Question

Write a 200-300 word entry describing in your own words (showing you understand the physiology) of how positive pressure therapy can affect one of the factors below:

Positive pressure effects on intrathoracic pressures

Pressure and distribution of airflow into the alveoli

Pressure, stretch, and the lung

Surfactant functions

Positive pressure and cardiac output

Pulmonary capillary blood flow

Positive pressure and the lymphatics

Positive pressure and organ system function

Answer 1

Non‐invasive ventilation refers to a number of respiratory support strategies commonly used in critical care settings.
NIV now, usually, refers to one of two forms of ventilation: continuous positive airway pressure (CPAP) and bi‐level
positive airway pressure (BiPAP).CPAP involves the application of a single level of positive airway pressure throughout
the respiratory cycle. Five centimetre H2O is considered a low level of CPAP and 15 cm H20 a high level in common practice.
CPAP circuits can be very simple: a source of gas under pressure applied to a face mask that makes an airtight seal around
the face with a valve that allows the air or oxygen to escape into the surrounds at the chosen level of pressure. Although
there is some benefit during the inspiratory phase of respiration, leading to an increase in tidal volume and improved ventilation,
the main benefit of CPAP is during expiration due to positive end expiratory pressure (PEEP). In some settings, the terms CPAP and
PEEP are used interchangeably, although strictly speaking CPAP is a ventilatory mode and PEEP is a physiological parameter.
There are two forms of PEEP: extrinsic (ePEEP) and intrinsic (iPEEP). ePEEP is applied to the airway externally, such as in
these forms of ventilation. iPEEP refers to the elevated alveolar and airway pressure that remains at the end of expiration due
to airway obstruction and incomplete expiration as occurs in asthma.

CPAP delivers a single level of positive pressure. In BiPAP, the patient's inspiratory effort is sensed and a higher inspiratory
level of pressure is delivered. BiPAP, bi‐level positive airway pressure; CPAP, continuous positive airway pressure; EPAP,
expiratory positive airway pressure; IPAP, inspiratory positive airway pressure; PEEP, positive end expiratory pressure; PS, pressure support.

BiPAP, also known as non‐invasive positive pressure ventilation (NPPV), involves the application of one level of pressure during expiration,
and another, higher, level of pressure during inspiration (Fig. 1). This is achieved by a microprocessor sensing a drop in the circuit's
pressure when the patient initiates a breath and switching to the inspiratory pressure. The expiratory pressure is variously referred to as CPAP,
PEEP or expiratory positive airway pressure (EPAP) – the last is the most accurate term but, probably, least commonly used. Confusion can arise
when describing the level of pressure during inspiration. Convention in traditional, invasive ventilation, is to refer to the pressure support (PS)
: the difference between the inspiratory and expiratory pressures. Sometimes the term IPAP – inspiratory positive airway pressure – is used.
IPAP refers to the absolute level of inspiratory pressure, which is the same as the level of EPAP plus the level of PS.

Both CPAP and BiPAP refer to modes of ventilation that rely on the patient breathing spontaneously. Thus, they can be described as
patient‐triggered, pressure‐controlled modes of ventilation.

Physiological effects-->>

IPAP or PS – the positive pressure applied during inspiration – decreases the inspiratory work of breathing and improves
tidal volumes.This, in turn, improves minute ventilation and the clearance of CO2 from the lungs.
PEEP, CPAP or EPAP – the positive pressure applied during expiration – has a range of beneficial effects. The major effect
is through either recruiting (opening) closed alveoli or preventing alveoli closing, thus making lung units available for gas
exchange.Gas exchange, especially the uptake of oxygen, is improved not only because there are more lung units available to do this,
but also because higher partial pressure of oxygen in the alveoli further favours diffusion into the blood: the A‐a gradient is improved.

Another beneficial effect of PEEP or, more correctly, ePEEP is to counter the effects of obstruction and reduce iPEEP.
In obstructive airways diseases, obstruction leads to incomplete expiration. Over many respiratory cycles, this leads to an increase in
alveolar pressure (iPEEP) and volume (referred to as dynamic hyperinflation). In an attempt to overcome this, the patient may start to
actively expire: instead of passive expiration due to chest wall elasticity, muscular effort is added to breathe out. This has the effect
of further increasing intrathoracic/intrapleural pressure. The higher pressures outside the small, collapsible airways further exacerbate
the airway collapse and obstruction, setting up a vicious cycle. Although it may seem counterintuitive to apply positive pressure during
expiration to someone who is having problems expiring, the principle of using ePEEP in this setting is to ‘splint the airways open’.
Ideally, ePEEP would be set at approximately 75% of iPEEP. This maintains a pressure gradient that allows flow out from the alveoli but,
hopefully, means that the pressure inside the collapsible airways is always greater than outside, preventing collapse.

Positive pressure applied in the thorax has many effects, both positive and negative, on the central circulation. Raised intrathoracic
pressure impairs venous return into the chest (to the right heart) but might be anticipated to aid blood flow out of the chest (i.e. left heart).
Left ventricular preload, transmural pressure and relative afterload are all decreased: these effects all improve stroke volume
(and, thus, cardiac output) without increasing myocardial oxygen consumption. This effect is most apparent in poorly functioning ventricles.
In normal ventricles, particularly in slightly hypovolaemic patients, the more predominant effect of raised intrathoracic pressure may be to
decrease venous return and, in turn, cardiac output.