Mechanical Ventilation Modes and Respiratory Math

Introduction to Respiratory Lab and Exam Content

• The upcoming exam will focus exclusively on topics covered in the laboratory sessions.
• It will include material discussed today, specifically centered on mechanical ventilation mathematics.
• Key focus areas include Pressure Control and understanding ventilator modes.

Understanding Assist Control (AC) Modes

• In Assist Control (AC) modes, every breath—whether machine-initiated or patient-triggered—is a mandatory breath.
• When patient-triggering occurs (the patient "asks" for more breaths), the ventilator delivers the exact same volume or pressure as the set mandatory breath.
• Volume Control Assist Control (VCAC):
  • Set parameters include: Tidal Volume ($V_T$), Respiratory Rate ($RR$), Oxygen Concentration ($FiO_2$), and Positive End-Expiratory Pressure ($PEEP$).
  • If the set rate is $12$ but the patient is breathing at $19$, the patient triggered $7$ extra breaths. However, all $19$ breaths look identical because they are all machine breaths.
• Pressure Control Assist Control (PCAC):
  • Set parameters are similar, but pressure is regulated instead of volume.
  • On a pressure-time graph, the breaths have a square shape.
  • The only visual difference on the graphic between a set breath and a patient-triggered breath is the presence of the patient trigger (indicated by a small deflection or marker before the breath), but the breath itself remains identical in pressure and duration.

Pressure Support Ventilation (PSV)

• Unlike AC, PSV is comprised entirely of spontaneous breaths.
• Set Parameters:
  • Pressure Support ($PS$).
  • PEEP ($PEEP$).
  • Oxygen Concentration ($FiO_2$).
• Graphic Characteristics:
  • Every breath will have a trigger in front of it on the waveform.
  • Breaths can be inconsistent in regularity and duration (time between breaths and the length of the breath itself).
  • The commonality is that they will all reach the same set inspiratory pressure targets.
• Clinical Application:
  • This is the mode used for extubating patients.
  • It is frequently compared to running a marathon. For an out-of-shape patient, breathing spontaneously is strenuous work. Patients often require short increments of PSV to build respiratory muscle strength, which is increased over time.
• Terminology and Variations:
  • PSV is to non-invasive ventilation what BiPAP is to non-invasive ventilation.
  • In clinical settings, it is often referred to simply as CPAP, although it is not identical to continuous positive airway pressure.
  • The Draeger ventilator refers to this mode as "Spontaneous CPAP."

Synchronized Intermittent Mandatory Ventilation (SIMV)

• SIMV is described metaphorically as the "baby" resulting from the marriage of Pressure Control Assist Control and Spontaneous (PSV) modes.
• It synchronizes mandatory breaths with the patient's effort.
• If a patient triggers a breath beyond the set mandatory rate, that breath is spontaneous rather than mandatory.
• Parameters for Ventilation:
  • Mandatory: Respiratory Rate ($RR$), Tidal Volume ($V_T$) or Pressure, Flow ($Flow$).
  • Spontaneous: Pressure Support ($PS$).
• Waveform Characteristics:
  • You will see a combination of shapes. Mandatory breaths in Pressure Control SIMV will be square/high, while spontaneous breaths will be square but generally smaller (supported by Pressure Support).
• Clinical Utility:
  • SIMV can serve as both a weaning mode and a mandatory support mode, depending on the set Respiratory Rate.

Categorizing Ventilator Settings: Ventilation vs. Oxygenation

• Oxygenation Settings:
  • Primary settings are always $PEEP$ and $FiO_2$.
  • $PEEP$ is functionally equivalent to terms like CPAP and EPAP.
• Ventilation Settings:
  • These differ based on whether the ventilation is mandatory or spontaneous.
  • Mandatory Ventilation Settings: Respiratory Rate ($RR$), Tidal Volume ($V_T$), and Flow ($Flow$).
  • Spontaneous Ventilation Settings: Pressure Support ($PS$).

Total Cycle Time (TCT) and Inspiratory Time (Ti)

• Total Cycle Time Formula:
  • $TCT = \frac{60\,\text{seconds}}{RR}$
  • Example: For a Respiratory Rate of $12$, the $TCT$ is $5\,\text{seconds}$.
• Inspiratory Time ($T_i$) in Volume Control:
  • $T_i$ is set indirectly through Tidal Volume and Flow.
  • Formula: $T_i = \frac{V_T}{\text{Flow in L/sec}}$
• Mathematical Conversions:
  • Tidal Volume must be converted to Liters (move decimal three places left or divide by $1000$). Example: $500\,mL = 0.5\,L$.
  • Flow must be converted from $L/min$ to $L/sec$ by dividing by $60$. Example: $45\,L/min = 0.75\,L/sec$.
• Example Calculation:
  • Given $V_T = 500\,mL$ ($0.5\,L$) and $Flow = 45\,L/min$ ($0.75\,L/sec$):
  • $T_i = \frac{0.5\,L}{0.75\,L/sec} = 0.6666\,seconds$.

Mathematical Relationships in Ventilation

• Tidal Volume and Inspiratory Time ($T_i$):
  • Direct Relationship: As $V_T$ increases, $T_i$ increases. (Metaphor: Loading more kids on a bus takes more time).
• Flow and Inspiratory Time ($T_i$):
  • Indirect Relationship: As Flow increases, $T_i$ decreases. (Metaphor: Traffic speed on I-84; faster flow means you reach the exit sooner).
• Respiratory Rate and Expiratory Time ($T_e$):
  • Respiratory Rate only changes the Expiratory Time ($T_e$). It does not directly change the Inspiratory Time ($T_i$).

Expiratory Time (Te) and I:E Ratios

• Expiratory Time Formula:
  • $T_e = TCT - T_i$
  • Example: If $TCT = 5\,sec$ and $T_i = 0.7\,sec$, then $T_e = 4.3\,sec$.
• I:E Ratio Formula:
  • Expressed as $1:n$, where $n = \frac{T_e}{T_i}$.
  • Example: $\frac{4.3}{0.7} \approx 6.1$. Ratio is $1:6.1$.
• Clinical I:E Standards:
  • Normal Ratio: Roughly $1:2$.
  • Restrictive Lung Disease: Roughly $1:2$.
  • Obstructive Lung Disease (CBABE - Cystic Fibrosis, Bronchitis, Asthma, Bronchiectasis, Emphysema): Ratios like $1:4$ or $1:5$ to allow for longer exhalation.

Impact of Setting Changes on Ratios

• Changing Respiratory Rate:
  • If RR is increased to $20$, $TCT$ becomes $3\,sec$.
  • If $T_i$ remains $0.7\,sec$, new $T_e = 2.3\,sec$ ($3 - 0.7$).
  • New Ratio: $\frac{2.3}{0.7} = 1:3.2$.
• The "Seesaw" Effect:
  • Because $TCT$ is a fixed length of time determined by the rate, any change to $T_i$ will indirectly change $T_e$ to balance the cycle. If you take time from one, it goes to the other.

Questions & Discussion

Question: In Pressure Control SIMV, are the first three breaths mandatory and the last three spontaneous, or is there a specific order?

Response: The waveform is determined by patient effort. Mandatory breaths will have the set square pressure shape. Spontaneous breaths occur whenever the patient triggers above the set rate; they are supported by Pressure Support and will also appear square but typically smaller. The order is entirely dependent on when the patient chooses to breathe relative to the machine's set timing. SIMV can be a weaning mode or a mandatory mode depending on the set rate.