Why did the patient's oxygen saturation drop to 91% after starting nitroglycerin (NTG) for hypertension during general anesthesia (GA) for a laparoscopic assisted vaginal hysterectomy (LAVH)?

Intraoperative Oxygen Desaturation Following Nitroglycerin Administration

The oxygen saturation drop to 91% after starting nitroglycerin was most likely caused by pulmonary vasodilation leading to increased ventilation-perfusion (V/Q) mismatch, as nitroglycerin dilates pulmonary vessels in poorly ventilated lung regions, shunting blood away from well-oxygenated areas.

Mechanism of Nitroglycerin-Induced Hypoxemia

Nitroglycerin causes both systemic and pulmonary vasodilation through its direct action on vascular smooth muscle. 1 While this is beneficial for reducing preload and afterload in cardiac conditions, it has important pulmonary consequences during general anesthesia.

Primary Mechanism: V/Q Mismatch

• Nitroglycerin abolishes hypoxic pulmonary vasoconstriction (HPV), a protective reflex that normally diverts blood away from poorly ventilated lung regions toward better-ventilated areas. 2

• When HPV is inhibited, blood flows to atelectatic or hypoventilated lung zones, creating increased intrapulmonary shunting and worsening oxygenation. 2

• This effect is particularly pronounced during general anesthesia, where patients already have reduced functional residual capacity and areas of dependent atelectasis, especially in the supine position required for LAVH. 3

Supporting Evidence from Clinical Studies

• A study comparing nitroglycerin to nitroprusside during general anesthesia found that both drugs significantly decreased PaO2 at comparable blood pressure reductions, confirming the pulmonary effects of nitroglycerin. 2

• The rapid onset (3-4 minutes) of nitroglycerin's effects matches the timeline described in your case, where desaturation occurred within 5 minutes of starting the infusion. 4

Additional Contributing Factors in This Case

Pneumoperitoneum Effects During Laparoscopy

• If pneumoperitoneum was already established for the laparoscopic portion of LAVH, this further compromises respiratory mechanics by elevating the diaphragm, reducing lung compliance, and worsening V/Q matching.

• The combination of pneumoperitoneum-induced respiratory compromise plus nitroglycerin-induced pulmonary vasodilation creates a particularly high-risk scenario for desaturation.

Excessive Preload Reduction

• Nitroglycerin's primary mechanism is venous capacitance vessel dilation, which reduces venous return and cardiac preload. 1

• Excessive preload reduction can decrease cardiac output by 13% or more, potentially reducing oxygen delivery despite adequate arterial oxygen content. 4

• The FDA label specifically warns that severe hypotension and shock may occur with even small doses of nitroglycerin, particularly in volume-depleted patients. 1

Clinical Management Approach

Immediate Actions

• Stop or reduce the nitroglycerin infusion immediately when oxygen saturation drops below 92-94%.

• Increase FiO2 to 100% to maximize oxygen delivery and overcome the increased shunt fraction. 3

• Apply recruitment maneuvers (sustained inflation to 30-40 cmH2O for 30 seconds) to re-expand atelectatic lung regions and improve V/Q matching.

• Ensure adequate intravascular volume before resuming nitroglycerin, as hypovolemia exacerbates the hypotensive effects. 1

Alternative Agents for Intraoperative Hypertension

For sudden intraoperative hypertension in this clinical scenario, consider alternative agents that don't impair HPV:

• Esmolol (short-acting beta-blocker) is particularly useful for managing intraoperative tachycardia and hypertension without pulmonary vasodilation effects. 3

• Labetalol or nicardipine are effective alternatives recommended by guidelines for intraoperative blood pressure control. 3

• Clevidipine is specifically mentioned for perioperative hypertension management without the pulmonary complications of nitroglycerin. 3

Dosing Considerations

• The dose used in your case (1 mL in 500 mL RL at 60 drops/min) likely delivered approximately 2-4 mcg/min, which is within the typical starting range but may have been excessive given the patient's response. 1

• When using non-absorbing tubing, initial dosing should start at 5 mcg/min with careful titration in 5 mcg/min increments every 3-5 minutes. 1

Important Caveats

• Nitroglycerin is often an agent of choice in patients with coronary ischemia, so if this patient had active ischemia, the risk-benefit calculation changes. 3

• The desaturation may resolve spontaneously within 27 ± 8 minutes after stopping the infusion, as nitroglycerin has a very short half-life of approximately 3 minutes. 1, 4

• Monitor for rebound hypertension after discontinuing nitroglycerin, which may require alternative antihypertensive therapy. 1