What are the causes of syncope and agonal breathing?

Causes of Syncope and Agonal Breathing

Syncope results from transient cerebral hypoperfusion causing sudden, self-limited loss of consciousness, while agonal breathing is an abnormal gasping respiratory pattern that occurs in 40-60% of cardiac arrests and signals severe cardiopulmonary compromise. 1, 2, 3

Syncope: Major Etiologic Categories

Reflex (Neurally-Mediated) Syncope

This is the most common cause of syncope across all age groups and clinical settings. 1, 4, 5

• Vasovagal syncope occurs with emotional stress, pain, medical settings, or prolonged standing, characterized by diaphoresis, warmth, nausea, pallor, and often preceded by identifiable triggers 6, 7
• Situational syncope triggered by specific actions: coughing, sneezing, micturition, defecation, post-exercise, post-prandial, swallowing, or gastrointestinal stimulation 1
• Carotid sinus syndrome occurs with mechanical manipulation of carotid sinuses, more frequent in older patients, defined by pause ≥3 seconds or systolic pressure drop ≥50 mmHg upon carotid sinus stimulation 6, 7
• Atypical forms present without apparent triggers, diagnosed by exclusion of structural heart disease and positive tilt testing 4

Orthostatic Hypotension

Defined as systolic BP drop ≥20 mmHg or diastolic BP drop ≥10 mmHg with standing. 6, 7

• Primary autonomic failure: pure autonomic failure, multiple system atrophy, Parkinson's disease with autonomic failure, Lewy body dementia 1, 4
• Secondary autonomic failure: diabetic neuropathy, amyloid neuropathy, uremia, spinal cord injuries 1, 4
• Drug-induced: vasodilators, diuretics, phenothiazines, antidepressants, alcohol 1
• Volume depletion: hemorrhage, diarrhea, vomiting, Addison's disease 1

Cardiac Arrhythmias

The second most common cause overall, particularly in older patients and emergency settings. 5

• Bradyarrhythmias: sinus node dysfunction (including bradycardia/tachycardia syndrome), atrioventricular conduction system disease 1
• Tachyarrhythmias: paroxysmal supraventricular and ventricular tachycardias 1
• Inherited syndromes: long QT syndrome, Brugada syndrome 1
• Device malfunction: pacemaker or ICD malfunction 1
• Drug-induced proarrhythmias 1

Structural Cardiac and Cardiopulmonary Disease

• Obstructive lesions: cardiac valvular disease (especially aortic stenosis), obstructive cardiomyopathy, atrial myxoma 1
• Acute conditions: myocardial infarction/ischemia, acute aortic dissection, pericardial disease/tamponade 1
• Pulmonary causes: pulmonary embolus, pulmonary hypertension 1

Cerebrovascular Causes

These are rare causes of true syncope. 1

• Vascular steal syndromes: occur when a blood vessel must supply both brain and arm 1
• Vertebrobasilar transient ischemic attacks (though these typically cause partial rather than complete loss of consciousness) 1

Agonal Breathing: Causes and Clinical Significance

Agonal breathing is characterized by slow, irregular gasping respirations that are ineffective for ventilation and occurs in 40-60% of out-of-hospital cardiac arrests. 2, 3

Primary Causes

• Cardiac arrest (most common): present in 46% of arrests from cardiac etiology versus 32% from other etiologies 3
• Severe cerebral hypoperfusion: occurs as brain stem function deteriorates 2
• Witnessed arrests: agonal respirations present in 56% of witnessed arrests versus only 16% of unwitnessed arrests 3
• Ventricular fibrillation: present in 56% of arrests with VF rhythm compared to 34% with non-VF rhythms 3

Clinical Implications

Agonal breathing is commonly misidentified by lay rescuers as normal breathing, leading to critical delays in CPR initiation. 2

• Callers describe agonal breathing variably as "abnormal breathing," "snoring respirations," or "gasping" 2, 3
• Presence of agonal respirations is associated with 27% discharge alive rate compared to 9% without them 3
• Agonal breathing diminishes the longer a person remains in cardiac arrest 2
• The risk of harm from providing chest compressions to an unconscious patient not in cardiac arrest is low, so any unconscious patient with absent or agonal breathing should receive immediate CPR 2

Critical Diagnostic Pitfalls

Neurological investigations are NOT routinely indicated for syncope evaluation. 1

• Neurological referral is warranted only when loss of consciousness cannot be attributed to syncope, when autonomic failure is suspected, or for cerebrovascular steal syndrome 1
• Avoid wasteful short-term ambulatory ECG monitoring and rarely positive neurologic tests (EEG, head CT/MRI) without head trauma or evident neurological signs 8
• Multiple causes often coexist in elderly patients, while reflex syncope predominates in young patients 5