What vital signs are typically observed in a patient with influenza?

Vital Signs in Influenza Patients

In patients with influenza, you should expect fever (typically >38°C), tachycardia (heart rate >100/min), tachypnea (respiratory rate ≥24-30/min), and normal to low blood pressure, with oxygen saturation potentially decreased if pneumonia develops. 1

Core Vital Sign Abnormalities

Temperature

• Fever >38°C (100.4°F) is the hallmark vital sign and a diagnostic criterion for influenza-like illness requiring antiviral treatment 1, 2
• Temperature >37.8°C indicates clinical instability and should prompt continued hospitalization 1
• Note that immunocompromised or very elderly patients may not mount an adequate febrile response despite active infection 1

Heart Rate

• Tachycardia >100 beats/min is common and represents one of the clinical instability criteria 1
• Fever increases heart rate by approximately 6-7 beats per minute for every 1°C rise in temperature 3
• Heart rate elevation should prompt assessment for cardiac complications and volume depletion 1, 4

Respiratory Rate

• Tachypnea ≥24 breaths/min indicates clinical instability and is part of the CURB-65 severity assessment 1
• Respiratory rate ≥30/min is a critical threshold that scores points in severity assessments and predicts severe disease 1, 5
• Fever increases respiratory rate by approximately 1.2-1.4 breaths per minute for every 1°C rise in temperature 3

Blood Pressure

• Systolic blood pressure <90 mmHg or diastolic <60 mmHg indicates severe illness and is part of severity scoring systems 1
• Hypotension should trigger assessment for septic shock, volume depletion, and consideration for ICU transfer 1

Oxygen Saturation

• Oxygen saturation <92% on room air is abnormal and requires supplemental oxygen therapy 1
• Oxygen saturation <90% represents clinical instability and should prompt continued hospitalization 1
• Pulse oximetry should be monitored in all hospitalized patients, with arterial blood gases obtained if saturation is <92% 1

Monitoring Frequency

Temperature, respiratory rate, pulse, blood pressure, mental status, and oxygen saturation should be monitored at least twice daily in hospitalized patients, with more frequent monitoring for those with severe illness or requiring oxygen therapy 1

Clinical Pearls and Pitfalls

Common Pitfalls

• Do not dismiss patients with normal vital signs if they have bilateral chest findings, as primary viral pneumonia can progress rapidly and should prompt hospital referral regardless of severity scores 1
• Mental status changes (confusion, disorientation) are a vital sign equivalent and score in severity assessments, indicating serious illness 1
• Elderly or immunocompromised patients may not exhibit typical fever despite severe infection 1

Additional Considerations

• The combination of respiratory rate ≥30/min, oxygen saturation <90%, and C-reactive protein ≥100 mg/L independently predicts severe influenza-associated illness 5
• NEWS2 score ≥7 demonstrates the highest accuracy (AUC 0.76) for identifying patients at risk of severe influenza, particularly in those under 65 years 5
• Patients meeting two or more instability criteria (fever >37.8°C, HR >100/min, RR >24/min, SBP <90 mmHg, O2 sat <90%, inability to maintain oral intake, abnormal mental status) should remain hospitalized 1