Dr Rajesh Swarnakar, Consultant Pulmonologist, Nagpur 

The world has never been more accessible. In 2024, the global aviation industry witnessed nearly 5 billion people traversing the skies, a testament to our interconnected world. This trend is particularly pronounced in burgeoning markets like India, where domestic air passenger traffic soared to over 220 million in the last financial year, marking a huge increase and highlighting a massive return to air travel. Yet, beneath the impressive statistics of this aviation boom lies a pressing and complex medical challenge: ensuring the safety of a growing number of passengers with pre-existing health conditions, particularly those with chronic pulmonary diseases, at 30,000 feet.  

The convergence of an aging global population and the increasing affordability of air travel has fundamentally altered the passenger demographic. Older individuals, who often manage multiple chronic health issues, are taking to the skies in unprecedented numbers. This demographic shift carries a greater propensity for medical impairment, transforming the aircraft cabin into a potential clinical setting. The controlled, but physiologically stressful, environment of a commercial airliner can turn a routine flight into a medical emergency for the unprepared.

The Hostile Sky: Physiology of Flight and Respiratory Stress

To understand the critical need for “Fit to Fly” assessments, one must first appreciate the unique environment inside an aircraft cabin. To maintain structural integrity and efficiency, commercial aircraft are pressurized, but not to sea-level pressure. During cruise altitude, the cabin pressure typically equates to being at an elevation of 6,000 to 8,000 feet above sea level. At this simulated altitude, the partial pressure of oxygen (PaO2) in the air drops significantly.

For a healthy individual, this triggers a minor compensatory increase in breathing and heart rate, which is often unnoticeable. However, for a patient with an existing pulmonary disease, this hypoxic (low-oxygen) environment can be dangerously debilitating. Conditions like Chronic Obstructive Pulmonary Disease (COPD), severe asthma, interstitial lung disease, cystic fibrosis, and pulmonary hypertension impair the lungs’ ability to oxygenate blood efficiently. The additional stress of cabin hypoxia can push these patients over the edge, leading to severe hypoxemia—a dangerously low level of oxygen in the blood—which can precipitate respiratory distress, cardiac strain, and other critical complications.

The High Stakes of In-Flight Emergencies

The real-world consequence of this physiological challenge is starkly evident in the data. Respiratory events are the fourth leading cause of in-flight medical emergencies. These are not rare occurrences; with the sheer volume of people flying, the actual number of inflight medical events has been gradually increasing over the years.

When a passenger experiences a severe medical event, the captain and crew face a critical decision. Often, the only safe course of action is an unscheduled diversion to the nearest suitable airport. This decision, while necessary for passenger safety, carries an enormous financial burden for airlines. The costs associated with dumping fuel, landing fees, ground services, passenger accommodation, and reassigning a new crew can range anywhere between **$30,000 and $725,000 per incident.** Beyond the financial toll, diversions are logistically nightmarish and deeply traumatic for the unwell passenger, the crew, and fellow travellers.

It is no surprise, therefore, that airlines are increasingly scrutinizing sick passengers on board, especially patients with oxygen requirement. The industry is moving towards a more proactive model of risk management, shifting the focus from responding to emergencies to preventing them altogether.

The “Fit to Fly” Assessment: A Proactive Shield

SCREENING FOR IN-FLIGHT HYPOXEMIA

Indications for screening — while exact criteria for screening for in-flight hypoxemia are lacking, the following are considered to be risk factors for in-flight respiratory symptoms or hypoxemia

• Respiratory conditions with potential for acute deterioration or need for medical intervention:
• Severe (FEV1 <50 percent predicted) or poorly controlled obstructive airways disease
• Symptomatic restrictive chest wall condition or respiratory muscle weakness (vital capacity <1 liter)
• Interstitial lung disease with SpO2 ≤95 percent or diffusing capacity of the lung for carbon monoxide (DLCO) ≤50 percent of predicted
• Pulmonary hypertension
• Hospitalization for respiratory illness within six weeks of intended air travel
• Requirement for continuous positive airway pressure (CPAP) or noninvasive ventilation
• Active cancer with lung involvement
• Comorbid conditions that may be worsened by hypoxemia (e.g., cardiac or cerebrovascular disease)
• Requirement for long-term oxygen therapy, CPAP, or noninvasive ventilation (NIV)
• Pneumothorax within six weeks of intended air travel, increased risk of pneumothorax (cystic lung disease or recurrent pneumothorax)
• Trapped lung with chronic pneumothorax
• Pulmonary embolism or deep venous thrombosis within six weeks or increased risk of venous thromboembolism
• Individuals with bothersome cardiorespiratory symptoms during prior air travel

The assessment typically involves several key components:

1. Detailed Clinical History and Examination: The physician conducts a thorough review of the patient’s pulmonary condition, including its severity, stability, recent exacerbations, and current medication regimen. Comorbidities, particularly cardiac issues, are also assessed due to the heart-lung interplay.
2. Resting pulse oximetry — for patients who do not require supplemental oxygen at rest, measurement of SpO2 is performed while the patient is resting and breathing room air.

The values described here presume that the patient is assessed at sea level.

• SpO2 ≥95 percent – For most patients with a resting room air SpO2 ≥95 percent at sea level, it is unlikely that their PaO2 will decrease below 55 mmHg (7.3 kPa) at cruising altitude, so in-flight oxygen is generally not needed. However, patients with a Medical Research Council dyspnea score ≥3 (calculator 1) should undergo further study, preferably with a six-minute walk test (6MWT) or shuttle walk test.

If the SpO2 is <84 percent during one of these tests, empiric supplemental in-flight oxygen or HAST (if hypercapnia is a concern) is advised.

• SpO2 92 to 95 percent – Patients with a resting room air SpO2 92 to 95 percent at sea level are assessed for risk factors (see ‘Indications for screening’ above) for in-flight hypoxemia and for desaturation on 6MWT to determine the need for further testing. Alternatively, the BTS guidelines suggest providing supplemental oxygen in flight or proceeding to HAST in patients with SpO2 <95 percent. In a study of patients with chronic obstructive pulmonary disease (COPD) and a SpO2 92 to 95 percent, 67 percent of those without risk factors for in-flight hypoxemia and 70 percent of those with risk factors developed desaturation to PaO2 <50 mmHg (6.67 kPa) during HAST , suggesting that risk factors may be inaccurate in predicting the likelihood of in-flight hypoxemia.

Pulse oximetry saturation values of 92 to 96 percent are also less likely to assure adequate oxygenation (i.e., an arterial saturation >88 percent) in Black individuals, where occult hypoxemia (i.e., an arterial saturation <88 percent) with SpO2 values of 92 to 96 percent can be threefold more common (i.e., 17 versus 6.2 percent) than in White individuals.

• SpO2 <92 percent – Patients with a resting room air SpO2 <92 percent at sea level are candidates for supplemental oxygen in-flight. The Global Initiative for Chronic Obstructive Lung Disease (GOLD) guidelines suggest prescribing oxygen at 3 L/min via nasal cannula or 31 percent by Venturi facemask without additional testing , although an alternative is to perform HAST on these patients to confirm the need for in-flight supplemental oxygen.

3. Spirometry and Lung Function Tests: Basic lung function tests provide a snapshot of the patient’s respiratory capacity and the degree of their airflow obstruction.
4. The Hypoxia Altitude Simulation Test (HAST): This is the cornerstone of a comprehensive assessment. The patient breathes a gas mixture that replicates the 15% oxygen concentration found at a cabin altitude of 8,000 feet. Their arterial oxygen saturation (SpO2) is continuously monitored via a pulse oximeter. If their saturation levels drop below a certain threshold (typically 85-90%), it indicates a high risk of in-flight hypoxemia.
5. Arterial Blood Gas (ABG) Analysis: In some cases, an ABG may be taken during the HAST to get a more precise measurement of oxygen and carbon dioxide levels in the blood.

5.6 Minute Walk Test:For patients with COPD and a resting SpO2 ≥92 percent, pre-flight (sea level) assessment with a 6MWT has been proposed to guide the need for HAST.

Patients with a resting SpO2 92 to 95 percent plus a 6MWT SpO2 <84 percent are prescribed supplemental oxygen for air travel without further testing. An oxygen flow rate of 2 L/min is reasonable.

Patients with a resting SpO2 of 92 to 95 percent and a 6MWT SpO2 ≥84 percent are referred for HAST.

Patients with a resting SpO2 >95 percent and a 6MWT SpO2 <84 percent are referred for HAST.

From Assessment to Action Plan

The outcome of a “Fit to Fly” assessment is not merely a “yes” or “no” for travel. It is a personalized safety plan. For patients who pass the assessment without issue, it provides peace of mind and medical clearance. For those identified as at-risk, the assessment provides a clear, life-saving prescription.

The most common intervention is in-flight supplemental oxygen. Commercial airlines do provide medical oxygen, but their policies and costs vary widely, and advance notice is almost always mandatory—often requiring a physician’s form completed weeks before the flight. The “Fit to Fly” assessment provides the precise data needed for this prescription, specifying the required oxygen flow rate (e.g., 2 or 4 liters per minute) to maintain safe saturation levels throughout the flight.

This prescribed oxygen is the key that unlocks safe travel. It mitigates the risk of a mid-air crisis, protects the patient’s health, and provides confidence. It also formally documents the passenger’s needs, preventing last-minute disputes or denials of boarding at the gate.

Conclusion: A Non-Negotiable Step in Modern Travel

In an era defined by mass mobility and an aging populace, the “Fit to Fly” assessment for pulmonary patients is evolving from a specialist’s recommendation to a standard of care. It is a critical junction where clinical medicine and aviation safety intersect. By identifying vulnerabilities before takeoff, this proactive evaluation empowers patients with chronic lung diseases to explore the world without jeopardizing their health. It protects airlines from devastating financial losses and operational disruptions. Ultimately, it ensures that the miracle of flight remains a safe and accessible option for all, clearing every passenger for a journey that is not just memorable, but also secure.