Influenza A 2009 (H1N1) virus in admitted and critically ill patients

The influenza A 2009 (H1N1) virus is a pandemic respiratory infection commanding international attention. More information is needed on patient demographics, illness severity, and risk indicators.

A total of 43 patients with H1N1 influenza A 2009 presenting to 2 urban academic medical centers during the first wave were assessed for demographics, triage vital signs, hemograms, and serum chemistries including lactate. Chest X-rays were assessed for infiltrate or effusion. Illness severity markers were recorded including anion gap (AG), strong ion gap (SIG), systemic inflammatory response syndrome score, shock index, confusion, uremia, respiratory rate, blood pressure, and age >= 65 years (CURB-65) score, and pneumonia severity index. Subgroup analysis was performed on asthmatic, pregnant, and intensive care unit (ICU) versus non-ICU patients.

Eighty-one percent of patients were women. Pregnancy (34.8%), asthma (39.5%), diabetes mellitus (18.6%), and sickle cell (6.98%) were the most frequent comorbidities. In all, 91% had positive influenza nasopharyngeal direct antigen test, while 9% tested positive only by viral culture or real-time reverse transcriptase polymerase chain reaction (rRT-PCR); 14% required ICU admission; and 20.8% had infiltrate on chest X-ray. A trend toward greater incidence of ICU admission existed among patients with elevated SIG (P = .08), however contrary to our prior studies in noninfluenza patients, an elevated SIG in the presence of normal AG and lactate measurements did not correlate with ICU admission.

A high percentage of patients with H1N1 presented with underlying comorbid conditions including asthma and pregnancy. Traditional markers of pneumonia severity including CURB-65 score, Pneumonia Severity Index (PSI), serum lactate, and AG did not correlate with ICU admission in patients with H1N1. Strong ion gap effectively identified significant acid-base disturbances not identified by lactate or AG, however the trend of greater ICU admission rates among patients with elevated SIG did not reach statistical significance. Further study is needed to identify clinical tools to aid in risk-stratifying H1N1 patients.