Increasing Respiratory Rate to 20-22 in Compensated Respiratory Acidosis: Risk of CO2 Washout
Yes, increasing the respiratory rate to 20-22 breaths/minute can cause CO2 washout in patients with compensated respiratory acidosis, potentially destabilizing their acid-base balance and causing harm. 1
Understanding Compensated Respiratory Acidosis
In patients with chronic hypercapnia (compensated respiratory acidosis), the kidneys have retained bicarbonate over hours to days to buffer the elevated CO2, resulting in a normal or near-normal pH despite persistently elevated PaCO2 (typically >6 kPa or 45 mmHg) and high bicarbonate levels (>28 mmol/L). 1 This represents a physiologic adaptation where the body has equilibrated to a new baseline CO2 level. 1
The Problem with Increasing Respiratory Rate
Mechanism of Harm
Bicarbonate-CO2 mismatch: When you increase the respiratory rate to 20-22 breaths/minute in these patients, you will increase minute ventilation and lower their PaCO2. 2 However, their bicarbonate level remains elevated because renal compensation takes days to reverse. 1
Resulting alkalosis: This creates a dangerous situation where the previously compensated patient now has high bicarbonate with lower CO2, resulting in metabolic alkalosis and potentially worsening their clinical status. 1
Evidence Against High Respiratory Rates
A critical study demonstrated that increasing respiratory rate to 30 breaths/minute in acute respiratory failure patients did not improve CO2 clearance and actually caused harm through: 2
- Increased alveolar deadspace ventilation (21% vs 14% at lower rates)
- Dynamic hyperinflation with intrinsic PEEP (6.4 cm H2O)
- Impaired right ventricular function with decreased cardiac index (2.9 vs 3.3 L/min/m²)
Guideline-Based Approach
For COPD Patients with Compensated Respiratory Acidosis
Target oxygen saturation of 88-92% rather than attempting to normalize CO2 through increased respiratory rate. 1
If PaCO2 is elevated but pH ≥7.35 with high bicarbonate (>28 mmol/L), the patient has compensated chronic hypercapnia—maintain their baseline, do not attempt rapid correction. 1
Blood gases should be rechecked at 30-60 minutes to ensure stability, not to drive aggressive ventilation changes. 1
For Mechanical Ventilation Settings
When managing these patients on ventilators, the BTS/ICS guidelines recommend: 1
- Respiratory rate: 10-15 breaths/minute for obstructive disease (not 20-22)
- Permissive hypercapnia with target pH 7.2-7.4 is acceptable and safer than aggressive CO2 reduction
- The higher the pre-morbid PaCO2 (inferred by high bicarbonate), the higher the target PaCO2 should be
When Decompensation Occurs
Only initiate aggressive ventilatory support (NIV) when pH <7.35 with PaCO2 ≥6.5 kPa and respiratory rate >23 breaths/minute persisting after optimal medical therapy. 1 This represents acute-on-chronic respiratory acidosis requiring intervention, not the compensated state.
Critical Pitfall to Avoid
Never attempt to rapidly normalize CO2 in chronically hypercapnic patients. 1 When hypercapnia is chronic, reducing bicarbonate buffering capacity requires a period of relative hyperventilation over days, not hours, with resulting urinary bicarbonate loss to reset central respiratory drive. 1 Acute correction risks:
- Metabolic alkalosis
- Cerebral vasoconstriction (CO2 is a potent cerebral vasodilator)
- Hemodynamic instability
- Patient-ventilator dyssynchrony
Bottom Line
Keep respiratory rates at 10-15 breaths/minute in patients with compensated respiratory acidosis from obstructive disease, accept their baseline elevated CO2, and focus on maintaining adequate oxygenation (SpO2 88-92%) rather than normalizing ventilation. 1