A low V/Q ratio occurs when pulmonary gas exchange is impaired a situation that typically causes hypoxemia. Carbon dioxide (CO2) excretion also is impaired, which can lead to increased partial pressure of CO2 in arterial blood (PaCO2). However, this is rare because increased PaCO2 typically triggers res- piratory stimulation and increased alveolar ventilation, which returns PaCO2 to normal. This scenario is most common in patients with chronic bronchitis, asthma, and acute pulmonary edema.