Respirology Primer 2006 University of Alberta Chapter One Introduction to the Pulmonary Reading Material Introduction to the Pulmonary Reading The reading material on this CD consists of a series of Of course, few algorithms can take into account all possibilities and chapters specifically written to help you manage common there will certainly be times when the algorithms cannot or should problems seen in respirology while on your rotation. Note not be followed rigorously. However, for the majority of cases, they some important points: will likely apply to a large degree. As well, the handouts often take into account local practice (i.e. Edmonton, specifically the University of Alberta Hospital), which is usually influenced by local resources, The Primer is not meant to substitute for textbook reading. It disease epidemiology, and local expertise. Thus, in areas where is assumed that, at the postgraduate level, you will already these factors may differ, what we do locally may not strictly be the know the basic facts of common respiratory diseases and that pattern of practice followed elsewhere (although it will likely be you have access to reference material to supplement your quite similar). learning. The purpose of the Primer is to provide you with a practical guide to help you manage the clinical pulmonary issues that you may encounter. Most of the topics intentionally deal with diagnosis rather than treatment, since the latter tends to be more individualized and can evolve relatively rapidly. Many of the sections utilize an algorithmic approach to problems. Unless otherwise specified, the algorithms have not necessarily been prospectively evaluated. However, they Feel free to direct feedback or suggestions for new topics in the represent a reasonable approach to problems seen by reading material to me. (email: [email protected] or practicing Respirologists and are based on clinical experience, 2E4.34 WMC) supplemented by the literature whenever possible. Chapter Two Basics on Non-Invasive Ventilation THE BASICS ON NON-INVASIVE POSI- TIVE PRESSURE VENTILATION by Dr. L. Cheung Figure 1: CPAP applies a continuous set pressure on 1. Introduction inspiration and expiration Case 1: A 68 year old male presents to the ER with an acute COPD exacerbation and has an ABG on 5 lpm which shows a PO2 of 68, PCO2 76, pH 7.26, HCO3 33. He is inspiration CPAP tachypneic and in moderate respiratory distress but is reasonably alert and awake. Is he a good candidate for continuous set non-invasive mechanical ventilation (NIMV)? presure Case 2: A 45 year old male presents with a 5 day history of increasing dyspnea due to community acquired pneumonia and is now in the ER requiring 15 lpm of oxygen to maintain his Oxygen saturation at just over expiration 89%. He is tachypneic and tachycardiac, and has CPAP increased work of breathing. Is he a good candidate for NIMV? continuous set Case 3: presure A 65 year old female is on the surgical ward post-op day 2 for a right hemicolectomy for colon cancer. She is now hypoxemic due to bilateral lower lobe atelectasis, requiring about 15 lpm of oxygen to maintain oxygen saturations of over 90%. However, she seems to be in Although one might think that this applied pressure would minimal respiratory distress, is speaking full sentences, help inspiratory effort, the effect is actually quite and is hemodynamically stable. Is she a good candidate negligible at the low levels previously mentioned (5 to 10 for NIMV? cm H2O). Thus, the main effect during inspiration is to Case 4: help “stent” the upper airway open in people with sleep A 70 year old male with end stage pulmonary fibrosis is apnea, but it provides little help for the inspiratory admitted due to severe hypoxemia requiring flush (> 15 muscles. lpm) oxygen by mask. He and his family know that he is going to die soon from his underlying disease. He has During expiration, however, the applied pressure acts as a increased work of breathing. Is he a good candidate for “back pressure”, and helps open collapsed alveoli by “palliative” NIMV? increasing functional residual capacity (FRC), the volume of gas remaining in the lungs after a normal tidal (The answers to these cases are described at the end of exhalation. This may improve oxygenation through this chapter) improved V/Q matching. The alveoli are the most vulnerable to collapse at end-exhalation since this is when This chapter will enable you to answer the following the lung volume is at it’s lowest. clinically important questions regarding NIMV 1. What is the difference between CPAP and BiPAP? Note, however, that if the pressure is set too high, it 2. What are the indications and contraindications for becomes uncomfortable during expiration – the patient each? feels like he can’t exhale – and it may actually impair 3. What are the typical settings that should be ordered? exhalation to the point where the patient is using energy trying to actively exhale against the high back pressure. 2. What is the Difference Between Thus, based on the above discussion, the indications for CPAP and BIPAP? CPAP would include either sleep apnea or hypoxemia due to atelectasis and low lung volumes resulting in very high A. CPAP O2 requirements in a patient who is otherwise stable. CPAP provides continuous positive airway pressure at low Although some would start CPAP for hypoxemia from levels, usually starting at 5 to 10 cm H2O. Because the other causes (eg. pneumonia with severe hypoxemia), pressure is applied continuously throughout the there are conflicting studies as to whether this improves ventilatory cycle, it occurs at a constant level throughout inspiration and expiration, as illustrated in Figure 1. THE BASICS ON NON-INVASIVE POSI- TIVE PRESSURE VENTILATION outcome. Part of the problem is that these patients can EPAP, which is lower than the inspiratory pressure. This is deteriorate quickly – if CPAP is applied, it should be done illustrated in Figure 2. in a monitored setting. Typical initial settings would be an EPAP of 4 cm H2O and Although CPAP starting pressures are usually 5 to 10 cm a starting IPAP of 8 cm H2O for an individual with an H20, higher pressures of up to 15 cm H20 may be required average sized body habitus and chest wall compliance. if a patient is obese (with low chest wall compliance, These settings are usually set low at first to get the higher back pressures are needed to open up the alveoli). patient used to the pressure and then titrated upwards as necessary. If a patient requires assistance with inspiration (eg. due to increased work of breathing, respiratory muscle fatigue, The IPAP is titrated by 2 cm H20 upward as necessary to a and hypercapnea from COPD), we need to increase the maximum of 20 cm H2O. Typical final settings for IPAP applied inspiratory pressure. But, as mentioned, if the would be about 12 to 18 cm H2O. same continuous positive airway pressure is simply increased, it may help inspiration but actually be Since the goal of the IPAP is to help inspiration, it is intolerable during expiration. For these instances, we titrated to improve the patient’s work of breathing and need to have bi-level positive airway pressure or BiPAP – sense of dyspnea, as well as to improve the PCO2 (by a higher pressure during the inspiratory phase to augment improving the tidal volumes during inspiration). inhalation and assist fatigued or weak inspiratory muscles, but a lower pressure during the expiratory Since the goal of the EPAP is to prevent alveolar collapse phase. at end-expiration (thus improving V / Q matching), it is titrated to improve the oxygenation. Final EPAP settings B. BIPAP are usually 4 to 8 cm H2O. BIPAP consists of an inspiratory positive airway pressure or IPAP and an expiratory positive airway pressure or Monitoring the effectiveness of BIPAP consists of clinical assessment and ABG’s, typically done just before the onset of BIPAP and then 1 hr, 6 hrs, and 24 hrs post- intiation of BIPAP (or more if necessary). Figure 2: BiPAP applies an inspiratory positive airway pressure to help fatigued or weak inspiratory muscles In general, indications for BiPAP would include during inspiration. However, BiPAP applies a lower expi- hypercapneic respiratory failure due to a number of ratory positive airway pressure during expiration - reasons including an acute exacerbation of COPD, enough to help keep the alveoli open but not so much as asthma, or cystic fibrosis, or acute, potentially reversible to impair exhalation. neuromuscular weakness in a patient who is still able to protect his airway. In general, hypoxemic respiratory failure is not really an indication for BiPAP. inspiration IPAP 3. Contraindications to Mask IPAP > EPAP Ventilation There are certainly situations where it is not safe to administer NIMV. These contraindications are listed below: • Cardiac or respiratory arrest • Severe encephalopathy - (eg. GCS < 10) Note, expiration EPAP however, that decreased LOC due to hypercapnea from acute COPD exacerbation might improve with NIMV. Thus, whether NIMV should be applied in this IPAP > EPAP setting should be decided on a case by case basis. As well, it is important to be reasonably sure that the hypercapnea is causing the decreased LOC and not THE BASICS ON NON-INVASIVE POSI- TIVE PRESSURE VENTILATION the other way around (ie. that decreased LOC is causing hypoventilation which, in turn, is causing 4.Answers to the Introductory hypercapnea - NIMV will not reverse the cause of the decreased LOC itself in these circumstances). Cases Case 1: • inability to protect the airway / high risk for aspiration This patient presents with hypercapneic respiratory failure • severe hemodynamic instability or unstable cardiac due to an acute COPD exacerbation and meets rhythm physiologic and clinical criteria for NIMV (hypercapnea, • active GI bleeding with hematemesis (vomiting blood respiratory acidosis, increased work of breathing, no into the mask would be bad) obvious contraindications).