CLIN MED III- INFECTIOUS DISEASE EXAM 1 –
COMPREHENSIVE STUDY GUIDE & REVIEW (HIGH-
YIELD TOPICS)
Systemic Inflammatory Response Syndrome (SIRS) – Overview
Definition:
SIRS (Systemic Inflammatory Response Syndrome) is a widespread and
exaggerated inflammatory reaction of the body to a harmful stressor such as
infection, trauma, burns, or pancreatitis.
Diagnostic Criteria
SIRS is diagnosed when two or more of the following abnormalities are present:
Temperature: Greater than 100.4°F (38°C) or lower than 96.8°F (36°C)
Heart Rate: Exceeds 90 beats per minute
Respiratory Rate: More than 20 breaths per minute or arterial PaCO₂ less than 32
mmHg
White Blood Cell Count: Above 12,000/mm³ or below 4,000/mm³
Key Concepts
Can SIRS occur without infection?
Yes — it can result from non-infectious causes such as trauma, pancreatitis, or
burns.
Relationship to Sepsis:
,SIRS may progress to sepsis when an infection is present.
Not all cases of SIRS are septic.
However, all cases of sepsis involve SIRS.
Most severe pathogens:
Gram-negative organisms tend to cause more severe sepsis.
Leading cause of sepsis:
Pneumonia
Common source in diabetics or catheterized patients:
Pyelonephritis, which often leads to SIRS or sepsis.
Organisms causing toxic shock-related SIRS:
Staphylococcus aureus and Streptococcus species (associated with cellulitis,
abscesses, and necrotizing fasciitis).
Clinical Patterns & Findings
Only ~25% of patients with SIRS in the emergency department have an infection.
Always search for the underlying cause.
,Vital Sign Trends in SIRS:
Fever or hypothermia (especially in elderly patients)
Tachycardia (compensatory response)
Tachypnea (may cause respiratory alkalosis)
Blood pressure: Often normal early on, but hypotension indicates possible shock.
In older adults:
Confusion or altered mental status may signal hypotension — monitor closely and
reassess frequently.
Physical Exam Clues by Source
Suspected Source Key Findings
Pneumonia Crackles, decreased breath sounds
Pancreatitis / Perforation Abdominal tenderness, guarding
Cellulitis / Wound infection Erythema, warmth
Pyelonephritis Costovertebral angle tenderness
Monitoring & Assessment
, Always trend vital signs over time rather than relying on a single measurement.
Review medications:
Beta-blockers can mask tachycardia
Antipyretics may suppress fever
Elderly patients often show atypical presentations — minimal or no fever and a
normal WBC count.
Signs of Organ Dysfunction (SIRS Progression)
Decreased urine output – indicates renal hypoperfusion
Altered mental status – reduced cerebral perfusion
Peripheral cyanosis or mottling – poor tissue oxygenation
Respiratory distress – may lead to ARDS
Essential Laboratory Tests
COMPREHENSIVE STUDY GUIDE & REVIEW (HIGH-
YIELD TOPICS)
Systemic Inflammatory Response Syndrome (SIRS) – Overview
Definition:
SIRS (Systemic Inflammatory Response Syndrome) is a widespread and
exaggerated inflammatory reaction of the body to a harmful stressor such as
infection, trauma, burns, or pancreatitis.
Diagnostic Criteria
SIRS is diagnosed when two or more of the following abnormalities are present:
Temperature: Greater than 100.4°F (38°C) or lower than 96.8°F (36°C)
Heart Rate: Exceeds 90 beats per minute
Respiratory Rate: More than 20 breaths per minute or arterial PaCO₂ less than 32
mmHg
White Blood Cell Count: Above 12,000/mm³ or below 4,000/mm³
Key Concepts
Can SIRS occur without infection?
Yes — it can result from non-infectious causes such as trauma, pancreatitis, or
burns.
Relationship to Sepsis:
,SIRS may progress to sepsis when an infection is present.
Not all cases of SIRS are septic.
However, all cases of sepsis involve SIRS.
Most severe pathogens:
Gram-negative organisms tend to cause more severe sepsis.
Leading cause of sepsis:
Pneumonia
Common source in diabetics or catheterized patients:
Pyelonephritis, which often leads to SIRS or sepsis.
Organisms causing toxic shock-related SIRS:
Staphylococcus aureus and Streptococcus species (associated with cellulitis,
abscesses, and necrotizing fasciitis).
Clinical Patterns & Findings
Only ~25% of patients with SIRS in the emergency department have an infection.
Always search for the underlying cause.
,Vital Sign Trends in SIRS:
Fever or hypothermia (especially in elderly patients)
Tachycardia (compensatory response)
Tachypnea (may cause respiratory alkalosis)
Blood pressure: Often normal early on, but hypotension indicates possible shock.
In older adults:
Confusion or altered mental status may signal hypotension — monitor closely and
reassess frequently.
Physical Exam Clues by Source
Suspected Source Key Findings
Pneumonia Crackles, decreased breath sounds
Pancreatitis / Perforation Abdominal tenderness, guarding
Cellulitis / Wound infection Erythema, warmth
Pyelonephritis Costovertebral angle tenderness
Monitoring & Assessment
, Always trend vital signs over time rather than relying on a single measurement.
Review medications:
Beta-blockers can mask tachycardia
Antipyretics may suppress fever
Elderly patients often show atypical presentations — minimal or no fever and a
normal WBC count.
Signs of Organ Dysfunction (SIRS Progression)
Decreased urine output – indicates renal hypoperfusion
Altered mental status – reduced cerebral perfusion
Peripheral cyanosis or mottling – poor tissue oxygenation
Respiratory distress – may lead to ARDS
Essential Laboratory Tests
