Inogen Inc. shares dropped in the extended session Tuesday after the maker of oxygen therapy systems missed Wall Street earnings targets and cut its outlook for the year. Inogen shares, which had been briefly halted, dropped 20% after hours, following a 4.2% decline to close the regular session at $91.14. The company reported first-quarter net income of $5.3 million, or 24 cents a share, compared with $10.8 million, or 48 cents a share, in the year-ago period. Revenue rose to $90.2 million from $79.1 million in the year-ago quarter. Analysts surveyed by FactSet had forecast earnings of 30 cents a share on revenue of $89.6 million. Inogen reduced its forecast revenue for the year to a range of $405 million to $415 million, down from a range of $430 million to $440 million. Analysts had forecast revenue of $433.8 million.