The clot CAUSES Dead space; which does not cause hypoxia/hypoxemia on its own.

ALWAYS ASK ABOUT: 

1. Resp Rate (PE creates dead space, reduced alveolar ventilation, CO2 increases and patient attempts to blow off CO2)
2. Blood pressure (?Obstructive shock from large saddle PE)
3. Tachycardia (Left heart trying to increase cardiac output because stroke volume low) 

1. Significant V/Q Mismatch (though usually dead space ventilation, but impaired pulmonary oxygen transfer can occur due to vascular re-distribution and release of vaso-active inflammatory molecules such as: serotonin, histamine, prostaglandins, bradykinin etc.)
2. Pain -> Splinting and atelectasis [caused by loss of surfactant and alveolar hemorrhage]
3. Inflammation causing infarction [however may be a matched defect] +/- superimposed pneumonia 
4. Increased pulmonary vascular resistance; potentiating right heart strain and cardiac stunting if (i.e PFO)

REFERENCES

1. Huet Y, Lemaire F, Brun-Buisson C, Knaus WA, Teisseire B, Payen D, Mathieu D. Hypoxemia in acute pulmonary embolism. Chest. 1985 Dec;88(6):829-36.
2. Santolicandro A, Prediletto R, Fornai E, Formichi B, Begliomini E, Giannella-Neto A, Giuntini C. Mechanisms of hypoxemia and hypocapnia in pulmonary embolism. Am J Respir Crit Care Med. 1995 Jul;152(1):336-47.
3. Torbicki A, Perrier A, Konstantinides S, Agnelli G, Galiè N, Pruszczyk P, Bengel F, Brady AJ, Ferreira D, Janssens U, Klepetko W, Mayer E, Remy-Jardin M, Bassand JP. Guidelines on the diagnosis and management of acute pulmonary embolism: the Task Force for the Diagnosis and Management of Acute Pulmonary Embolism of the European Society of Cardiology (ESC). Eur Heart J. 2008 Sep;29(18):2276-315. 

Aspiration events have a gravity-based predilection, meaning the lobes/ lung segments in the most dependent positions are likely affected. Also, the right main bronchus has a larger luminal diameter and more vertical trajectory than the left main bronchus making it more susceptible to aspirated content.

Lobes of the lungs most likely to be affected by aspiration include:

Upright: The lower lobes (Right>Left)

Supine: Superior segments of the lower lobes (Right>Left) or posterior segment of the RIGHT upper lobe. This is typically in patients with altered LoC (i.e Alcoholics, Intubated patients etc.).  

REFERENCES:

1. Kuhajda, Ivan et al. “Lung Abscess-Etiology, Diagnostic and Treatment Options.” Annals of Translational Medicine 3.13 (2015): 183. PMC. Web. 14 Sept. 2018.
2. Kim M, Lee KY, Lee KW et-al. MDCT evaluation of foreign bodies and liquid aspiration pneumonia in adults. AJR Am J Roentgenol. 2008;190 (4): 907-15.
3. Moreira Jda S, Camargo Jde J, Felicetti JC, et al. Lung abscess: analysis of 252 consecutive cases diagnosed between 1968 and 2004. J Bras Pneumol 2006;32:136-43.

Ground glass opacities are hazy increases in lung opacity without obscuration of the underlying parenchymal vessels.

Whereas, Consolidation is an increase in lung opacity that does obscure the vessels.

REFERENCES

1. radiologyassistant.nl
2. Chest Radiology (Seventh Edition), 2019

It is common dogma on the wards that oxygen therapy for chronic CO2 retainers should be targeted between 88-92% during an COPD exacerbation.

The mechanism often quoted is the “hypoxic drive to breath”. The idea is that COPD patients tend to have chronically elevated levels of carbon dioxide due to the nature of their illness. As such, administration of oxygen to these patients with COPD can be dangerous.

Their chronically elevated carbon dioxide levels result in loss of the hypercapneic mediated respiratory drive and they rely solely on their “hypoxic” drive to breath. The patient’s chemo-receptors are already tolerant of high levels of carbon dioxide.

However, also contributory to the desaturation seen with higher oxygen levels is the Haldane effect. The Haldane effect states that deoxygenated hemoglobin has a higher affinity for CO2 because it is a better proton acceptor than oxygenated hemoglobin.

Therefore, increasing oxygen concentration in the blood by giving patients supplemental oxygen means carbon dioxide molecules will be displaced in favour of the oxygen, thereby reducing alveolar expulsion. 

Finally, another important mechanism is: hypoxic mediated vasoconstriction (aka V/Q mismatch).

When alveolar oxygen tension is reduced (i.e bronchoconstriction, mucus plugging), it induces vasoconstriction of pulmonary capillaries supporting the effected alveoli. This is meant to counteract possible shunting and normalize the V/Q ratio, a mechanism called hypoxic pulmonary vasoconstriction (see figure). The strongest determinant for hypoxic pulmonary vasoconstriction is alveolar partial pressure of oxygen.

Therefore, providing a high fraction of inspired O2 (FiO2) will increase O2 tension in alveoli with a low level of ventilation (i.e scarred alveoli in setting of COPD), inhibiting hypoxic pulmonary vasoconstriction. As a result, alveoli with relatively impaired ventilation (which would be vasoconstriction normally) are no longer, leading to an increase in V/Q mismatch.

Therefore in summary (Abdo et al. 2012):

“In patients with COPD, hypoxic pulmonary vasoconstriction is the most efficient way to alter the V/Q ratios to improve gas exchange. This physiological mechanism is counteracted by oxygen therapy and accounts for the largest increase of oxygen-induced hypercapnia. A titrated oxygen therapy to achieve saturations of 88% to 92% is recommended in patients with an acute exacerbation of COPD to avoid hypoxemia and reduce the risk of oxygen-induced hypercapnia.”

REFERENCES

1. Abdo WF, Heunks LM. Oxygen-induced hypercapnia in COPD: myths and facts. Critical Care. 2012;16(5):323. doi:10.1186/cc11475.
2. Aubier M, Murciano D, Milic-Emili J, Touaty E, Daghfous J, Pariente R, Derenne JP. Effects of the administration of O2 on ventilation and blood gases in patients with chronic obstructive pulmonary disease during acute respiratory failure. Am Rev Respir Dis. 1980;16:747–754.
3. organ, G.E., Mikhail, M.S., Murray, M.J. Clinical Anesthesiology: Fourth Edition. Chapter 22 Respiratory Physiology: The Effects of Anesthesia. Pg. 565.

During a Respiratory exam, the question of paradoxical breathing commonly comes up. The more accurate and correct term is actually abdominal paradox (as paradoxical breathing may also refer to the deranged breathing seen during an injury resulting in flail chest).

During a normal respiratory cycle, in the supine position, the anterior abdominal wall displays a prominent outward movement during inspiration. This is secondary to diaphragm movement and displacement of abdominal viscera. The opposite is seen during expiration.

However, during abdominal paradox, as the diaphragm fatigues, it is unable to contract and move downwards. Instead of the expected outward movement of the abdominal wall, there is an inward movement instead (hence the term paradox).

MECHANISM: The paradoxical inward movement of the abdominal wall during inhalation is due to the cephalic (upward) movement of the fatiguing diaphragm in response to the negative intra-thoracic pressure generated by the inspiratory action of the neck and intercostal muscles.

EXAMPLE: Video

 REFERENCES

1. Chapter 9. Pulmonary Evaluation. Lawrence P. Cahalin. Cardiovascular and Pulmonary Physical Therapy: An Evidence-Based Approach.
2. Civetta, Taylor, & Kirby’s: Critical Care, 4th Edition Section II – Monitoring Chapter 20 – Bedside Assessment and Monitoring of Pulmonary Function and Power of Breathing in the Critically.
3. Chapter 143: Pump Failure: The Pathogenesis of Hypercapnic Respiratory Failure in Patients with Lung and Chest Wall Disease. Steven G. Kelsen. Fishman’s Pulmonary Diseases and Disorders, 5e.