Shock is a term used to describe the sudden onset of low blood flow to tissues in the body. This can happen for a variety of reasons, such as trauma, blood loss, or cardiac arrest. When tissues experience low blood flow, cells become deprived of oxygen and nutrients. This can cause cells to undergo metabolic changes that lead to acidosis, which is a process that reduces pH.
What happens during shock?
During shock, blood pressure drops dramatically. This reduces overall blood flow and oxygen delivery to tissues and organs. Cells become ischemic, meaning they are deprived of oxygen and nutrients. Ischemic cells switch to anaerobic metabolism, meaning they produce energy without using oxygen. Anaerobic metabolism produces lactic acid as a byproduct, which lowers pH.
In addition, when blood flow is reduced, carbon dioxide clearance from tissues is also reduced. Carbon dioxide reacts with water to form carbonic acid, which lowers pH. The kidneys also play an important role in regulating pH. During shock, the kidneys have impaired function and cannot excrete acids normally, leading to acid accumulation.
Finally, catecholamines and stress hormones are released in response to shock. These hormones further increase cellular metabolism and acid production. All of these metabolic effects combine to reduce pH during shock.
How does pH change during different types of shock?
There are several classifications of shock based on underlying cause:
- Hypovolemic shock – This results from blood or fluid loss, such as in hemorrhage or severe dehydration. The primary driver of acidosis is decreased tissue perfusion leading to anaerobic cell metabolism.
- Cardiogenic shock – This is caused by the heart’s inability to pump sufficient blood. Like hypovolemic shock, decreased perfusion drives acidosis.
- Septic shock – This is caused by a serious infection and the inflammatory response it triggers. Septic shock involves metabolic acidosis from hypoperfusion as well as lactic acid production by immune cells.
- Anaphylactic shock – Caused by a severe allergic reaction, anaphylactic shock involves dilation of blood vessels leading to hypotension. Metabolic acidosis occurs due to hypoperfusion.
- Neurogenic shock – This is caused by an injury to the nervous system. Loss of vascular tone leads to reduced blood pressure and thus, shock. Metabolic acidosis again arises from poor perfusion.
In all forms of shock, the severity of metabolic acidosis correlates with the severity and duration of low blood flow. The more severe the ischemic insult, the greater the increase in lactic acid and other acids that lower pH.
How much does pH typically change in shock?
In one study examining pH changes during hemorrhagic shock in animal models, pH declined from a normal value of around 7.4 down to 7.1-6.9 after significant blood loss. After 90 minutes of sustained shock, pH declined further to 6.8-6.6 in severe cases. The degree of pH change correlates with the amount of blood loss and resulting hypotension and hypoperfusion.
In clinical studies of trauma patients, the mean pH upon hospital admission in patients with hemorrhagic shock is around 7.1-7.2. The range extends from 6.8 in profound shock to 7.3 in moderate cases. The lower the pH at admission, the higher the correlation with mortality risk.
In cardiogenic shock, admission pH averages around 7.3 but can be as low as 7.0 in advanced cases. Any pH under 7.2 indicates a significant degree of metabolic acidosis.
In septic shock, admission pH tends to be lower than other forms of shock, averaging 7.2-7.3. This reflects the additional acid burden arising from inflammation and infection. Septic shock can induce severe acidosis with pH as low as 6.8.
Overall, most forms of shock induce a pH reduction of about 0.2-0.3 pH units on average. In severe or prolonged cases, the pH reduction can be 0.5 units or more. Rapid correction of hypoperfusion and restoration of oxygenation is critical to stabilize pH.
How does the body compensate for reduced pH during shock?
The body has several mechanisms to counteract a falling pH and mitigate metabolic acidosis. These include:
- Buffering by bicarbonate – Bicarbonate is part of the body’s extracellular and intracellular buffer systems that absorbs protons to moderate pH changes.
- Increased ventilation – Hyperventilation blows off carbon dioxide, reducing the amount that can be converted to carbonic acid.
- Increased renal acid excretion – The kidneys compensate by increasing excretion of acid equivalents.
- Release of alkaline reserves – The body mobilizes alkaline reagents such as phosphate and proteins.
Despite these compensatory mechanisms, the degree of acidosis induced by shock can overwhelm the body’s capacity to effectively correct it. Rapid hemodynamic stabilization and treatment of the underlying cause is necessary to allow the body to regain pH control.
How is pH monitored during shock treatment?
Given the danger of metabolic acidosis, continuous monitoring of pH is a key part of shock treatment. pH is typically monitored using the following methods:
- Arterial blood gas – This provides the most accurate pH measurement from arterial blood. It also measures CO2 and bicarbonate.
- Venous blood gas – Less invasive but provides an approximation of pH status from venous blood.
- Capillary blood sampling – Involves a finger stick for capillary blood draw. Does not give precise arterial pH.
- Indwelling catheters – Allow continuous blood sampling from an arterial or venous line to enable pH trend monitoring.
Treatment targets restoring pH to a normal physiological range of 7.35-7.45. Greater priority is placed on pH normalization compared to other acid-base parameters. Treatment escalates in cases of severe or refractory acidosis threatening end-organ function.
How is pH corrected when too low during shock?
Treating the underlying cause of shock is the most direct way to stabilize pH. However, when acidosis is severe or shock persists, active efforts are made to counteract the low pH including:
- Infusion of bicarbonate – This provides additional buffering capacity and can directly raise blood pH.
- Increased ventilation – Uses higher ventilator settings to blow off more CO2.
- Catecholamines – Medications like dopamine and norepinephrine improve perfusion and oxygen delivery.
- Renal replacement therapy – Helps remove acid equivalents when kidney function is impaired.
Careful monitoring of pH is needed during correction to avoid overshoot alkalosis. Once pH is stabilized, ongoing care focuses on treating the shock state and gradually restoring normal acid-base balance.
Key takeaways
- Shock universally leads to metabolic acidosis due to reduced tissue perfusion.
- All classifications of shock can lower pH, with average reductions of 0.2-0.3 units.
- Compensatory mechanisms such as bicarbonate buffering mitigate but do not fully correct acidosis.
- pH monitoring is vital during shock treatment to guide appropriate interventions.
- Restoring adequate perfusion and oxygenation is key to stabilizing pH.
Conclusion
Shock has a direct and often severe impact on pH balance in the body. The metabolic changes triggered by low blood flow and oxygen delivery uniformly lower pH. The degree of pH reduction correlates with the severity and duration of shock. Compensatory mechanisms provide partial mitigation but do not override the acidifying effects. Prompt normalization of perfusion and treatment of underlying shock causes are needed to stabilize pH. With aggressive resuscitation, the body can regain acid-base equilibrium, but left unchecked, shock can induce life-threatening acidosis. Close monitoring and management of pH is a crucial part of the clinical strategy during all forms of shock.
