What is the most detrimental effect of tachycardia on a patient with cardiac problems?

The most detrimental effect of tachycardia on a patient with cardiac problems is the increased oxygen demand that accompanies an elevated heart rate. When the heart beats faster, it requires more oxygen to sustain its higher workload. This increased demand can be particularly harmful for patients with existing cardiac conditions, as their hearts may already be compromised and have a reduced capacity to meet this demand effectively.

In patients with heart disease or other underlying cardiac issues, the additional strain from tachycardia can lead to symptoms such as chest pain, shortness of breath, or even heart failure. If the oxygen supply cannot meet the increased demand due to impaired coronary circulation or heart function, it might result in ischemia, which can worsen the patient's overall condition.

The other options, while relevant in the context of cardiac problems, do not capture the primary and immediate physiological risk posed by tachycardia. Increased blood pressure can occur but is not as directly linked to the immediate threat as the heart's oxygen requirements. Increased stress and anxiety are psychological responses that can exacerbate a patient's condition but are not direct physiological consequences. Decreased cardiac functioning is a result of various factors but stems from the increased oxygen demand and overall stress on the heart during episodes of tachycardia.