Respiratory Therapy – Humidity, VOCP Priorities, Ventilator I:E Ratios, Lung Volumes & Disease Patterns

Humidity Fundamentals

• Absolute Humidity (Content)

  • Actual mass of water vapor present in a gas, expressed in \text{mg}\,L^{-1}.
  • Example used in lecture : given “content = 30” \text{mg}\,L^{-1}.

• Capacity

  • Maximum water vapor a gas can hold at a stated temperature.
  • At normal body temperature ( 37\,^{\circ}\text{C} ) the physiological capacity is always 43.8\,\text{mg}\,L^{-1}.
  • “43.8” is only applied to gases delivered to/inside people, not to generic test-questions unless the stem explicitly places you at body conditions.

• Relative Humidity (RH)

  • Definition: \text{RH}(\%) = \dfrac{\text{Content}}{\text{Capacity}} \times 100.
  • If capacity = 60 and content = 30 → \text{RH}=50\%; do not force 43.8 into such problems.
  • Clinical shortcut: when evaluating humidification devices for patients, always benchmark content against 43.8 mg/L (100 % RH at body temp).

Clinical Prioritization : VOCP

• “Road-map” for every respiratory scenario – tackle problems in this exact order:
  1. V = Ventilation (move CO₂ out / air in)
  2. O = Oxygenation (raise PaO₂ / SpO₂)
  3. C = Circulation (cardiac output, blood pressure)
  4. P = Perfusion (blood flow reaching tissue)
• Rationale
  • Ventilation first because oxygen cannot help a patient who is not exchanging gas; primary indicator is P{\text{a}}CO{2}.
  • Oxygenation addressed only after ventilation stabilised.
  • Circulation/perfusion useless if lungs are not loading O₂ or removing CO₂.
• Scenario illustration
  • 29-year-old, MVC, 36 weeks pregnant, agonal breathing, pinpoint pupils, hypotension.
  • If “V” abnormal → treat ventilation (e.g., bag-mask, intubation) before giving O₂, fluids, drugs.

Cardiopulmonary Resuscitation & the VOCP Logic

• CPR appears to start with C (compressions) but still respects VOCP:
  • In-hospital arrests usually monitored; once heart stops we assume last breaths carried adequate O₂ into arterial blood.
  • Therefore ventilation/oxygenation briefly “parked”; pressing threat is circulation.

Respiratory Cycle & I:E Ratio

• One complete cycle (inspiration + pause + exhalation) = 100 % of time.
• Normal I:E ≈ 1:2 – 1:3; can see 1:4 occasionally.
  • Convert ratio to % of cycle:
  • 1:2 → \frac{1}{3}\;=\;33\% inspiration, \approx66\% exhalation.
  • 1:3 → 25\% : 75\%.
  • 1:1 → 50\% : 50\%.
  • 1:4 → 20\% : 80\%.
• Ventilator control
  • RT chooses I:E; physicians rarely order it unless requesting a deliberate inverse-ratio setup.
  • Inverse I:E (2:1, 3:1, 4:1) prolongs inspiratory time to improve O₂ diffusion in severe hypoxemia/ARDS.
  • Requires heavy sedation/paralysis; feels extremely unnatural (demonstrated by 4-s inhale / 1-s exhale exercise in class).

Inspiratory Flow-Rate Dynamics

• Flow setting (L·min⁻¹) on the ventilator is inversely proportional to inspiratory time.
  • ↑ Flow ⇒ breath delivered faster ⇒ shorter I-time ⇒ longer E-time.
  • ↓ Flow ⇒ longer I-time ⇒ shorter E-time.
• No physician order ­– flow is 100 % RT responsibility.
• Everyday analogy : driving 65 mph vs 50 mph to mom’s house (same distance, slower speed → more time).

Lung Volumes & Capacities (Graphical Boxes)

• Residual Volume (RV)
  • Gas that never leaves lungs (measurable only indirectly or post-mortem).
• Vital Capacity (VC)
  • \text{VC} = \text{max inhalation} - \text{max exhalation}.
  • Patient instructions : “Blow out all the way, now inhale as deep as possible; exhale into spirometer.”
  • Tracked frequently in neuromuscular disorders (e.g., ALS) to anticipate respiratory failure.
• Total Lung Capacity (TLC)
  • \text{TLC} = \text{VC} + \text{RV}.
• Inspiratory Capacity (IC)
  • Maximum volume inhaled after a normal exhalation.
  • \text{IC} = \text{VT} + \text{IRV}.
• Functional Residual Capacity (FRC)
  • Gas remaining after a normal exhale.
  • \text{FRC} = \text{ERV} + \text{RV}.
  • Clinically crucial; changes flag many pathologies.
• Inspiratory Reserve Volume (IRV)
  • “Extra” air you can still inhale at the top of a normal breath (the surprised-gasp example).
• Expiratory Reserve Volume (ERV)
  • Additional volume you can force out after a normal exhalation.
• Tidal Volume (VT)
  • Ordinary quiet breath in/out.

Equational Relationships Mentioned

\begin{aligned}
\text{TLC} &= \text{VC} + \text{RV} \
&= \text{IC} + \text{FRC}\[4pt]
\text{FRC} &= \text{ERV} + \text{RV}
\end{aligned}

Pulmonary Function Testing & Clinical Application

• Spirometers not computer-linked; RT manually enters values into Electronic Medical Record (EMR).
• Reproducibility: trend VC q4 h in ALS, Guillain-Barré, Myasthenia gravis to detect crisis (e.g., fall from 4.2\,L \rightarrow 3.5\,L).
• Correct patient coaching vital – wrong instructions turn a VC into an IC and mislead care.

Obstructive vs Restrictive Lung Disease

• “Highly-compliant” analogy: person overly compliant in an abusive relationship – seems harmless but harmful in reality.
• Mixed patterns possible (e.g., COPD patient with severe scoliosis).

Key Numerical & Statistical References

• Physiological water-vapor capacity at body temp: 43.8 mg L⁻¹.
• Normal I:E ratios: 1:2 – 1:3.
• Part-to-percent conversions (memorise):
  • 1:2 → 33 / 66 %; 1:3 → 25 / 75 %; 1:1 → 50 / 50 %; 1:4 → 20 / 80 %.
• Example VC table from lecture (average adult male, \approx6 L total):
  • \text{VT}=0.5 L, \text{IRV}=2.5 L, \text{ERV}=1.2 L, \text{RV}=1.8 L.
  • Confirms 0.5+2.5+1.2+1.8 = 6.0 L (TLC).

Mnemonics, Analogies & Study Tips

• VOCP – write it everywhere; one cohort printed it on miniature rum bottles!
• Flow vs Time – remember the speed-limit / travel-time analogy.
• Inverse I:E – practise 4-s inhale / 1-s exhale to feel how unnatural it is; patients must be sedated.
• Obstruction = Flow, Restriction = Volume – two buckets on PFT cheat sheet.
• Abusive-relationship analogy for excess compliance; helps to recall that compliant lungs are not always desirable.

These notes encapsulate every primary and secondary concept, example, equation, and clinical pearl presented in the transcript. Use them as a standalone study guide for respiratory physiology, ventilator management, humidity control, and pulmonary function testing.