Pathophysiology of dyspnoea in acute pulmonary embolism: A cross-sectional evaluation

Prognostic role of different temporal patterns of dyspnoea onset in patients with acute pulmonary embolism

AIMS

Dyspnoea is a well known symptom of acute pulmonary embolism (PE). We assess the prognostic role of different patterns of dyspnoea onset regarding in-hospital mortality, clinical deterioration and the composite of the outcomes in PE patients, according to their haemodynamic status at admission.

METHODS

Patients from the prospective Italian Pulmonary Embolism Registry (IPER) were included in the study. At admission, patients were stratified, according to their haemodynamic status, as high- (haemodynamically unstable) and non-high-risk (haemodynamically stable) patients.

RESULTS

Overall, 1623 consecutive patients (mean age 70.2 ± 15.2 years, 696 males), with confirmed acute PE, were evaluated for the features of dyspnoea. Among these, 1353 (83.3%) experienced dyspnoea at admission. No significant differences were observed regarding in-hospital mortality and the composite outcome of in-hospital mortality and clinical deterioration between patients with and without dyspnoea. However, in non-high-risk patients, clinical deterioration was more frequently observed when dyspnoea was present compared with absence of dyspnoea (P = 0.002). Multivariate Cox regression analyses showed that non-high-risk patients had an increased risk of clinical deterioration when experiencing dyspnoea within 24 h [hazard ratio (HR): 1.57, 95% confidence interval (CI): 1.49-1.65, P < 0.0001] and between 25 h and 7 days before admission (HR: 1.66, 95% CI: 1.58-1.77, P < 0.0001), independently of age, sex, right ventricular dysfunction, positive cardiac troponin and thrombolysis.

CONCLUSIONS

Non-high-risk PE patients experiencing dyspnoea within 7 days before hospitalization had a higher risk of clinical deterioration compared with those without and, therefore, they may require more aggressive management.

Silent Hypoxemia in COVID-19 Pneumonia

In patients suffering from Coronavirus Disease 2019 (COVID-19), dyspnoea is less likely to occur despite hypoxemia. Even if the patient develops severe hypoxemia, it cannot be detected from subjective symptoms. In other words, it becomes more serious without the person or the surroundings noticing it. Initially less talked about, hypoxemia without dyspnoea (silent hypoxemia or happy hypoxia: hypoxemia that does not coincide with dyspnoea) is now experienced in many institutions. Dyspnoea is defined as "the unpleasant sensation that accompanies breathing." Dyspnoea occurs when afferent information is transmitted to the sensory area. Receptors involved in the development of dyspnoea include central and peripheral chemoreceptors, chest wall receptors, lung receptors, upper respiratory tract receptors and corollary discharge receptors. In the present study, we considered mechanisms mediating the silent hypoxemia through three cases experienced at our hospital as a dedicated coronavirus treatment hospital. We have treated about 600 people infected with COVID-19, of which about 10% were severe cases. In the present study, the patients' condition was retrospectively extracted and analysed. We investigated three typical cases of COVID-19 pneumonia admitted to our hospital (men and women between the ages of 58 and 86 with hypoxemia and tachypnoea). Silent hypoxemia is not entirely without dyspnoea, but hypoxemia does not cause dyspnoea commensurate with its severity. The virus may have specific effects on the respiratory control system. In our cases, respiratory rate significantly increased with hypoxemia, and hyperventilation occurred. Therefore, information about hypoxemia is transmitted from the carotid body. Since hyperventilation occurs, it is suggested that information is transmitted to effectors such as respiratory muscles. The fact that these patients did not feel the unpleasant sensation indicates that information is not accurately transmitted to the sensory area of the cerebral cortex. These cases suggest that there may be a problem somewhere in the path from the respiratory centre to the sensory area.

Why COVID-19 Silent Hypoxemia Is Baffling to Physicians

Patients with coronavirus disease (COVID-19) are described as exhibiting oxygen levels incompatible with life without dyspnea. The pairing-dubbed happy hypoxia but more precisely termed silent hypoxemia-is especially bewildering to physicians and is considered as defying basic biology. This combination has attracted extensive coverage in media but has not been discussed in medical journals. It is possible that coronavirus has an idiosyncratic action on receptors involved in chemosensitivity to oxygen, but well-established pathophysiological mechanisms can account for most, if not all, cases of silent hypoxemia. These mechanisms include the way dyspnea and the respiratory centers respond to low levels of oxygen, the way the prevailing carbon dioxide tension (PaCO2) blunts the brain's response to hypoxia, effects of disease and age on control of breathing, inaccuracy of pulse oximetry at low oxygen saturations, and temperature-induced shifts in the oxygen dissociation curve. Without knowledge of these mechanisms, physicians caring for patients with hypoxemia free of dyspnea are operating in the dark, placing vulnerable patients with COVID-19 at considerable risk. In conclusion, features of COVID-19 that physicians find baffling become less strange when viewed in light of long-established principles of respiratory physiology; an understanding of these mechanisms will enhance patient care if the much-anticipated second wave emerges.

Cardiac arrest due to pulmonary embolism

Pulmonary embolism (PE) is a potentially life threatening clinical condition that is fairly non-specific in presentation. Massive pulmonary embolism (PE) without cardiac arrest has been associated with a mortality rate of 30%. However, when cardiac arrest ensues, mortality may be as high as 95%. Since outcomes of cardiac arrest following PE are generally dismal, any available potentially life-saving measure must be instituted when the diagnosis of PE is suspected. Despite a lack of randomized controlled trials guiding the management of suspected PE in the cardiac arrest victim, thrombolysis and other therapies have been associated with good outcomes in the handful of published case reports and other small studies.