5.4 Respiratory Physiology: Acid-Base Balance, Oxygenation, Lung Mechanics & Troubleshooting
Key Takeaways
Winter's formula (expected PaCO2 = 1.5 × HCO3 + 8 ± 2) tells you whether a metabolic acidosis is appropriately compensated or has a coexisting respiratory problem.
The anion gap (Na − [Cl + HCO3]) separates high-gap acidoses (lactate, ketones, toxins, uremia, inborn errors) from normal-gap acidoses (diarrhea, large saline infusions, renal tubular acidosis).
Arterial oxygen content is 1.34 × Hb × SaO2 + 0.003 × PaO2, so anemia or abnormal hemoglobin can cause tissue hypoxia despite a normal PaO2.
Hypoxemia from shunt responds poorly to added oxygen, whereas hypoventilation and V/Q mismatch usually improve with supplemental oxygen.
Acute deterioration in a ventilated child is worked through with DOPE: Displacement, Obstruction, Pneumothorax, and Equipment failure. Gastric distension and breath stacking are added checks.
Respiratory Physiology for Transport: Blood Gases, Oxygenation & Troubleshooting
Transport teams interpret blood gases at the referring bedside and often again in the vehicle. They make ventilator and fluid decisions from those numbers. This section gives the physiology behind the ventilation strategies in Sections 5.2 and 7.1–7.3.
Reading a Blood Gas in Four Steps
| Parameter | Typical normal arterial range | Neonatal/transport notes |
|---|---|---|
| pH | 7.35–7.45 | Many NICUs accept 7.25–7.35 during permissive hypercapnia |
| PaCO2 | 35–45 mmHg | Preterm permissive targets often 45–55 mmHg |
| HCO3 | 22–26 mEq/L | Lower in preterm infants because of immature renal bicarbonate handling |
| PaO2 | 80–100 mmHg (older child, room air) | Neonatal targets are usually 50–80 mmHg to avoid hyperoxia |
| Base excess | −2 to +2 | A base deficit beyond about −10 signals significant metabolic acidosis |
- Acidemia or alkalemia? Look at the pH.
- Primary process? If PaCO2 moves in the opposite direction to pH, the process is respiratory. If HCO3 moves in the same direction as pH, it is metabolic.
- Is compensation appropriate?
- Metabolic acidosis: Winter's formula, expected PaCO2 = 1.5 × HCO3 + 8 (± 2). If measured PaCO2 is higher than expected, there is an added respiratory acidosis (for example, a tiring child). If it is lower, there is an added respiratory alkalosis.
- Metabolic alkalosis: PaCO2 rises about 0.7 mmHg per 1 mEq/L rise in HCO3.
- Acute respiratory acidosis: HCO3 rises about 1 mEq/L per 10 mmHg rise in PaCO2 (about 3.5–4 in chronic states such as bronchopulmonary dysplasia).
- Acute respiratory alkalosis: HCO3 falls about 2 mEq/L per 10 mmHg fall in PaCO2.
- In metabolic acidosis, calculate the anion gap: Na − (Cl + HCO3). A typical normal is about 8–12 mEq/L (laboratory dependent).
| High anion gap | Normal anion gap (hyperchloremic) |
|---|---|
| Lactic acidosis (shock, hypoxia, sepsis) | Diarrhea (bicarbonate loss) |
| Ketoacidosis (DKA, starvation) | Large-volume 0.9% saline resuscitation |
| Toxins (salicylates, methanol, ethylene glycol, iron) | Renal tubular acidosis |
| Uremia | Early renal failure, ureteral diversions |
| Inborn errors of metabolism (organic acidemias) | Carbonic anhydrase inhibitors |
Worked example: A 9-month-old in shock has pH 7.21, PaCO2 26 mmHg, HCO3 10 mEq/L, Na 138, and Cl 108. The anion gap is 138 − 118 = 20, so this is a high-gap metabolic acidosis, probably lactic. Expected PaCO2 is 1.5 × 10 + 8 = 23 (range 21–25). The measured 26 is slightly high. That can signal early respiratory fatigue, and the infant needs close watching before a long transport.
Sample sources
- Venous gases run a pH about 0.03–0.05 lower and a PCO2 about 4–6 mmHg higher than arterial gases in stable patients. They are useful for pH and PCO2 trends.
- Capillary gases (warmed heel) track pH and PCO2 but not PO2.
- End-tidal CO2 is usually a few mmHg below PaCO2. The gap widens when dead space rises (low cardiac output, overdistension, pulmonary embolism), so a falling EtCO2 in a crashing patient may mean falling perfusion, not better ventilation.
Oxygen Content, Delivery & the Hemoglobin Curve
- Arterial oxygen content (CaO2) = (1.34 × Hb × SaO2) + (0.003 × PaO2). Almost all oxygen is carried by hemoglobin.
- Oxygen delivery (DO2) = cardiac output × CaO2. A child with Hb 6 g/dL and SaO2 100% can deliver less oxygen than a child with Hb 14 g/dL and SaO2 85%.
- Fetal hemoglobin shifts the dissociation curve left (P50 about 19 mmHg versus about 27 mmHg in adults). The fetus loads oxygen well at low PO2, but release to tissues is less.
- Right shift (easier unloading): acidosis, hypercarbia, fever, and higher 2,3-DPG. Left shift: alkalosis, hypothermia, fetal hemoglobin, carbon monoxide, and methemoglobin.
Why a Patient Is Hypoxemic: Five Mechanisms
| Mechanism | Example in transport | A–a gradient | Response to added O2 |
|---|---|---|---|
| Low inspired PO2 | Unpressurized flight at altitude | Normal | Good |
| Hypoventilation | Opioids, PGE1 apnea, exhaustion | Normal | Good (but CO2 keeps rising) |
| V/Q mismatch | Bronchiolitis, pneumonia, atelectasis | Increased | Good |
| Shunt | PPHN, cyanotic heart disease, consolidated lung | Increased | Poor |
| Diffusion limitation | Interstitial edema, severe lung injury | Increased | Moderate |
The alveolar-arterial (A–a) gradient is PAO2 − PaO2, calculated from the alveolar gas equation in Section 3.1. A normal gradient with hypoxemia points to hypoventilation or low inspired oxygen.
Lung Volumes, Compliance & Resistance
- Functional residual capacity (FRC) in newborns is about 25–30 mL/kg. The infant's very compliant chest wall offers little outward recoil, so FRC is defended actively by grunting (partial glottic closure), rapid respiratory rates, and diaphragmatic braking. Intubation removes grunting, so PEEP must replace it.
- Tidal volume is about 4–6 mL/kg in preterm infants on ventilators and about 6–8 mL/kg in healthy lungs. Anatomic dead space is roughly 2–2.5 mL/kg. Added circuit dead space (flow sensors, connectors) matters more when tidal volumes are small.
- Compliance (C = change in volume ÷ change in pressure) falls in RDS, pulmonary edema, and pneumothorax. It rises abruptly after surfactant.
- Resistance rises with small airways, secretions, bronchospasm, and narrow endotracheal tubes (resistance varies with 1/radius⁴).
- Time constant = compliance × resistance (Section 5.2). Stiff lungs empty fast, while obstructed lungs need long expiratory times.
- Oxygenation indices: P/F ratio (PaO2 ÷ FiO2), S/F ratio, oxygenation index (OI = mean airway pressure × FiO2 × 100 ÷ PaO2), and oxygen saturation index (OSI, using SpO2). An OI above 40 has traditionally prompted neonatal ECMO referral (Section 6.6).
Principles of Ventilation and Non-Invasive Support
Conventional and high-frequency ventilation are covered in Section 5.2. The NCC outline also lists non-invasive and alternative respiratory support:
| Modality | How it helps | Transport cautions |
|---|---|---|
| CPAP (bubble or ventilator) | Maintains FRC, splints airways, reduces apnea | Nasal prong fit, open-mouth leaks, gastric distension (place an orogastric tube) |
| NIPPV / BiPAP | Adds inspiratory pressure to offload work of breathing | Mask leaks, aspiration risk, patient tolerance |
| Heated humidified high-flow nasal cannula (HFNC) | Washes out nasopharyngeal dead space, gives modest distending pressure, and delivers humidified gas | Delivered pressure is not measured; flows often 1.5–2 L/kg/min in infants with bronchiolitis (Section 11.2); prongs must not occlude nares |
| Heliox (helium-oxygen mixtures, e.g., 70:30) | Low density reduces turbulent-flow resistance in upper airway obstruction and severe asthma | Limits FiO2 to about 0.3; requires calibrated equipment |
| Inhaled nitric oxide | Selective pulmonary vasodilation (Section 5.3) | Never interrupt abruptly |
Know the failure signs of non-invasive support: rising PaCO2, recurrent apnea, FiO2 climbing above about 0.4–0.6, worsening work of breathing, and altered mental status. Intubating before departure is safer than intubating in a moving vehicle.
Troubleshooting: DOPE
When a ventilated patient suddenly desaturates, loses end-tidal CO2, or shows rising pressures, take the patient off the ventilator and hand-bag while you work through:
| Letter | Problem | Clues | Action |
|---|---|---|---|
| D | Displacement (extubation or mainstem) | Loss of EtCO2 waveform, changed tube depth, asymmetric chest rise | Check depth, laryngoscopy, reintubate or withdraw |
| O | Obstruction (secretions, kink, biting) | High peak pressure, poor chest rise, resistance to bagging | Suction, check tube and circuit, bite block |
| P | Pneumothorax | Unilateral breath sounds, hypotension, transillumination | Needle decompression, then chest tube (Section 4.2) |
| E | Equipment failure | Empty cylinder, disconnected circuit, ventilator alarm | Hand-bag from a separate oxygen source; check gas and power |
Two more checks complete the list: stomach distension (decompress with a gastric tube) and stacked breaths or auto-PEEP in obstructive disease (disconnect and allow exhalation; see Section 11.2).
A 2-year-old with gastroenteritis has pH 7.27, PaCO2 27 mmHg, HCO3 12 mEq/L, Na 136 mEq/L, and Cl 114 mEq/L. Which interpretation is correct?
High anion gap metabolic acidosis with an additional respiratory acidosis
Mixed metabolic alkalosis and respiratory acidosis
Primary respiratory alkalosis with metabolic compensation
Normal anion gap metabolic acidosis with appropriate respiratory compensation
A term infant with severe PPHN remains at SpO2 70% despite FiO2 1.0. Which mechanism of hypoxemia best explains the poor response to supplemental oxygen?
Low inspired oxygen from cabin altitude
Right-to-left shunt through the ductus arteriosus and foramen ovale
Alveolar hypoventilation
Diffusion limitation from a thickened alveolar membrane
During flight, an intubated 5-year-old suddenly desaturates, the waveform capnogram flattens, and the ventilator alarms for low exhaled volume. What is the first action?
Increase the ventilator rate and FiO2
Obtain a chest radiograph after landing before making changes
Give a sedative bolus for presumed ventilator dyssynchrony
Disconnect from the ventilator, hand-bag with 100% oxygen, and assess for displacement, obstruction, pneumothorax, and equipment failure
Sections you finish are checked off in the contents.