Prehospital Academy Lesson 11.13
Advanced Respiratory Physiology and Acid-Base Balance
Advanced Airway Physiology and Acid-Base Balance
Introduction
In this lecture, we will take a deeper dive into the respiratory system, looking at advanced respiratory physiology and acid-base balance. A comprehensive understanding of these concepts is critical for advanced-level EMS providers to recognize and manage respiratory emergencies effectively.
Respiration: External and Internal
Respiration is the process of exchanging oxygen and carbon dioxide. There are two types of respiration:
External respiration: The exchange of oxygen and carbon dioxide between the alveoli and the blood in the pulmonary capillaries.
Internal respiration: The exchange of oxygen and carbon dioxide between the systemic circulation and the body's cells.
Oxygen is essential for cellular metabolism, specifically for the production of adenosine triphosphate (ATP) through glycolysis and the Kreb's cycle. Carbon dioxide, a waste product of this process, can significantly impact the body's pH and overall homeostasis.
Pathophysiology of Respiration
Disruptions in pulmonary ventilation, oxygenation, or respiration can have immediate and life-threatening effects. As an EMS provider, it is crucial to recognize these disruptions and intervene promptly. Time is critical:
60 seconds of hypoxia: Cardiac irritability can occur.
4 minutes of hypoxia: Brain damage is possible.
10 minutes of hypoxia: Irreversible brain damage may occur.
This is why our assessment algorithms always begin with Airway, Breathing, and Circulation (ABC). Every cell in the body needs a constant supply of oxygen, which requires adequate perfusion. The pulmonary and circulatory systems work together to meet these cellular demands.
Ventilation-Perfusion (VQ) Mismatch
For the respiratory and circulatory systems to function correctly, ventilation (air flow) and perfusion (blood flow) must be directed to the same place at the same time. This is known as the ventilation-perfusion (VQ) ratio. A VQ mismatch occurs when this relationship is disrupted. This can happen in several ways:
Blood passes over the alveolar membranes without gas exchange occurring.
Carbon dioxide is recirculated into the bloodstream.
Blood is not directed to an area of the lungs that is being adequately ventilated (this is referred to as a shunt).
Factors Affecting Ventilation
Several factors can affect ventilation, leading to a VQ mismatch. These can be categorized as intrinsic or extrinsic.
Intrinsic Factors:
The tongue: The most common airway obstruction. Simple maneuvers like the head-tilt, chin-lift, or jaw-thrust, as well as airway adjuncts like OPAs and NPAs, can correct this.
Allergic reactions and infections: Swelling of the glottic opening can inhibit the passage of air.
Unresponsiveness: Can lead to the tongue obstructing the airway.
Extrinsic Factors:
Trauma: Significant facial trauma and swelling can obstruct the airway, often requiring advanced interventions like a cricothyrotomy.
Foreign body airway obstructions: Mechanically occlude the airway.
Factors Affecting Oxygenation and Respiration
Other factors can affect oxygenation and respiration more specifically:
Ambient Air:
High altitudes: The concentration of oxygen decreases as altitude increases, leading to inadequate oxygen supply for respiration.
Closed environments and toxic gases: Gases like carbon monoxide can displace oxygen, preventing gas exchange.
Conditions Affecting the Lungs:
Pulmonary edema: Fluid in the alveoli prevents diffusion and external respiration.
Submersion injuries and toxic inhalation: Can damage alveolar tissue.
Conditions Affecting Internal Respiration:
Hypoglycemia: Inadequate sugar levels can reduce respiratory effort.
Infection: Increases the body's metabolic demand for oxygen.
Hormonal imbalances: Can lead to conditions like ketoacidosis.
Circulatory Compromise
Inadequate perfusion at the cellular level will prevent oxygen from reaching the tissues, regardless of how well the lungs are oxygenating the blood. This leads to anaerobic metabolism, which produces less ATP and creates harmful metabolic byproducts.
Heart conditions: Can reduce blood flow to tissues.
Blood loss (anemia): Reduces the oxygen-carrying capacity of the blood.
Shock states: Hypoperfusion prevents oxygen from reaching the tissues.
Assessing Oxygenation and Ventilation: Capnography
End-tidal carbon dioxide (EtCO2) monitoring, or capnography, is a valuable tool for assessing the body's ability to oxygenate, ventilate, and engage in cellular respiration.
CO2 as a byproduct: CO2 is a byproduct of cellular respiration. Its presence or absence tells us how well the body is metabolizing oxygen and glucose.
Waveform capnography: The shape of the waveform can indicate conditions like bronchial spasms or lower airway constriction.
Normal EtCO2: In a healthy patient, the target EtCO2 is between 35 and 45 mmHg.
Hypercapnia (high EtCO2): Indicates inadequate CO2 off-gassing, which may require an increase in the ventilation rate.
Hypocapnia (low EtCO2): Indicates excessive CO2 removal, which may require a decrease in the ventilation rate.
Acid-Base Balance
The body uses the concentration of CO2 to determine the rate of spontaneous ventilations. CO2 dissolved in the blood makes the blood more acidic (lowers the pH). This change in pH is detected by the brain and influences the respiratory rate.
Oxyhemoglobin Dissociation Curve: This curve describes the tendency of oxygen to bind to hemoglobin in the lungs and release from hemoglobin at the tissues.
Shift to the left (increased pH): Oxygen has a higher affinity for hemoglobin.
Shift to the right (decreased pH): Oxygen is more easily released from hemoglobin to the tissues.
Homeostasis: The Role of the Respiratory and Renal Systems
The respiratory and renal systems work together to maintain the body's pH balance (homeostasis).
Respiratory system: Makes rapid adjustments to CO2 levels by changing the rate and depth of ventilation.
Renal system: Makes slower, long-term adjustments by producing bicarbonate.
Acid-Base Imbalances
Respiratory acidosis: Occurs due to hypoventilation, leading to CO2 retention and a decrease in pH. This can often be corrected by increasing the ventilation rate.
Respiratory alkalosis: Occurs due to hyperventilation, leading to excessive CO2 elimination and an increase in pH.
Metabolic acidosis and alkalosis: Changes in pH driven by the metabolic system.
Conclusion
As an advanced EMS provider, a thorough understanding of the intricate relationship between ventilation, oxygenation, respiration, and acid-base balance is paramount. This knowledge allows for the accurate assessment and effective management of patients with respiratory emergencies, ultimately improving patient outcomes.
Critical Thinking Questions
A patient presents with shallow, slow breathing following an opioid overdose. Explain the likely impact on their EtCO2 and arterial pH, and describe the immediate interventions an advanced EMS provider should consider.
During a cardiac arrest, capnography shows a sudden drop in EtCO2 from 40 mmHg to 10 mmHg. What could this indicate, and how would it influence your resuscitation efforts?
Differentiate between respiratory and metabolic acidosis. Provide an example clinical scenario for each and outline the primary treatment goals in an EMS setting.
Discuss how factors such as high altitude or carbon monoxide poisoning affect oxygenation at a cellular level, even if a patient's ventilation appears adequate.
Explain the significance of the oxyhemoglobin dissociation curve in the context of a patient experiencing severe hypothermia (e.g., core body temperature of 28°C). How might this affect tissue oxygen delivery?