Hypotension during anesthesia often occurs because reduced systemic vascular resistance and blocked sympathetic nervous system by anesthetic drugs. In patients who are taking hypertension medication, blood pressure drops are exaggerated by inadequate compensation mechanism due to decrease of blood vessel elasticity and desensitization of baroreceptors. In one-lung ventilation (OLV) during thoracic surgery, persistent perfusion of non-ventilatory lungs can lead to increased intra-pulmonary shunt and hypoxemia. As a compensatory mechanism, the gravitational effect and hypoxic pulmonary vasoconstriction occur. Among these, hypoxic pulmonary vasoconstriction is associated with pulmonary vascular resistance. Norepinephrine and vasopressin, which are commonly used in patients with hypotension, affect systemic and pulmonary vascular resistance. However, no studies have been done on lung oxygenation and pulmonary mechanics of these vasoactive drugs in patients undergoing surgery on one lung. Therefore, the purpose of this study is to investigate the effects of vasoactive drugs, norepinephrine and vasopressin, in patients with hypertension.

Inclusion Criteria:

1. Adult patients aged 40-80 years who are planning to have thoracoscopic single lobectomy with unilateral lung ventilation during surgery.
2. Patients taking hypertension drug CCB(calcium channel blocker), ARB(angiotensin II receptor blocker), ACEi(ACE inhibitor, angiotensin converting enzyme inhibitor) at least 4 weeks.
3. American Society of Anesthesiologists (ASA) classification 2~3

Exclusion Criteria:

1. patients with heart failure (NYHA class III~IV)
2. patients who are having moderate obstructive lung disease or restrictive lung disease
3. Low DLCO (< 75%)
4. patients with pulmonary hypertension (mean PAP>25mmHg)
5. patients with liver disease (AST level ≥100 IU/mL or ALT ≥ level 50 IU/L) or kidney disease (Creatine level ≥ 1.5 mg/dL)
6. body mass index (BMI) > 30 kg/m2
7. patients who cannot read explanation and consent form
8. patients who are pregnant

Patients, care givers and outcomes assessors are blinded. The investigator should not be included in the blind because the drug must be rate controlled during investigations.