Chapter 142: Hypocarbia and Hypercarbia

INTRODUCTION

For most patients receiving general or regional anesthesia, the arterial carbon dioxide tension (Paco₂) should be maintained within normal physiologic limits (35-45 mm Hg). Alterations in homeostasis may lead to hypercarbia or hypocarbia.

HYPOCARBIA

Presentation

Hypocarbia, or hypocapnia, occurs when levels of CO₂ in the blood become abnormally low (Paco₂ <35 mm Hg). Hypocarbia is confirmed by arterial blood gas analysis. Hypocarbia, especially if only transient, is usually well tolerated by patients. Deliberate hyperventilation, leading to hypocarbia, is often used to decrease intracranial pressure in neurosurgical patients.

Causes

A. Increased Carbon Dioxide Elimination

1. Hyperventilation

   • Excessive minute ventilation in mechanically ventilated patients

   • Increased minute ventilation in spontaneously ventilating patients

     • Response to metabolic acidosis

     • Pain

     • Pregnancy

     • CNS pathology (infection, tumors)

2. Decreased dead space ventilation

3. Decreased CO₂ rebreathing

B. Decreased Pulmonary Perfusion

1. Decreased cardiac output

   • Hypovolemia

   • Hypotension

   • Cardiac arrest

2. Pulmonary embolism

C. Decreased Carbon Dioxide Production

1. Hypothermia

2. Deep anesthesia

3. Hypothyroidism

4. Decreased metabolism

D. Airway/Equipment Problems

1. Esophageal intubation

2. Accidental extubation or circuit disconnection

3. Air entrainment (eg, cuff leaks)

4. Dilution with circuit gases

Physiologic Effects

1. Cardiovascular:

   • Decreased myocardial oxygen supply

   • Increased coronary vascular resistance

   • Increased risk of coronary artery vasospasm

   • Increased coronary microvascular leakage

   • Increased myocardial oxygen demand

2. Neurologic:

   • Decreased cerebral blood flow

   • Decreased cerebral oxygen delivery

   • Decreased cerebral blood volume

   • Decreased intracranial pressure

3. Metabolic/hematologic:

   • Respiratory alkalosis

   • Increased intracellular calcium concentration

   • Increased platelet count and aggregation

Management

1. Assess oxygenation status

2. Obtain arterial blood gas to confirm capnography results

3. Since the most common cause of hypocarbia during surgery is iatrogenic hyperventilation, the first step in management should focus on decreasing minute ventilation

4. Assess and restore circulation if the problem involves decreased cardiac output

HYPERCARBIA

Presentation

Hypercarbia, or hypercapnia, occurs when levels of CO₂ in the blood become abnormally high (Paco₂ >45 mm Hg). Hypercarbia is confirmed by arterial blood gas analysis. When using capnography to approximate Paco₂, remember that the normal arterial–end-tidal carbon dioxide gradient is roughly 5 mm Hg. Hypercarbia, therefore, occurs when PETco₂ is greater than 40 mm Hg.

In the awake or sedated patient, signs and symptoms include dyspnea, sweating, muscle tremors, flushed skin, headache, lethargy, and confusion. Spontaneously breathing patients develop tachypnea while mechanically ventilated patients may overbreathe the ventilator.

In patients breathing room air or low inspired oxygen concentrations, severe hypercarbia leads to severe hypoxemia. According to the alveolar gas equation, a patient breathing room air with Paco₂ of 90 ...