The Mechanical Ventilator and Lung Mechanics

Flashcards covering the mechanical functions of ventilators, various modes of ventilation, pulmonary mechanics, and clinical indicators for respiratory failure based on lecture notes.

Negative pressure ventilation (NPV)

A type of ventilation that attempts to mimic the function of respiratory muscles to allow breathing through normal physiological mechanisms; examples include the Iron lung and Chest cuirass.

Positive pressure ventilation (PPV)

Mechanical ventilation used to deliver air into the patient's lungs by way of an endotracheal tube or positive pressure mask.

High frequency jet ventilation (HFJV)

A type of ventilation with rates between $100$ to $400-600\,breaths/min$, often used in NICU for PPHTN.

High frequency oscillatory ventilation (HFOV)

A type of ventilation operating at $4000\,breaths/min$ used for sputum clearance.

Pneumatically powered vents

Ventilators that use one or two $50\,psi$ gas sources and internal reducing valves, utilizing pneumatic or fluidic mechanisms (like the Coanda effect) to control gas flow.

Coanda effect

Also known as the wall attachment phenomenon, it occurs when turbulent jet flow causes a drop in lateral pressure, creating a low-pressure vortex bubble that bends the jet toward a wall.

Open Loop control system

A system that is not microprocessor controlled where the operator sets a control and that value is delivered without responding to changing patient conditions.

Closed Loop Control system

An intelligent system that compares a set variable to a measured control variable and responds to changes in patient condition.

Double circuit vent

Commonly seen in the OR, this ventilator uses an electrical compressor to force pressure into a bellows chamber containing the gas mix for the patient.

Single circuit vent

Standard in most ICUs where gases follow one continuous path, often using a piston moving into a cylinder during inspiration.

Rotatory drive piston

A drive mechanism that produces a sine waveform where flow is slow at the start, high-speed mid-inspiration, and slow at the end of inspiration.

Patient Wye

The connection component between the inspiratory line, the patient, and the expiratory line.

HME (Heat and Moisture Exchanger)

A device located on the patient connector that provides approximately $70\%$ relative humidity but can pose potential resistance issues.

Pneumotachometer

A device used to measure flow past a known resistance based on Poiseuille's law.

Ventilation

The movement of air into and out of the lungs.

Respiration

The actual exchange of oxygen and carbon dioxide between an organism and its environment.

External respiration

The diffusion of oxygen and carbon dioxide between the alveoli and the pulmonary capillaries.

Internal respiration

The exchange of oxygen and carbon dioxide between the systemic capillaries and the cells of the body.

Type 1 Respiratory Failure

Hypoxic respiratory failure characterized by a shortage of $O_2$ at the tissue level, often associated with lung fibrosis.

Type 2 Respiratory Failure

Hypercapnic or ventilatory failure where the patient cannot blow off $CO_2$, defined as $PaCO_2 > 50\,mmHg$.

Total Cycle Time (TCT)

The duration of a full breath cycle, including inspiration and exhalation, calculated as $60\,s / RR = TCT$.

Auto/Intrinsic PEEP

Trapped gas and leftover pressure keeping the alveoli open, created naturally by the lungs when exhalation time is too short.

Transpulmonary pressure ($P_{TP}$)

The pressure across the entire lung surface used to keep alveoli open, calculated as alveolar pressure minus intrapleural pressure ($P_{pl}$).

Transairway pressure ($P_{ta}$)

The pressure gradient required to produce airflow in the conductive airways, calculated as airway pressure ($P_{aw}$) minus alveolar pressure.

Peak Inspiratory Pressure (PIP)

The highest pressure reached during inspiration, influenced by resistance and lung compliance; ideally kept below $30$ to $35\,cmH_2O$.

Plateau Pressure ($P_{plat}$)

The true pressure in the alveoli at the end of inspiration when airflow stops, measured using an inspiratory pause.

Driving Pressure

The stress applied to the entire respiratory system felt at the alveoli, calculated as $P_{plat} - PEEP$; the ideal range is below $15\,cmH_2O$.

Compliance

The relative ease with which a structure distends, calculated as the change in volume divided by the change in pressure ($C = V / P$).

Static compliance ($C_s$)

Measured during non-changing lung conditions (inspiratory hold), calculated as $C_s = V_t / (P_{plat} - EEP)$.

Elastance

The tendency of a structure to return to its original form after being stretched.

Time constants

The product of compliance and resistance, determining how long it takes to inflate or deflate the lung; usually $5$ time constants are required for full inflation/deflation.

Minute volume ($V_e$)

The total volume delivered in a minute, calculated as $V_e = V_t \times RR$.

Trigger

The phase variable that begins the inspiration, which can be time (machine triggered) or patient-initiated (pressure, flow, or volume).

Limit variable

The maximum value a variable can reach during inspiration (pressure, volume, or flow) that does not end the breath.

Cycle variable

The variable used by the ventilator to end inspiration and begin exhalation, such as volume, time, flow, or pressure.

Rise Time

In Pressure Support Ventilation, the adjustment of the slope or time required for the ventilator to reach the set pressure at the beginning of inspiration.

Proportional Assist Ventilation (PAV)

A mode where pressure, flow, and volume delivery are proportional to the patient's spontaneous effort, functioning as a positive feedback system.

Maximum inspiratory pressure (MIP)

Also called Negative Inspiratory Force (NIF), it is the most negative pressure generated during forceful inspiration; values below $-20\,cmH_2O$ indicate a critical need for intubation.