Respiratory Therapy 50C - Chapter 47: Physiology of Ventilatory Support

Flashcards covering the physiology of ventilatory support, including pressure gradients, mechanical ventilation modes, lung mechanics, and clinical complications based on Chapter 47.

Pawo

Pressure at the airway opening, also referred to as mouth pressure.

Palv

Alveolar pressure.

Ppl

Intrapleural pressure; it is slightly negative at the end of exhalation during spontaneous breathing.

Pbs

Atmospheric pressure or pressure at the body surface.

Transairway pressure ($P_{TA}$)

The pressure gradient between the airway opening and the alveoli, calculated as $P_{awo} - P_{alv}$.

Transthoracic pressure ($P_{TT}$ or $P_W$)

The pressure gradient between the alveoli and the body surface, calculated as $P_{alv} - P_{bs}$.

Transpulmonary pressure ($P_L$ or $P_{tp}$)

The pressure gradient between the airway opening and the pleural space, calculated as $P_{awo} - P_{pl}$.

Transrespiratory pressure ($P_{TR}$)

The pressure gradient between the airway opening and the body surface, calculated as $P_{awo} - P_{bs}$.

Transalveolar pressure ($P_L$)

The pressure gradient between the alveoli and the pleural space, calculated as $P_{alv} - P_{pl}$.

Trans-chest wall pressure ($P_{TCW}$)

The pressure gradient between the pleural space and the body surface, calculated as $P_{pl} - P_{bs}$.

Minute ventilation ($\dot{V}_E$)

The total volume of gas exhaled per minute, calculated as $V_T \times f$.

Alveolar ventilation ($\dot{V}_A$)

The volume of gas participating in gas exchange, calculated as $(\dot{V}_{CO2} \times 0.863) / PaCO_2$.

$\dot{V}/\dot{Q}$ ratio in PPV

Positive Pressure Ventilation increases ventilation to nondependent (upper) lung regions while blood flow is greatest in dependent regions, often worsening the ratio and increasing $P(A - a)O_2$.

Dead space ratio ($V_D/V_T$)

The portion of ventilation that does not participate in gas exchange; increases can be caused by excessive PEEP, pulmonary embolus, or emphysema.

Alveolar Air Equation

$PAO_2 = ((P_B - PH_2O) \times FiO_2) - (PaCO_2/R)$, used to predict how $PaO_2$ responds to changes in $FiO_2$.

Oxygen Content ($CaO_2$)

The total amount of oxygen carried in the arterial blood; it decreases if Hemoglobin, arterial saturation, or $PaO_2$ decreases.

Tissue oxygen delivery ($DO_2$)

The total amount of oxygen delivered to the tissues per minute; it depends on both arterial oxygen content and Cardiac Output.

Time Constant ($TC$)

Determined by lung compliance ($0.1\,L/cm\,H_2O$) and airway resistance ($2.5\,cm\,H_2O/L/s$); one constant for a normal patient is $0.25\,seconds$.

Plateau pressure ($P_{plat}$)

The pressure measured during an inflation hold indicating alveolar pressure; it should be kept less than $28\,cm\,H_2O$ to minimize lung injury.

Airway Resistance ($Raw$)

Calculated as $(PIP - P_{plat}) / flow$ when using a square flow waveform.

Mean Airway Pressure (MAP)

The average pressure across the respiratory cycle; it has a direct relationship with and is most closely tied to oxygenation.

Lower Inflection Point ($P_{flex}$)

The point on a Pressure-Volume curve where PEEP should be set approximately $2\,cm\,H_2O$ above to ensure lung recruitment.

VC-CMV

Volume-controlled continuous mandatory ventilation; volume is guaranteed while airway pressure varies, indicated when precise minute ventilation is desired.

PC-CMV

Pressure-controlled continuous mandatory ventilation; pressure is guaranteed while tidal volume varies, resulting in more even gas distribution if resistance is an issue.

PC-IRV

Pressure-controlled inverse ratio ventilation; pressure-controlled ventilation with an I:E ratio greater than $1:1$.

IMV

Intermittent mandatory ventilation; a partial support mode that allows spontaneous breathing between mandatory breaths delivered at a set rate.

APRV

Airway pressure release ventilation; a mode where the patient breathes spontaneously at a high PEEP level with brief periods of release to a low PEEP level.

PSV

Pressure support ventilation; a patient-triggered, pressure-limited, flow-cycled mode used to overcome the work of breathing caused by the endotracheal tube.

PAV

Proportional assist ventilation; the ventilator performs a fixed proportion of work based on patient effort, elastance, and resistance.

NAVA

Neurally adjusted ventilatory assist; a mode that responds to changes in diaphragmatic EMG activity to improve synchrony.

ATC

Automatic tube compensation; calculates and adjusts pressure to eliminate the work of breathing imposed by the artificial airway, sometimes called 'electronic extubation'.

PRVC

Pressure-regulated volume control; a mode that adjustments pressure level based on the previous breath to maintain a target tidal volume.

ASV

Adaptive Support Ventilation; a pressure-targeted mode that minimizes work of breathing and auto-PEEP by optimizing tidal volume and frequency.

Cerebral Perfusion Pressure (CPP)

The pressure gradient driving blood flow to the brain; it decreases if MAP decreases or ICP increases.

Modified Ramsay Sedation Scale

A scale used to monitor the level of sedation in patients, ranging from 1 (Agitated) to 5 (Unable to be assessed/paralyzed); a level of 2-3 is target for MV.

VALI

Ventilator-associated lung injury; pulmonary complications related to high pressure.

VILI

Ventilator-induced lung injury; lung damage caused by overdistention (volutrauma), repeated collapse/reexpansion (atelectrauma), or inflammatory mediators (biotrauma).

Auto-PEEP

Air trapping that occurs if expiratory time is too short, increasing work of breathing and making it difficult for the patient to trigger the ventilator.

VAP

Ventilator-associated pneumonia; a nosocomial infection prevented by keeping the head of the bed elevated $30\text{ to }45\,degrees$ and performing oral hygiene.