When a monitor does not read SpO2, common factors include poor sensor placement, low perfusion, motion artifacts, or low hemoglobin levels. To troubleshoot, ensure the sensor is properly placed, check for adequate circulation, or try a different finger or toe. In case the monitor still fails, alternatives like a handheld pulse oximeter or arterial blood gas (ABG) test can help. A plethysmograph visualizes blood volume changes, useful in assessing oxygen levels. Vasospasm, constriction of blood v