The ventilator detects the patients spontaneous breathing, waits until the patient exhales before delivering another mechanical breath and thus “synchronizes" the ventilator to the spontaneous breathing.It is primarily a volume assist mode.

Pressure

Not too much pressure: Think of lungs as balloons. If pressurized too much, a balloon bursts. The same will happen to our lungs. Thus, it is extremely critical to not over-pressurize our lungs.

Not too little either: If a balloon loses all its air, it shrivels up. This should not happen to our lungs, as it causes “wounds” and lung trauma.

Pressure Range: Max 40 cm of H2O column during forced inhalation above atmospheric pressure; Min 10 cm of H2O column during forced exhalation above atmospheric pressure. Note: Atmospheric pressure varies from place to place.

This is a very small gauge pressure. We repeat: This is a very small gauge pressure. One should be very careful with pressure control while designing ventilators. 

Pros and cons of pressure control

  • Airway pressures are limited, regardless of changes in resistance or compliance, thus preventing damage to alveoli due to shearing forces.
  • If the resistance of the lung rises due to disease in the patient’s airway or chest wall, the set pressure limit being the same (or potential being limited) the delivered tidal volume (charge) will fall.

BiPAP, CPAP, the coronavirus and D1

  • BiPAP or Bi level positive airway pressure is essentially the pulse that D1 applies. BiPAP is typically applied through a mask.
  • CPAP is continuous positive airway pressure and is also typically applied through a mask.
  • In both BiPAP and CPAP the exhalation happens into the hospital room. In the case of the coronavirus, this is very dangerous to the medical professionals in the room. MASK-based ventilation should only be used in a negative-pressure room with medical professionals in well-protected suits.
  • D1 is meant for BiPAP but to be administered NOT with a mask. It should be administered using the standard ventilator interface. NO EXHALATION INTO THE ROOM unless precautions as in point 3 above are taken.