From the Guidelines
We aim to maintain arterial carbon dioxide (CO2) levels between 35-45 mmHg in ventilated patients because this range represents normal physiological values that optimize patient outcomes, as supported by the most recent guideline from 2023 1. Maintaining CO2 within this range helps ensure proper acid-base balance in the body, as CO2 directly affects blood pH. When CO2 levels rise above 45 mmHg (hypercapnia), respiratory acidosis can develop, potentially causing confusion, headaches, and in severe cases, coma. Conversely, when CO2 levels fall below 35 mmHg (hypocapnia), respiratory alkalosis can occur, leading to symptoms like lightheadedness, numbness, and seizures. Additionally, appropriate CO2 levels ensure adequate cerebral blood flow, as CO2 is a potent vasodilator of cerebral blood vessels. Maintaining this target range also helps prevent ventilator-induced lung injury by avoiding excessive ventilation pressures that would be needed to achieve very low CO2 levels. For patients with specific conditions like ARDS or increased intracranial pressure, CO2 targets may be adjusted, but 35-45 mmHg remains the standard goal for most ventilated patients to maintain physiological homeostasis, as also suggested by older guidelines from 2010 1. Some key points to consider when managing CO2 levels in ventilated patients include:
- Avoiding hyperventilation, which can cause cerebral vasoconstriction and worsen global brain ischemia 1
- Maintaining normocapnia, with a target PaCO2 of 35-45 mmHg, to ensure proper acid-base balance and cerebral blood flow 1
- Adjusting CO2 targets for patients with specific conditions, such as ARDS or increased intracranial pressure, to optimize outcomes 1
- Using lung-protective ventilation strategies, such as low-tidal-volume ventilation, to prevent ventilator-induced lung injury 1.
From the Research
Ventilation Management in Patients
- The goal of keeping CO2 levels between 35-45 mmHg in ventilated patients is to avoid hypercapnia, which is the elevation of partial pressure of carbon dioxide (PaCO2) above 45 mmHg in the bloodstream 2.
- Hypercapnia can lead to compromise at the cardiovascular, cerebral, metabolic, and respiratory levels, resulting in high morbidity and mortality 2.
- To manage CO2 levels, it is essential to understand the pathophysiological mechanisms of hypercapnia, including decreased minute volume, increased dead space, or increased CO2 production per second 2.
Lung Recruitment and Ventilation Strategies
- Lung recruitment is a crucial component of lung-protective ventilatory strategies, which can have a positive effect on outcomes in patients with adult respiratory distress syndrome/acute lung injury 3.
- Techniques such as high-pressure recruitment maneuvers and prone positioning can help open the lung and improve oxygenation 3, 4.
- However, these techniques must be used early in the course of adult respiratory distress syndrome/acute lung injury and in combination with appropriate levels of positive end-expiratory pressure to be effective 3, 4.
CO2 Production and Measurement
- CO2 production (VCO2) and oxygen consumption (VO2) can be measured in mechanically ventilated patients, but it is essential to consider the effects of changing minute ventilation, body temperature, and sedation levels on CO2 stores 5.
- Measuring VO2 and VCO2 requires a stable period of at least 60 minutes of stable body temperature and awareness, and ventilatory variables should not be changed substantially during this period 5.
Clinical Outcomes and Ventilation Strategies
- The use of prone position ventilation combined with recruitment maneuvers can improve clinical outcomes in patients with severe acute respiratory distress syndrome, including shorter length of ICU stay, fewer ventilation days, and decreased 28-day mortality 6.
- However, the optimal ventilation strategy may vary depending on the individual patient's condition and response to different techniques, and further research is needed to determine the best approach 6.