Continuous Positive Airway Pressure Therapy By Ahmad Younis Professor of Thoracic Medicine Mansoura Faculty of Medicine
AMBIENT AIR PRESSURE • It is the pressure around us wherever we are, at sea level or the top of a high mountain. • The weight of earth's atmosphere creates 'air pressure', which we don't feel because it's evenly distributed throughout our lungs. • This pressure can be expressed in several different units of measurement based on the following conversions: 1 mmHg = 1.36 cmH2O = 0.133 k Pa (kilo-pascal) = 1.33 hpa (hecto-pascal)
In this figure , blue is mercury, and the distance from 'B' to 'A' is the barometric or air pressure. If the open pan of mercury is at sea level then the height of the column (from 'A' to 'B') is 760 mm Hg and the sea level air pressure is said to be '760 mm Hg'.
The ambient air pressure decreases with altitude, simply because as you go higher, there is less quantity of air to weigh on the surface. This is shown in this figure , where PB is the barometric pressure at a given altitude. (PO2 is the partial pressure of oxygen at that altitude )
Mechanisms of breathing • We breathe by contracting respiratory muscles (mainly the diaphragms) to expand the thorax and thereby create a slightly negative airway pressure relative to ambient pressure. • This slightly negative pressure -- about -3 cm H2O at rest -- allows fresh air to enter our lungs and supply the blood with oxygen. • Then we relax the respiratory muscles, and in so doing exhale to create a slightly positive pressure relative to ambient (+3 cm H2O); this allows stale air full of carbon dioxide to leave our lungs and enter the atmosphere.
To simplify the numbers for these pressure changes we always reference ambient pressure to zero. This has two great advantages: 1-We don't have to use large numbers to show the change in airway pressures during breathing; 2-Though the ambient pressure changes with altitude (lower the higher up you go), zero as the reference point can be used at any pressure. In other words, since the ambient pressure is distributed evenly thoughout out lungs, zero can be the reference point for any altitude.
Non Invasive Positive Pressure Ventilation • Noninvasive ventilation (NIV) refers to the provision of mechanical ventilation (MV) through the patient’s upper airway by means of a mask without the use of an invasive artificial airway (endotracheal tube or tracheostomy) . • NIV has long been used as the standard method to treat patients with chronic respiratory failure (CRF) related to chest wall diseases, neuromuscular disorders, or central hypoventilation . • It has been shown to be effective in treatment of different forms of acute respiratory failure (ARF)
Non Invasive Positive Pressure Ventilation for SDB • Studies have shown CPAP to increase upper airway size, especially in the lateral dimension. • Positive intraluminal pressure expands the upper airway (pneumatic splint) and increase in lung volume due to CPAP (due to a downward pull on upper airway structures during lung expansion “tracheal tug”), may also increase upper airway size and/or stiffen the upper airway walls, making them less collapsible
Positive airway pressure (PAP) can • 1-Bring the AHI down to below 5 to 10/hr in the majority of patients. • 2-Improves arterial oxygen saturation and decreases respiratory arousals. • 3-Increase the amount of stage N3 and stage R. • NB : • 1-An occasional patient with very severe apnea will have a large REM or stage N3 sleep rebound on the first night of PAP treatment. • 2-The most difficult problem with PAP treatment is that adherence is suboptimal in a large percentage of patients.
Mechanism of upper airway occlusion in obstructive sleep apnea and its prevention by continuous positive airway pressure: “pneumatic splint” effect.
Change in the upper airway of a normal individual after application of CPAP of 0 cm H2O (A) and CPAP of 15 cm H2O (B). The airway increases in size mainly in the lateral dimension.
CPAP • CPAP was developed in 1981 by Professor Colin Sullivan (Royal Prince Alfred Hospital in Sydney, Australia) for treating patients with severe sleep apnea. • Within a few years, CPAP was commercially available in the U.S., and replaced tracheostomy as treatment of choice for severe OSA. • Without doubt ,CPAP was the catalyst for widespread development of sleep labs to diagnose OSA and for the evolution of sleep medicine as a recognized medical specialty.
CPAP • With CPAP the patient is exposed to an airway pressure above the ambient or room air pressure, which is always referenced to zero. • A CPAP of 5 cm H2O means the patient is continually breathing against an airway pressure 5 cm H2O above the ambient or 'zero' pressure. • The pressure curve looks the same as if breathing at ambient pressure, so that inspiratory and expiratory pressure are still below and above the baseline, respectively. • Because the pressure is set at some specific level above ambient (usually in the range of 5 to 15 cm H2O) CPAP can be thought of as 'uni-level' positive airway pressure (though it is never called that), to distinguish it from bilevel positive airway pressure or BiPAP.
Top: Normal pressure curve (pressure measured at the mouth level) breathing at ambient ("0") pressure; airway pressure is @ -3 cm H2O at peak of inspiration (I) and @ +3 cm H2O at peak of expiration (E).Bottom: Pressure curve when CPAP = 5 cm H2O; the baseline pressure against which the patient breathes is raised 5 cm H2O above ambient.
CPAP as the name implies, requires the airway pressure to be constant between inspiration and expiration. • Such a pressure is achieved by a servo-controlled air compressor that maintains the airway pressure as closely to the prescribed pressure despite the pull (inspiration) and push (exhalation) of the patient. • The maintenance of such pressure within an FDA-specified pressure range (for example, ± 1.5 cm H2O of the set pressure) is necessary as a quality-assurance measure that would ensure that the device maintains a certain prescription pressure for the patient. • Such a pre-specified error range is generally greater with larger tidal volume (VT) or inspiratory effort from patient, faster respiratory rate, and at higher prescription pressure settings, because the device would need to be more rapidly responsive to the perturbations in the airway pressure at such extremes to maintain the pressure at the prescribed level.
Representative tracings of flow, tidal volume, and airway pressure (Paw) during administration of continuous positive airway pressure (CPAP) and bi-level PAP
Physiological effects of positive airway pressure (PAP) therapy. PAP therapy splints the upper airway (black crosses and arrows), achieves positive intra-thoracic pressure (white crosses), decreases venous return, increases lung volume, decreases after- load, and can increase cardiac output. The bidirectional vertical arrows signify the traction on the upper airways affected by the increase in end-expiratory lung volume. Such a traction effect can assist in the splinting open of the upper airway.
CPAP treatment • Non-acute setting: Treatment of obstructive sleep apnea. • Acute setting: Pulmonary edema or COPD exacerbation, when there is hypoxemia but not CO2 retention. Note: CPAP by face mask = PEEP in the intubated patient.
BiPAP treatment • Non-acute setting 1) When CPAP doesn't work for OSA (need high pressure or not tolerant due to high expiratory pressure). 2) For patients with chronic CO2 retention who also have OSA. 3) For patients with neuromuscular disease who need some assistance with nocturnal ventilation. • Acute setting: Pulmonary edema or COPD exacerbation, when there is CO2 retention and a desire to avoid indotracheal intubation. Note: BiPAP by face mask = PSV + PEEP in the intubated patient.
How is the pressure applied non-invasively? • Via a tight fitting mask attached in such a way that air can be blown into the nose or the nose and mouth. • The mask connects to a hose that is attached to a CPAP machine . • The mask choices are the same whether the patient is using CPAP or BiPAP. • Generally there are 3 types: nasal mask, nasal pillows, and full face mask.
The nasal mask (left) and nasal pillows (middle) and full face mask (left)
A: Mouth piece devices B: Mouthpiece with lip-seal. C: Patient using an angled mouthpiece D: Patient using mouthpiece with lip-seal .
Interfaces • Nasal pillow masks are often better tolerated than traditional nasal masks by patients with claustrophobia and are useful in patients with a mustache or edentulous patients who have no dental support for the upper lip. • For patients who have severe nasal congestion or open their mouths during PAP treatment, oronasal (full face masks) and oral interfaces are available • If the patient gets up to use the bathroom during the night, we encourage disconnection of the hose from mask rather than taking off the mask. Masks that are removed in the middle of the night are often not replaced.
Measures for full face mask ( height from under lower lip to bridge of the nose and width of mouth)
A: Mask with inflatable cushion. B: Mask with foam-filled cushion. C: Mask with inner lip that fits to the face when pressure is applied to the mask. D: Mask with a gel-filled cushion.
Forehead spacer designs to decrease the risk of facial skin breakdown. Left : Gel spacer. Center: Foam spacer. Right: Adjustable forehead arm.
Facial skin breakdown secondary to mask used for noninvasive positive-pressure ventilation.
Rebreathing • The interface can affect the degree of rebreathing during NPPV if the ventilator circuit has a leak port for exhalation. • In a lung-model study, a lower volume of rebreathed CO2 with the exhalation port in the mask is found as compared to the exhalation port in the circuit. also an oronasal mask with the exhalation port in the mask decreased the total dynamic dead space, compared to having the leak port in the circuit. • With a nasal mask, the patient can exhale through the mouth, which should decrease rebreathing.
Confusing Points Clarified • CPAP does not, technically, provide 'ventilation' to the patient. • It sets a single higher ambient pressure against which the patient breathes, but does not augment alveolar ventilation. • If your goal is to improve someone's PaCO2 non-invasively (i.e, to treat hypercapnia), CPAP is not the method of choice; instead, BiPAP is recommended.
Another clarification about CPAP is that it is a generic term, not any manufacturer's trademark, like BiPAP and ASV. • It is offered on machines from multiple companies, all of whom may use the term 'CPAP'. • Manufacturers may embellish their CPAP with little twists which are patented, and seldom adequately explained. An example is Respironics' CFlex and CFlex+. They are 'pressure relief' modes that abruptly drop the pressure in the transition from inspiration to expiration, to a sharper degree than would occur with passive exhalation. • CFlex comes in 3 levels, 1, 2 and 3, representing roughly 1, 2 or 3 cm H2O drop in pressure. CFlex+ is supposed to be an advance over regular CFlex.
Flexible Pressure Two manufacturers of PAP devices have developed flexible PAP 1- Philips-Respironics provide several comfort options (Cflex, Cflex+, and Aflex) 2- ResMed devices offer expiratory pressure relief (EPR). In Cflex, expiratory pressure drops at the start of exhalation but returns to the set CPAP at end-exhalation. The amount of drop (Cflex 1, 2, 3) is determined by a proprietary algorithm.
Cflex+adds a smoothing of the transition from inhalation to exhalation. • Aflexis a form of APAP that provides a 2 cm H2O lower end-expiratory pressure than the inspiratory pressure (in addition to the features of Cflex* A form of expiratory pressure relief is available For both BPAP and autoBPAP devices, (Biflex). The technology provides a smoothing of transition from IPAP to EPAP as well as expiratory pressure relief during the EPAP cycle (Biflex 1, 2, 3).
C-Flex+ is new enhancement to comfort relief for advanced CPAP units (REMstar Pro and Auto) when in fixed CPAP mode. Like C-Flex, C-Flex+ provides flow-based pressure relief at the beginning of exhalation. Like A-Flex, C-Flex+ softens the pressure transition from inhalation to exhalation to provide additional comfort in fixed-CPAP mode.
Ramp • Most PAP devices, with the exception of certain APAP devices, allow the patient to trigger the ramp option. • In the ramp option, the pressure starts at a preset level—usually a low level of CPAP—and then slowly increases to the treatment pressure (CPAP) over the set ramp time • Some APAP devices have a “settling time” at a low pressure before the device starts auto-adjusting pressure
Humidification • Most PAP devices come with the option of an integrated heated humidification system. • They can be used in the cool humidity mode if desired. • Heated humidity can deliver a greater level of moisture than cool humidification and may be especially useful in patients with mouth leak or nasal congestion. • Mouth leak can cause a dramatic fall in relative humidity and a loss of humidity from the upper airway/CPAP system, thus drying the nasal or oral mucosa. • Use of heated humidification is recommended to improve CPAP utilization. In the clinical guidelines for titration, having HH available for titration was recommended
OXYGEN AND YOUR PAP UNIT • Your tubing is connected to the large end on the tee adapter and the small tubing from your oxygen system is connected to the small nipple on the tee adapter. • Always turn your CPAP or bi-level unit ON before turning ON the oxygen flow. • Always turn OFF the oxygen before turning OFF the CPAP or bi-level unit