Chapter 11: Ventilation Study Guide

What is ventilation?

Ventilation is the process of moving gas (usually air) in and out of the lungs

What is respiration?

The physiologic process of using O₂ by the tissues at the cellular level (Process of gas exchange)

Two components that make up one respiratory cycle

Inspiration and Expiration

What is VOCP?

The order in properly assessing a patient. Ventilation, Oxygen, Circulation, and Perfusion.

What is TIdal volume (TV or VT) ?

The volume of gas that moves in and out of the respiratory tract (A normal breath)

Normal Tidal Volume in adults?

500 mL (.5L)

How is Tidal Volume important?

The VT refreshes the gas present in the lung, removing CO₂ and supplying O₂ to meet metabolic needs.

What is an I:E ratio

Ratio of inspiratory time to expiratory time.

For example - 1:2 (33% Inspiration; 66% Expiration)

How does INCREASING the flow rate on the ventilator affect the I:E ratio?

What about if you DECREASE the flow rate?

What happens to the PCO2?

When flow rate is increased, I-time shortens (decreases) and E-time lengthens. (more E-time means less PCO2)

When flow rate is decreased, I-time is lengthened and E-time shortens. (more I-time means more PCO2)

What is normal I:E ratio?

How about longer exhalation?

Equal phases?

Or inverse ratio ventilation?

Normal - 1:2

Longer exhalation - 1:3 or 1:4

Equal phases - 1:1

Inverse ration ventilation - 3:1

VE (Volume Expired) Equation

VT x RR or F (Frequency)

What is Total Lung Capacity (TLC)?

Total amount of air in lungs after maximal inspiration ≈ 6.0 L.

What is Vital Capacity (VC)?

Maximum amount of air exhaled after maximal inspiration ≈ 4.8 L.

What is inspiratory capacity (IC)?

Maximum air inhaled after normal exhalation ≈ 3.0 L.

What is Functional Residual Capacity (FRC)?

Amount of gas in lungs after a normal exhale ≈ 3.0 L.

What is inspiratory reserve volume (IRV)?

Extra air inhaled after a normal breath ≈ 2.5 L.

What is expiratory reserve volume (ERV)?

Extra air exhaled after a normal exhale ≈ 1.8 L.

What is tidal volume (TV)?

Normal breath ≈ 0.5 L.

What is residual volume (RV)?

Air left in lungs after maximal exhalation ≈ 1.2 L.

What is ΔPressure (ΔP)?

Force generated by the respiratory muscles or a mechanical ventilator during inspiration. This is a pressure gradient required for airflow.

What is ΔVolume (ΔV)?

Change in volume. (e.g., VT = amount of air inspired in a usual breath).

What is Elastance (E)?

Distensibility (stiffness) of the lungs and thorax. Highly elastic lungs are restrictive diseases, or diseases of volume.

Hooke's Law states:

The tension (force) developed in an elastic structure is directly proportional to the amount of stretch (deformation) applied. The pressure it takes to inflate my lungs is directly related to the deformity of my lungs

What is hysteresis?

Tidal history of the object. An object that is inflated will never return to how it was before it was inflated. Common in newborns. When we apply it to the lungs, hysteresis refers to the difference between inspiratory and expiratory pressure-volume curves exhibited by the lung.

What is compliance?

Compliance means how easily the lungs expand. Highly compliant lungs are obstructive diseases, or diseases of flow. Normal compliance = 0.2 L/cmH2Ops.

What is Resistance (R) or Raw?

Gas flow through the airways causes frictional impedance called flow resistance. Resistance to ventilation by movement of gas through the airways is called airway resistance (Raw).

Higher turbulence also means higher resistance.

Where does 80% of airway resistance (Raw) occur?

What about the other 20%?

Approximately 80% of resistance occurs in the nose, mouth, and large airways (where flow is mainly turbulent).

20% of total resistance occurs in small airways (< 2 mm diameter) (where flow is mainly laminar).

What is normal Raw?

0.5-2.5 cm H₂O/L/sec.

What is surface tension and what does it affect?

Attraction of like molecules at the surface of a liquid that causes the surface to contract and resist expansion.

Affects elastance, compliance, and Raw.

Surfactants importance to surface tension and where is it produced?

If surfactant is present, surface tension decreases. Alveolar Type II cells produce surfactant.

Can gas flow occur without pressure gradients?

NO!

Note: During normal breathing, the glottis remains open.

What is Transalveolar Pressure Gradient (PTA) and what does it represent?

𝑃𝑇𝐴 = 𝑃𝐴 − 𝑃𝑝𝑙

Pressure difference between the alveoli (PA) and the pleural space (Ppl).

What it represents: It is the distending pressure across the alveoli. Reflects the force keeping alveoli open.

What is Trans-Chest Wall Pressure Gradient (PTCW) and what does it represent?

𝑃𝑇𝐶𝑊 = 𝑃𝑝𝑙 − 𝑃𝑏𝑠

Pressure difference between the pleural space and the body surface (atmospheric pressure).

What it represents: The elastic recoil of the chest wall. The pressure required to expand or compress the thorax

What is Transpulmonary Pressure Gradient (PTP) and what does it represent?

Transpulmonary pressure at end-inspiration is the largest pressure gradient during normal breathing.

𝑃𝑇𝑃 = 𝑃𝐴𝑂 − 𝑃𝑝𝑙

Pressure difference between the airway opening (PAO) and the pleural space (Ppl).

What it represents: • The total distending pressure of the lung (airways + alveoli). Clinically most important pressure when evaluating lung mechanics.

What is Transrespiratory Pressure Gradient (PTR) and what does it represent?

𝑃𝑇𝑅 = 𝑃𝐴𝑂 − 𝑃𝑏𝑠

Pressure difference between the airway opening and the body surface.

What it represents: Pressure across the entire respiratory system (lungs + airways + chest wall). Equals the total pressure generated by respiratory muscles or a ventilator.

What is the Transthoracic Pressure Difference (PTT) and what does it represent?

𝑃𝑇𝑇 = 𝑃𝐴 − 𝑃𝑏𝑠

Pressure difference between the alveoli and the body surface.

What it represents: Pressure needed to expand both the lung and chest wall together. Reflects total elastic recoil of thorax + lungs.

What happens during Inspiration?

Pleural pressure becomes more negative. PTA and PTP increase. PA drops slightly below PAO and air flows into lungs.

What happens during expiration?

Muscles relax and elastic recoil raises PA above PAO. Air flows out. Expiration is positive.

How is stridor characterized?

High-pitched "barking seal" sound. Associated with laryngeal and tracheal obstruction. Caused by upper airway narrowing. Stridor indicates upper airway obstruction, not lower airway disease (like asthma).

What is Croup (Laryngotracheobronchitis)

Typically affects young children (infants to ~4 years old). Caused most commonly by viral infection (Parainfluenza). Swelling below the glottis (subglottic edema).

What is Epiglottitis?

More common in older children (historically >4 years old). Previously caused by Haemophilus influenzae type B (Hib). Now rare due to vaccination.

What is a Transmural Pressure Gradient?

pressure inside the airway − pressure outside the airway. During normal exhalation, elastic recoil of the lungs increases alveolar pressure. Air then flows out because alveolar pressure becomes greater than atmospheric pressure. The transmural pressure helps determine whether airways stay open or collapse.

What is FEV1?

Forced Expiratory Volume in 1 second. Measuring flow

What is FEF 25-75%?

Forced expiratory flow 25-75%, Flow during the middle half of expiration (reflects small airway function).

< 80% of predicted value = medical issue.

What happens during forced exhalation?

Intrathoracic pressure becomes positive. Pleural pressure increases significantly. Airway compression can occur (dynamic compression). The equal pressure point (EPP) forms where airway pressure equals pleural pressure

What is the work of breathing (WOB)?

Cost of breathing ≈ 5% of total oxygen consumption (at rest). It can increase to 40% with sick patients

What patterns of breathing are seen in patients with restrictive diseases?

Rapid and shallow breathing.

What are some Restrictive diseases

Pulmonary Fibrosis, ARDS, Sarcoidosis, Kyphoscoliosis, Neuromuscular disorders

What patterns of breathing are seen in patients with obstructive diseases?

Prolonged expiration

What are some Obstructive diseases

Asthma, COPD, Bronchiectasis, Cystic Fibrosis, Bronchiolitis. and Atelectasis

Upper airway inflammation means what?

Prolonged Inspiration

What happens during weaning from mechanical ventilation?

Patient must assume full work of breathing. O₂ consumption increases. If respiratory muscles fatigue → weaning failure.

Aerobic metabolism

Presence of O2, biproduct is CO2

Anaerobic Metabolism

Without presence of O2, biproduct is lactic acid.

Do sick lungs have more time for filling and emptying gas?

YES!

Patients will also have abnormal time constants.

Time constant

Resistance × Compliance (R × C).

What is air trapping?

Incomplete exhalation before next breath begins. Leads to dynamic hyperinflation. Increases work of breathing. Can cause hypotension in ventilated patients

Normal V/Q ratio?

Respiratory Quotient?

Normal Ratio - 4 = ventilation, 5 = perfusion

RQ- 0.8

What do we assume if V/Q is above 1?

Pulmonary Embolism

Lower VQ =

Higher VQ =

Lower VQ means ventilation issue

Higher VQ means perfusion issue

What is PEEP?

Positive End-Expiratory Pressure.

• Maintains alveolar recruitment.

• Prevents collapse.

• Improves oxygenation.

Anatomic dead space

Gas left in conducting airways after inspiration.

1 mL/lb of IBW (2.2 mL/kg)

Alveolar Dead Space

When alveoli are ventilated but not perfused Gas reaches alveoli but there's no blood flow available for gas exchange.

Physiologic dead space

Physiologic dead space = Anatomic dead space + Alveolar dead space.

𝑉𝐷(𝑝ℎ𝑦𝑠) = 𝑉𝐷(𝑎𝑛𝑎𝑡𝑜𝑚𝑖𝑐) + 𝑉𝐷(𝑎𝑙𝑣𝑒𝑜𝑙𝑎𝑟)

What is the Bohr Equation?

𝑉𝐷/𝑉𝑇 = 𝑃𝑎𝐶𝑂2 − 𝑃𝐸𝐶𝑂2/𝑃𝑎𝐶𝑂2

What is PaCO2?

How about PeCO2?

PaCO₂ = Arterial CO₂ tension.

PECO₂ = Mixed expired CO₂ tension.