The Breath

Definition of a Breath

  • Breath: Defined as one cycle of inspiratory time followed by expiratory time.

    • Inspiratory Time (Ti): Duration from the beginning of inspiratory flow to the beginning of expiratory flow.

    • Expiratory Time (Te): Duration from the beginning of expiratory flow to the beginning of the next inspiratory flow.

Breath Visualization During Mechanical Ventilation (MV)

  • Types of Breath Waveforms (waveform shapes):

    • Rectangular Waveform: Also known as square wave or constant waveform.

    • Descending Ramp: Known as a decelerating ramp.

    • Ascending Ramp: Known as an accelerating ramp.

    • Sinusoidal Waveform: Often referred to as the sine wave.

Phase Variables of the Breath

  • Trigger: The variable that initiates inspiratory flow (starts inhalation).

  • Control: The variable that regulates the size of the breath (tidal volume).

  • Cycle: The variable that transitions the breath into exhalation (ends inhalation).

  • Baseline: The variable applied during the expiratory phase (PEEP).

Total Cycle Time (TCT)

  • TCT: The total time in seconds per breath cycle, given by the formula:

    • TCT=Ti+TeTCT = Ti + Te

    • To calculate frequency of breaths per minute: 60 seconds/breath / frequency (breaths/min) = total cycle time (seconds)

    • Example Calculation:

    • If the rate is 20 breaths/min: 60 / 20 = 3 seconds

      • Each breath cycle takes 3 seconds to complete, which includes both inhalation and exhalation.

      • A breath is delivered every 3 seconds.

Calculating Inspiratory (Ti) and Expiratory (Te) Time

  • Example:

    • Tidal Volume (VT): 500 mL (or 0.5 L)

    • Flow Rate: 60 L/min (or 1 L/sec)

    • Calculation of Ti

      • Tidal Volume (VT) / Flow Rate = Ti

      • 0.5 L / 1 L/sec = 0.5 second inspiratory time (Ti)

  • Calculation of Expiratory Time (Te):

    • If the frequency is 10 breaths/min and Ti is 1.0 seconds:

    • First Calculate TCT

      • 60/10 = 6 seconds

      • TCT - Ti = Te

        • 6 - 1 = Te 5 seconds

Relationship Between Volume & Flow

  • Flow Equation:

    • Flow = Volume (L) / Time (sec)

    • Example Calculation:

      • VT = 0.5L and Ti 1 sec:

      • 0.5 / 1 = 0.5 L/sec

      • Multiply by 60 to convert to L/min

      • 0.5 × 60 = 30 L/min

  • Flow and Ti Relationship:

    • Increasing flow decreases Ti and vice versa.

Normal Adult Values

  • Tidal Volume (VT): 5 - 7 mL/kg Ideal Body Weight (IBW).

  • Minute Ventilation (VE): Normal range is 5 - 6 L/min (up to 10 L/min).

  • Breath Frequency (f): Normal rate 12 - 18 breaths/min; > 35 breaths/min indicates respiratory distress.

Work of Breathing (WOB)

  • WOB is the effort required to generate a breath and maintain airway patency.

  • Factors involved in WOB include:

    • Pressure gradients for air movement and alveolar distention.

    • The relationship between elastic load (lung compliance) and resistive load (airway resistance).

  • Normal WOB: Typically < 1 joule/L, ideal range is 0.3 – 0.7 joules/L.

Equation of Motion

  • Components of Equation:

    • The ventilator and the patient act as "workers."

    • Elastic Load: Energy needed to overcome the elastic properties of the lung.

    • Resistive Load: Energy needed to overcome airway resistance.

  • Unbalanced workload implies excessive effort necessary for the patient, indicating abnormal compliance or increased resistance.

Airway Resistance (Raw)

  • Definition: The measurement of airflow resistance through the airways which is greatly influenced by airway diameter.

  • Effects of Airway Diameter:

    • Decreasing diameter increases Raw, which increases workload.

    • Conversely, increasing diameter decreases Raw and workload.

Factors Increasing Airway Resistance

  • Bronchoconstriction

  • Secretions

  • Airway Edema (inflammation)

  • Foreign Bodies

  • Artificial Airway

  • Obstructive Lung Diseases: COPD, Asthma, Cystic Fibrosis, and Bronchiectasis are linked to increased Raw.

Measuring Airway Resistance

  • Peak Inspiratory Pressure (PIP): The highest pressure achieved during inspiration.

  • Plateau Pressure (Pplat): Reflects alveolar pressure during inspiratory hold (static state, pressure measured during a period of zero airflow).

  • To ensure safety:

    • Ideal PIP < 35 cmH2O

    • Plateau 30 cmH2O


Positive End-Expiratory Pressure (PEEP)

  • Purpose: To prevent alveolar collapse by applying baseline pressure during exhalation (mimicking transpulmonary pressure gradient).

  • Types of PEEP:

    • Extrinsic PEEP: Set on the ventilator.

    • Intrinsic PEEP: Auto-PEEP caused by air trapping.

Calculating Compliance

  • Measure of Compliance: Change in volume per change in pressure. The equation used revolves around the exhaled tidal volume (Vte).

  • Static Compliance: Measured during no airflow conditions, indicating the force required to maintain lung inflation when flow is zero.

  • Dynamic Compliance: Measured during airflow, thus including airway resistance adjustments.

  • Trends in compliance must be observed to recognize improvements or the need for interventions.

    • Normal compliance ~ 60 – 100 mL/cmH2O; < 30 mL/cmH2O is critically low

Factors Decreasing Compliance

  • Restrictive Lung Conditions:

    • Atelectasis

    • Pneumonia

    • Pulmonary Edema

    • ARDS

    • Pulmonary Fibrosis

  • Thoracic Compliance Issues: Deformities and abdominal problems (obesity, ascites).

  • Importance of Relationships:

    • Reduced compliance results in increased WOB as the system requires higher pressure gradients to keep alveoli distended

    • decreased compliance = decreased volumes

PIP & Plateau

  • increased resistance = increased PIP, Plateau stays the same

  • decreased compliance increased PIP and Plateau